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
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.
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.
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
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
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
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.
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
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
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
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.
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.
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.
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
(left). The pin was only supported on the outside half
of the lifter
bore and sometimes caused a failure. In 2000, a
that extends to the other side of the lifter bore was
(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
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
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.
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
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
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
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
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
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
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
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
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.
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
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
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
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.
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
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
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.
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
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 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.
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
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
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!
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