XL Camshaft Installation


Installing new bumpsticks into your bike is an important part of almost every high performance project, but it's really important that you do it right. That means checking a number of clearances and making adjustments as needed. None of this is difficult at all, everything described here is all well within the reach of most do-it-yourselfers and requires only inexpensive tools. Although this guide is written around the XL motor, most of what's presented here applies to Big Twin motors as well.


Exactly what will need to be clearanced, and how much, is directly a function of the cams chosen. Some cams likely won't need any special attention at all; these are commonly called "bolt-in" cams by their manufacturers. And while you can certainly get good results with stock heads using bolt-in cams, head work often improves the heads in ways that require more cam to take advantage of. So to get to that next level, a larger set of cams and the clearancing that goes with them becomes important.


The parameters of the cam grind that will have a direct affect on the amount of clearancing required are:


TDC Lifts: This is the amount the valves are open when the piston is at TDC during overlap, i.e. between the exhaust and intake strokes. During the other TDC, the one that occurs between the compression and power strokes, both valves are closed. At overlap TDC however, both valves are slightly open and this is the point where piston to valve contact can occur. Higher TDC lifts naturally make this contact more likely. Having both valves open a large amount at the same time can also cause problems with valve to valve clearance.


Maximum Lift: This is the maximum amount each valve gets opened. A common misconception is that max lift causes valve to piston clearance issues. This isn't true at all, because maximum lift always occurs with the piston well down the bore. However, it causes the valve springs to compress farther, possibly too far, and the resultant bind will bend pushrods or break rocker arms. Even if the springs don't bind, without adequate coil bind clearance the spring's life will be shortened dramatically. High lift can also cause the valve spring retainer to run into the valve guide seal that sits atop the valve guide, again damaging parts. And high lift often causes the rocker arms to contact the rocker box tops, especially on 03 and earlier motors. Down at the gearcase, a high lift cam's taller lobes can run into the bottom of the tappet bosses as well as the pinion bearing race. Finally, the tappets themselves can run out of travel in their bore with taller lobes, causing them to run into the tappet anti-rotation pins. So while high lift won't cause piston to valve clearance issues, it most certainly can cause a range of other clearance problems.


Base Circle: One alternative to using a higher lobe to get more lift is to make the cam with a smaller base circle. So in other words, rather than lift the tappet higher, a small base circle cam lets the tappet down farther, which increases lift because lift is the difference between the base circle and the nose of the lobe (times the rocker ratio). So this solves or makes less severe the gearcase clearance issues as well as the tappet anti-rotation pin clearance issue. However, a small base circle cam has a more severe angle of attack on the tappet roller, and is therefore a less desirable arrangement from a geometry point of view. For this reason, some cam grinders don't use a small base circle even in very high lift cams. Smaller than stock base circles are most commonly found in the larger Andrews grinds as well as the SE .551 lift "E" cam grind.


So let's go through each of the checks and see what to look for and how to make adjustments as necessary, as well as spot check our cam timing. Step one though is the ...


Disassembly:  Before you can remove the gearcase cover and remove the cams, you need to relieve the pressure from the cams. Anytime a valve is open you've got some pretty substantial pressure on that camshaft, and there's always at least one valve open somewhere on an HD twin.


If you've got adjustable pushrods and collapsible covers, you can probably accomplish this without pulling the rocker boxes. Simply shorten each pushrod to close the valve and relieve the pressure. However, if you've got the stock setup, the rocker boxes need to come off. Pull a rocker box top and then position the engine (a swingarm stand works well) such that both valves are fully closed. You want the motor sitting about halfway between the point where the intake valve closed and the exhaust valve starts to open while turning the engine forward. Be careful not to position it in between the exhaust close point and the intake open point; that's overlap and there's no spot there where both are fully closed. Now remove the lower rocker box. Repeat the process for the other cylinder.


Now go downstairs to the gearcase. You may have to remove the exhaust system and footpeg and/or brake pedal assembly to get enough access, depending on your bike. For an '03 or earlier, you'll want to remove the timing cover, remove the timing plate, and the rotor cup. Now remove all the cover screws and gently work the cover off. I generally push on the #2 cam, through the timing hole, to make it stay in the engine as I do this. Once you get the cover to pull free from the engine, you'll notice that it's still attached by the timing pickup plate wires (03 and older only) as well as a vent hose coming off the back of it. Often you can simply leave these attached and rotate the cover backwards out of the way.


Your old cams will now slide out. Wrap them in some rags or something to keep them from getting banged up.


Lobe Swing Clearance: The first thing to make sure of is that each of the 4 new cams will spin freely in it's cam bushing without running into anything. You'll want to remove the pinion gear before doing this check, so that you can spin the #2 cam. Place each cam lobe into it's cam bushing in the right crankcase half and try to turn it around. Make sure it has at least .020 of clearance between the lobe and the bottom of the tappet bosses.


Providing lobe swing clearance on an XL motor. If the engine is apart, this is the

best way to do it, on a milling machine. NRHS can handle this service for you.


If you have to clearance the tappet bosses on an assembled engine, first carefully mask off the entire gearcase so that no chips or filings can find their way into the crankcase or the pinion bearing or the oil pump. Use a die grinder or a Dremel tool. Thoroughly clean out all the chips and filings.


This clearancing job was done by hand with a die grinder. Do it carefully and you can get

good results.


Another common area to have interference is between the lobe and the pinion bearing race, on the number 2 and 3 cams. This is especially an issue on 2000 and later motors.



To gain clearance for the pinion race, I recommend chamfering the back side of the cam lobe instead of grinding on the race. Carefully grind a 45 degree chamfer that protrudes onto the lobe surface no more than about .100". It seems odd to narrow the cam lobe, but I've done this many many times and it's never caused an issue. Be sure to deburr it so you don't have a burr reaching up that may damage the tappet roller. Grinding on that pinion race, particularly on an assembled engine, is difficult and carries too much risk, I much prefer modifying the lobe like this.


Chamfering the inside top edge of the lobe is simplest and least risky method of

gaining a little clearance to the pinion race. 2000 and later motors are more likely

to need this modification.


Cam Bushing Clearance: One of the side effects of a dropping the lifter farther to gain more lift (as is done with small base circle cams) is that sometimes the lifter will make contact with the cam bushing when on the base circle of the cam (i.e. valve fully closed). Make absolutely sure the tappet roller is still resting on the cam lobe and not hanging up on anything! This is easy to check by positioning the cam with the base circle toward the tappet, and then pulling the cam out while pushing down on the tappet. You should see and feel the tappet fall down slightly as it comes off the lobe base circle. If it doesn't, it's not resting on the lobe. Correct this by slightly grinding on the bottom side of the tappet axle support.


Gear Fit: This is the next thing you may need to address. In 1999 and older motors, each of the cams was factory selected from seven different sizes to fit your motor precisely, providing quiet operation. The #2 cam, which has two gears on it, literally has 7 different sizes for each of those gears, creating 49 possible combinations for that gear alone. Furthermore, your pinion gear (which meshes with the larger gear on the #2 cam) came in 7 different sizes. So the number of possible combinations of cam gears in a given motor is staggering.


Each of the cams was color coded from the factory. There's a colored dot somewhere on the cam gear that corresponds to which of the seven possible sizes it is. It's pretty easy for these marks to disappear in normal operation, cleaning, and handling of the cams so be sure to take a look at it and record the colors for each cam, as well as the pinion gear, in case you want this information later.


Cam gear sizes can also be measured with a tenth-reading micrometer and a set of gauge

pins. The service manual lists the sizes for each color code.


Aftermarket cams for these '99 and older motors are made slightly on the loose end of the spectrum. In most motors, they'll slide in and work fine, although they'll usually be a little more rattly than the factory fitted cams. A little rattling won't hurt anything, what you're concerned about here is that they don't fit too tightly. Cams that fit too tight will whine, but worse than that, they cause localized gear tooth heating that can lead to tooth failure. So this is not a step you want to skip.


First put all four of the new cams into the gearcase cover and make sure they turn without any binding. You may want to put some oil on the teeth, without it you'll get stiction even with loose fitting cams. Now put all 4 cams into the motor (without the pinion gear in place), and repeat the same check. As a final check, put the cover on (without the pinion gear), put the timing rotor onto the end of the #2 cam, and make sure you can rotate the cams freely.


Always make sure your new cams spin freely in both the gearcase and the gearcase cover.

To spin them in the gearcase, remove the pinion gear as shown above.


In the ideal world you'll also check the fit between the #2 cam and the pinion gear. Of particular concern is excessive pinion shaft runout which is not all that uncommon especially with bolt together flywheel assemblies. It's not a bad idea to rig up a dial indicator and take a look at the runout. Assuming it's within spec, about all you can do is put it all together and make sure the flywheel and cams spin freely. This can be tough to do on an assembled engine, though, because you've got friction from the rings and the primary/transmission as well.


99 and older cam gears use a coarser tooth pitch (left) than the later motors

(right). Engines can be updated or backdated by changing the pinion gear.


If you have an '00 motor, you may notice that the tooth pitch on the pinion gear & it's mating gear on the #2 cam have a finer tooth pitch than the other gears. This is called a "high contact ratio" gear cut and it eliminated the need to fit this connection. In 2001, this change was propagated to all of the cam gears. Cam sets specified to fit '00 and later motors may have high contact ratio at the pinion connection only, or may have them on all the connections. If you have the latter, in theory you don't have to worry about all this gear fit stuff.


However, a number of companies don't offer their more aggressive cams in high contact ratio, claiming that the finer tooth pitch is a weaker arrangement. I've personally never had a failure with one, but I'll take their word for it anyway. To put these cams into your 00-up motor, you'll have to backdate your pinion gear. Older style pinion gears are readily available from NRHS and will go right in to your newer motor, allowing you to use these more aggressive cams.


You can also update your '99 or older motor to high contact ratio if you'd like. All it takes is an update of the pinion gear. The high contact ratio pinion gear as found on newer engines is also readily available from NRHS.


Despite the fact that you don't have to worry about fit when using high contact ratio cams, I can tell you that many many times I've found HCR equipped motors to be very rattly. Especially after a cam change. Especially when using the factory "E" cams. You pays your money and you takes your chances.


Another solution to the whole cam gear fit issue is to use Red Shift cams. Red Shift cams are made by grinding the lobes off your old factory cams and welding new lobes onto them. This preserves the factory fit. And Red Shift offers some really good grinds, although none of them are of the "bolt-in" variety. NRHS is a dealer for Red Shift cams and can fix you right up.


Done carefully, the drive gear on the #2 cam can be pressed on or off. This is

useful when using 91-99 cams in 00-up motors or vice-versa, or just to retain

your factory gear fit on this connection. We sometimes move the gear slightly

one way or the other to alter the power range of the cams.


Finally, be aware that the larger gear on your #2 cam can be pressed off and moved to your new cam set, if you desire, so that you can use your old pinion gear even when changing a new motor to the old style cams or vice-versa. Just be sure you know how to press it on there and keep the cam timing correct. Later I'll go over a simple method for spot-checking cam timing, it'd be good to know how to do this if you're going to mess with that gear. We often move the gear intentionally just to alter the cam timing and the power characteristics of the motor.


Oil Pump Drive Gear: Directly under the pinion nut, there's a worm gear that meshes with your oil pump gear. This gear can be pulled off without too much trouble once you've got the pinion gear off. Take a close look at this gear. Some motors love to wear it out prematurely. If you see the teeth going sharp on you, get a new one now, BEFORE it fails. Look at the matching gear on the oil pump as well. If there's wear, this may be a good time to change the oil pump. If you have a '97 or older bike, you can upgrade to the '98 and newer pump which has a much larger scavenge section. It also has a scavenge inlet inside the cam box, allowing you to plug the drain between the cam box and the crankcase thus decreasing the oil drag on the flywheel assembly. NRHS can supply this pump for you, or even better, a Pro-Flow pump.


Pinion Nut: The way the pinion gear is retained on XL motors since 1987 is a bit problematic. You've probably noticed by now that there's a woodruff key under your oil pump drive gear that keeps it rotationally attached to the pinion shaft. Just a small portion of that woodruff key sticks out and provides the same service to the pinion gear. Well, it's not nearly as uncommon as it should be for the tip of that woodruff key to shear off, allowing your pinion shaft to spin freely of your pinion gear, and that'll quickly send pistons crashing into valves. Not a good thing. This especially happens with the big valve springs that often go with high performance cams.


Only that little tip of the woodruff key that protrudes from under the oil pump drive gear

rotationally locates the pinion gear. Heavy valve springs can sometimes cause it to shear,

sending pistons crashing into valves. Loctite red and 70 ft-lbs on the pinion nut will make

this failure extremely unlikely, however. Also, always carefully check your oil pump

drive gear for wear. If you see the teeth starting to sharpen, replace it.


The good news is that you can make the failure very unlikely by simply tightening the pinion nut to 70 ft-lbs. Now there's enough clamp load on the pinion gear that it relieves some of the burden from the woodruff key. Be sure and use Loctite red on this fastener as well.


Tappet Anti-Rotation Pins: In '99 and older motors, the tappet anti-rotation pins are kind of a weak design. They're only supported on the outboard side of the tappet bore. This can sometimes lead to failures, particularly if you've got marginal clearance to the tappet body (more on that in a minute). The first thing you ought to do here in any 1991-1999 motor is to update these pins to the new style that came out in 2000.


91-99 motors used a short tappet pin with triangular retainer

(left). The pin was only supported on the outside half of the lifter

bore and sometimes caused a failure. In 2000, a screw-in pin

that extends to the other side of the lifter bore was used instead

(right). We highly recommend updating all 91-99 motors to this new

style pin. NRHS can handle this service for you.



The '00 and up pins (part number 18532-00, 4 needed) are threaded into the case rather than retained by the little triangle piece. To update a '91-'99 motor, first carefully drill the pin hole deeper using a 3/16 (.1875") drill bit. Drill it to a depth that'll be adequate for the full length of the pin, but not so deep you break into the cylinder area. A little tape wrapped around the drill bit at the appropriate depth level works well to tell you when to stop. Be sure to drill as straight as absolutely possible, you don't want that pin to go sideways in there as it'll bind up the tappet. This whole operation is best accomplished on a good drill press or milling machine, but it can be done successfully on an assembled engine using a hand drill if you're careful. Then drill the outboard portion of the hole with a #7 drill bit to a depth adequate for the threaded portion of the pin, and carefully tap the hole to 1/4-20NC. Use plenty of lubrication and a sharp tap to reduce the risk of breaking off the tap, that can be a real nightmare to fix.


This is what it looks like when a 91-99 engine has been updated

to the 00-up style tappet anti-rotation pins.


Okay, you've got the tappet pins updated, the cam box is all clearanced and your cams fit good. Next up is a check of the tappet anti-rotation pin clearance. If you pulled your tappets you probably noticed that the front and back sides have flat areas on them. The tappet anti-rotation pin  goes against one of these flats to keep the tappet from turning in it's bore.


The problem comes in because on the factory tappets, the flat area often doesn't extend down far enough. As a result, the tappet body can run into the pin at full lift. This makes for a noisy valvetrain and can even cause a failure of the pin.


Aftermarket tappets (right) often provide more anti-rotation pin clearance than the

stock piece (left). The stock tappet can be modified to provide this clearance, however.


To check this, rotate the motor such that the lobe is at full lift. Stick a magnet down into the lifter bore and pull the tappet up until it stops against the pin. It needs to pull up at least .060" to have enough running clearance. To gain more clearance, the flat area on the tappet needs to be machined farther down the lifter body.  High performance tappets sold by NRHS are already equipped with a longer flat area to accommodate high lift cams.  Tappets are really hard and difficult to hold straight for machining so we recommend getting aftermarket lifters.


Before buttoning up your cam box, make sure everything is spotlessly clean and also lubricate your cam gears and lobes thoroughly. We recommend Red Line assembly lube, available from NRHS.


Cam Timing: Itís very simple to perform a spot-check of your cam timing, this is particularly valuable in cases where the alignment marks on the cams leave room for interpretation. With the cams installed but no pushrods in place, rotate the engine while watching the tappets on one cylinder. Position the engine approximately halfway between the point where the intake valve closes and the exhaust valve opens; this will ensure the lifters are on the base circle of the cams. Do not try to do this between the exhaust close and intake open event, thatís the overlap period and there is no point where both valves are closed. Once the motor is positioned correctly, use a dial caliper positioned as a depth gauge and measure the distance between the top of the tappet block and the top of the tappet. Record these numbers. Now position that cylinder at TDC on overlap, i.e. where the exhaust is closing and the intake is opening, and repeat the measurements. For each tappet, subtract the overlap measurement from the base circle measurement and multiply the result by 1.625. You have just measured your TDC lift.


Compare these measurements to the TDC lift specifications for your cams. They wonít be exactly the same, but they should be pretty close. If there is a large discrepancy, open the gearcase back up and investigate the possibility that youíre a tooth off in your cam installation. If your exhaust TDC lift is larger than the spec and your intake TDC lift is smaller than the spec, it indicates your cams are retarded, i.e. everything is happening later than designed. Likewise, if your exhaust TDC lift is smaller than the spec and your intake TDC lift is larger than the spec, it indicates your cams are advanced, i.e. everything is happening sooner than designed.  Repeat the measurements for the other cylinder.


Pushrods: A strong, high quality pushrod that doesn't deflect is essential for an engine using high performance valve springs, which your cams may call for. In addition to strength, look for pushrods with a ball type tip on the top side, as the semi-circle type tips can dislodge from the rocker arm pocket at the severe angles associated with very high lifts. All NRHS pushrods are equipped with a ball style tip.


Left to right: stock, large diameter aftermarket, tapered aftermarket, NRHS

Hurricane. The factory pushrod is light but flimsy, it's not suitable for high

spring pressures or high rpm. Avoid the semi-circle tips as shown on the

second pushrod, they can dislodge from the rocker socket at high lift. Also, large

diameter pushrods frequently rub in places and cause noise. Tapered

pushrods lessen the chances of rub, but some are still too big, notice the rub

mark on this one. Hurricane pushrods are strong and stiff and use an extra

strong round ball tip, and don't have rub issues.


Many, many cases of excessive valvetrain noise can be traced to "pushrod rub". This happens when a pushrod is too large in one or more areas and rubs against an area of the pushrod tube. It's a common problem on large diameter or tapered style pushrods, both in the area where the pushrod passes through the head into the rocker box, as well as the area just above the adjuster where the pushrod tube necks down. NRHS pushrods are carefully designed to minimize rub problems while providing superior stiffness and rigidity.


The most fundamental parameter of a pushrod to get correct, though, is the length. Several things can affect the length of pushrods you need, including head milling, thin gaskets, small base circle cams, sinking of valves, and aftermarket rocker arms.


Really what you're trying to do is get the correct tappet plunger preload or running clearance, depending on the type of tappet you're using. The three common types of tappets are:


1) Hydraulic: This is what your bike came with stock. It's equipped with a tappet plunger that has about .200" of travel. Ideally, you'd like the tappet plunger preloaded about halfway through it's travel, or about .100", when you assemble the engine. If you're more than .050" away from being centered, we recommend that you alter your pushrod length to get closer to the middle of the plunger travel. NRHS offers non-adjustable pushrods in both stock and minus .040" lengths; the latter is commonly used with heads that have been milled and/or thin gaskets to optimize squish and compression.


Adjustable pushrods can also be used with hydraulic tappets. Adjustable pushrods will require a special collapsible pushrod cover kit available from NRHS. Simply raise the pushrod cover to gain access, and then extend the pushrod adjustment by finger until all of the slack is taken up between the tappet and the rocker arm. Now extend the pushrod by an additional .100" to preload the plunger. The easiest way to do this is to count the turns of the adjuster. With a 32 thread per inch adjuster as used on all NRHS Hurricane pushrods, 3 turns will preload the plunger about .100". Tighten the locknut securely.


Anytime a hydraulic tappet is used, whether with adjustable or non-adjustable pushrods, it's very important to let the tappet "bleed down" before rotating the engine. It takes some time after assembly for the oil to bleed from the cavity underneath the plunger, and until it does, valves are being held open. Rotating the engine before the tappet has bled down and closed the valves can cause valve to piston contact and tear up parts. If you can access the pushrod with your fingers, you can tell when the tappet has bled down because the pushrod can easily be rotated.


2) Solid: This type of tappet has no plunger, it's just a solid piece of steel with an oil hole in it. It's often used in high performance motors where absolutely no lifter collapse or "pump-up" at high rpm can be tolerated. However, it requires frequent adjustment and is therefore not generally used on street motors.


A solid tappet essentially requires an adjustable pushrod to set the running clearance correctly. The running clearance is measured between the valve tip and the rocker arm tip using a feeler gauge. Contact the cam manufacturer for a running clearance recommendation when using solid type tappets.


3) Hydrosolid: This is a hybrid between the hydraulic and solid type tappet. The plunger has a much reduced travel (on the order of .070") and is designed to be operated in a bottomed or near bottomed state. So lifter collapse is prevented, and yet hydraulic action remains to provide some level of self adjustability.


Like a solid tappet, a Hydrosolid essentially requires an adjustable pushrod. To adjust the pushrod when using a Hydrosolid, extend the pushrod about .100" or so beyond the zero lash condition, opening the valve in the process. Now wait at least 10 minutes for the tappet to bleed down. The pushrod should remain tight, which tells you the plunger is bottomed and the valve is still being held open. Now shorten the pushrod until you just feel it go loose as the valve closes. Tighten the locknut at this length. Some people shorten the pushrod an additional quarter turn or so from this spot which can sometimes make the valvetrain run quieter.


If you can beg, borrow, or steal this tool for installing and removing pushrod clips on

collapsible pushrod covers, do it! Better yet, buy it from NRHS. It greatly simplifies a job

that's otherwise a huge PITA.


High quality tappets of all three types are offered by NRHS, as well as collapsible pushrod tube kits and high quality pushrods.


Rocker Box Clearance: Upstairs in the rocker box, the extra lift of your cams might try to make the pushrod side of the rocker arm get friendly with the top of the rocker box. This is especially a problem on '03 and older Sportster motors ('02 and older Buell motors). Fortunately, this is an easy thing to check with some simple baking clay available from your neighborhood hobby store. On some motors, you can also have a clearance issue on the valve end of the rocker arm when the valve is closed. This happens when the valves have been sunk a fair bit, or when longer than stock valves have been used to accommodate long travel springs. Some aftermarket rocker arms are shaped in such a way that it aggravates this problem. The S&S rocker arms preferred by NRHS are the best ones we've found for minimizing rocker box clearance issues.


Contact between the rocker arms and the rocker box tops not only causes noise, but it also can cause some difficult to diagnose leaks. I once had a customer bring me his bike, which had a chronic leaking rocker box top that the local dealer had tried but failed to fix. I popped the top off, saw the two little indents from the rocker arms, ground those areas out a little bit, put it back together, and it never leaked again. The customer thought I was magic or something, but in fact the dealer tech just didn't work much with high performance cams and didn't know what to look for.


High lift cams will almost always need a little grinding on the underside of the rocker

box top. XB or 04-up type rocker box tops won't likely need this, however.


Although not specifically related to cam install, I'm going to mention spring clearance anyway, just because it's such a critical issue. High performance springs are almost always larger in diameter than the factory springs and they'll make contact with the walls of the rocker box if adequate clearancing isn't performed. This is not a nuisance noise thing, this is a critical critical critical clearance. If the spring contacts the rocker box wall, it forces the valve to land unevenly on the valve seat. That will cause valve seat recession and subsequent poor valve seal and loss of compression. Always always always make absolutely sure your springs don't touch your rocker boxes anywhere!


Clearance your rocker boxes enough to make SURE your valve springs won't

make contact. This is not a noise nuisance thing; you will cause damage to

your valve seats if you don't provide enough clearance here.


Coil bind clearance: As the cam lobe opens the valve and compresses the spring, it's critical that the spring not go into a coil bind condition, or even really come very close to it for that matter. .050" of coil bind clearance is considered a minimum, and we like to see street motors set up at .070" or more to promote good valve spring life.


So how do you check this? Well, you can calculate it if you know the coil bind height spec of the spring, the installed height, and the lift of the cam. Every spring has a coil bind height, i.e. a height at which the spring cannot be compressed any further. For NRHS .600 and .650 lift springs (orange and purple stripe respectively), this height is 1.125". For NRHS .700 lift springs (green stripe), it's 1.175". If we prepared your heads, we can let you know the height at which we installed your springs, we keep meticulous records on every head we ship. Subtract the cam lift from the installed height, and then subtract the coil bind height to get the clearance. For example, if you're using a .575 lift cam and we set up your purple stripe springs at 1.850" installed height, your coil bind clearance is 1.850 - .575 - 1.125 = .150". This is a good clearance that will give long spring life.


This fixture from Trock Cycle allows us to precisely open either or both valves to the exact

lift desired. It's used for flow testing as well as checking valve to valve clearance as well

as total valve travel available.


A more direct (and accurate) way to make the measurement is to attach something like a Trock fixture to the head. This is a tool that lets you turn a screw to open the valve and precisely measure with a dial indicator exactly how far you've got it open. Simply open until you feel bind and note how much travel you had available. Subtract your cam lift from this number and you have your clearance. Note that it's possible the bind occurred on the retainer to guide clearance instead of coil bind, though. So you're really checking both of these things at once.


All NRHS heads are flow tested before being shipped, and as part of the flow test, we open the valves as far as .700. If you contact NRHS, we can verify for you that we indeed opened the valves at least this far, and even tell you what the flow number was. Only the largest cams will have any clearance concerns on a valve with .700" of travel.


Retainer to Guide Seal clearance: The bottom side of the valve spring retainer often comes in close proximity to the valve guide seal at full lift. You need at least .020" of clearance here. To measure this, attach the valve spring retainer to the valve stem, using the valve locks, without the spring in place. Hold the valve spring retainer up such that the valve is fully closed and using the other hand, position a caliper between the stem seal and the bottom side of the retainer adjacent to the valve stem seal. Subtract the cam lift spec from this number to get the clearance.


Checking retainer to guide clearance. Subtract the cam lift from the

measurement to determine the clearance. You need at least .020". This

head is equipped with NRHS Hurricane guides which are shorter than

stock; the caliper is showing .780" of travel available which allows

up to a .760" lift cam.


Retainer to guide seal clearance is most often adjusted by shortening the valve guide. NRHS bronze guides (included on most Stage 1, 2, and 3 head porting services) are much shorter than factory guides and afford a great deal more room, and thus it's not likely you'll have this issue using our guides. Using factory guides with high lift cams, however, it's definitely a concern and needs to be carefully checked.


Spring Pressure: For proper control of the valvetrain, the valve springs must provide adequate pressure to keep the lifters following the cam lobes and prevent any slack from occurring in the valvetrain. The valvetrain cannot be reasonably expected to survive if this requirement is not met.


How much spring pressure is needed? The cam manufacturer will generally specify this, and they'll almost always specify the "seat pressure", i.e. the pressure with the valve closed. That's because on the seat is where the pressure is at it's lowest. But be sure to get an rpm spec to go with that seat pressure. If you're planning to turn more rpm than the cam manufacturer is specifying the pressure for, you'll need more pressure.


We use a high quality spring tester to measure seat pressure, open pressure,

and coil bind height.


Low cost spring testers that work in a vise are also available. They do a reasonable job

and are popular with do-it-yourselfers.


I'm not a fan of excessive pressure, it prematurely wears things out. But even worse is not enough pressure. Even if you're not going into full float, inadequate pressure causes seat bounce, and that can really tear things up as well as rob horsepower. If in doubt, err on the side of more pressure.


This tool is used for measuring spring installed height. Correct spring installed

height is critical to ensure proper spring pressure and enough travel. The

spring is shimmed as needed to achieve the desired height.


If NRHS prepared your heads for you, and you let us know what cams you're using, we've sprung the heads accordingly and we can provide the data if you need it.


Valve to Valve Clearance: As mentioned previously, during overlap both valves are slightly open. In a head like a Harley has, where the valves are not parallel to each other, having both valves open at the same time reduces the distance in between them.


For safe operation, it's desirable to have .060" or so of valve to valve clearance, as measured with both valves open to their TDC lift spec (the Trock fixture described earlier is a good tool for holding the valves at precisely the right position - alternatively the springs can be removed and the valves positioned by hand and measured with a caliper).


Adjusting valve to valve clearance is generally done by sinking the valve in the head, which is done by cutting the seat farther down. Sinking the valves hurts low lift flow, though, because it shrouds the valves. When we have to do it, we always "unshroud" the valve with a special cutter that removes chamber material around it so that flow is restored. The sinking and unshrouding of the valve then makes the chamber bigger, which in turn lowers compression unless the head is also milled. And milling the head introduces it's own set of issues, you really never want to mill a head any more than necessary. So sinking the valves excessively causes a chain of undesirable events; we often refer to it as "molesting the heads to make them work". Done in moderation, say up to about .050", sinking valves is manageable. Beyond that, it starts causing more issues than it solves and you're better off to seek a solution through a smaller exhaust valve, for example.


It's not unusual at all for people to run valve to valve clearance tighter than .060" as well. So long as you know you have adequate spring pressure to control the valvetrain at the rpm you intend to run, down to .045" or so is common and not a problem, and often far preferable to molesting the heads. I know an XR750 racer who routinely runs it as tight as .030" and turns rpm's approaching 9,000. If you've got good control of things, you can get away with this.


Valve to valve clearance checking and adjustment is something NRHS does at all levels of head preparation, so if we set up your heads for you and you told us what cams you're using, you're good to go.


Valve to Piston Clearance: Similar to valve to valve clearance, this is also a TDC lift driven clearance, although it's not something we can check for you when preparing your heads.


During overlap, i.e. that window of time when the exhaust valve is almost closed and the intake valve starts opening, the piston passes through top dead center (TDC). Since both valves are open a little at this time, clearance to the piston becomes a concern.


This is best checked with clay in the valve pockets, the same baking clay mentioned earlier. Don't get crazy with the clay; a little bit around the perimeter of the valve pocket is fine. You're looking for actually two things: depth clearance and perimeter clearance, the latter of which we commonly call "eyebrow" clearance. NRHS pistons all come equipped with large and deep valve pockets so this will be less of a problem if you use our pistons. However, making the pockets big enough to accommodate EVERY cam and valve size would excessively weaken the piston for the 99.9% of the folks who run more typical stuff, so we can't guarantee everything will be okay. You have to check it, and it's the engine builder's responsibility.


Position the motor at TDC on the compression stroke and place the clay in the pockets, and then rotate the motor forward a little (into the intake stroke) to take the piston an inch or so down the bore. Now bolt on the head (with the head gasket) and the rocker box with the pushrods in place. Adjust the pushrods and let the tappets bleed down as needed. Now rotate the motor around, through the overlap TDC and back to the compression stroke (but not all the way up the compression stroke, just enough to close the intake valve. Pull the head back off and carefully measure the clay.


For pocket depth clearance, you want at least .060" on the intake valve and .090" on the exhaust valve. The reason for this difference is that the piston chases the exhaust valve closed, but the intake valve chases the piston down. So valve float can cause exhaust valve to piston contact, but not intake valve to piston contact. All you're looking for on the intake valve is enough clearance to accommodate the thermal expansion of the piston.


This piston got a little bit friendly with the valve because it didn't have quite enough

"eyebrow" clearance. This wasn't enough to bend the valve, however.


For eyebrow clearance, you really need .050" on both valves to make sure you don't get contact. Often on race motors we simply can't put that much into the pocket without excessively weakening the piston, and we're not alarmed if we pull the motor apart later and see evidence of minor contact. Letting the valve do the final little bit of clearancing for you is actually not a bad strategy for getting precisely how much clearance you need without weakening the piston more than necessary. So long as you don't bend the valve that is.


If you followed all of these steps and made your measurements carefully, you can rest assured that your cam install will be successful. If you have any questions or concerns at all, call us. We'd much rather help you through the process than help you repair a failure!


Return to tech articles index