Checking Valve to Piston Clearance


Piston to valve clearance is a critical dimension, as not enough clearance here can cause bent or even broken valves. But few people take the time to measure and adjust it. It's not difficult to do, and I highly recommend checking this if the TDC (top dead center) lift figures on your cams are on the large side, say over .220" on either or both valves.


The TDC lift spec tells how much each valve is open at overlap TDC, i.e. the TDC that occurs between the exhaust and intake strokes. Both valves are slightly open at this point. Since the piston is passing through top dead center with both valves open, this is where you may run into issues. The other TDC, between the compression and power strokes, both valves are closed.


A quick and dirty method for checking valve to piston clearance is to attach a Trock fixture or similar to the cylinder head, and with the engine positioned at TDC, open each valve until contact just occurs, measure how much lift it took, and subtract the TDC lift figure. The result is your clearance. A lot of people do it this way, but there are some drawbacks. The biggest one in my mind is that the method only checks for depth clearance. Equally important and at least as likely to be an issue when using large valves is what we call "eyebrow clearance", the clearance between the perimeter of the valve and the i.d. of the valve pocket. The other disadvantage to the static method is that it's not giving a dynamic dimension. Since the valves and the pistons are both moving in the real operation of the motor, the point where they're closest together is not necessarily right at TDC.


So what I'm going to show here is how to "clay" a motor. In my opinion, this is a better way to do it. The first thing you need to do is position the motor correctly:



In this picture, Jon has a ratchet and socket on the final drive pulley nut, the motor is in gear, and he's turning the motor over to position it. If your motor is in the bike you can do much the same thing, turning the rear wheel instead. Get the rear wheel up off the ground by placing a small scissor jack under the frame, or better yet, use a bike stand or swingarm stand.


So where do you position it? We want to start this process at "compression TDC", i.e. top dead center (TDC) between the compression and power strokes. At compression TDC, both valves are closed. Conversely, as mentioned, at overlap TDC both valves are slightly open. For putting the clay in, we want to start at TDC with both valves closed, then we'll roll it through overlap TDC to compress the clay.


To determine which TDC you're on, watch the lifters as you turn the engine. When you see the intake lifter going down, i.e. intake valve closing, the next TDC is the compression TDC.


Okay, now that we're at TDC in between the compression and power strokes, we're going to pack some clay into the valve pockets.



The best thing I've found for this job is oven-bake clay from my local hobby store. Peel off a little bit, pack it into each valve pocket, and then ...



Take a sharp knife and trim away the excess. Too much clay in the pockets will make it get between the valve and seat when we turn the motor over.



You want it to look about like this. Just around the perimeter of the pockets. Now turn the motor slightly forward and ...



Put the head gasket into place. Something to watch for here:



These Cometic gaskets are multiple layers of steel that are riveted together. Notice how this cylinder has a notch on each side to accommodate the rivets. Most cylinders aren't like this. You need to make sure that rivet doesn't get sandwiched between the head and cylinder and keep the head from sitting flat. It's usually only one side, the spark plug side, that has an issue. We generally just grind off the rivet and it's little tab. The alignment dowels on the cylinder will hold it all in alignment anyway.


Okay, now that the gasket is in place, carefully put the head on, without scratching up it's deck, and torque it into place:



Be sure to follow the order as specified for the gaskets, or if the gaskets don't have an instruction sheet, go by the service manual. For the Cometic MLS/EST gaskets, torque in steps of 9, 14, 22, 35, and finally 42 ft-lbs.



Most high performance springs will require you to clearance your rocker box. This is really important, because if that spring or retainer hits the rocker box, it makes the valve land unevenly on the seat, which can cause seat recession and loss of compression. Make SURE you get this right. Remove the rocker arms before you do this, clean it thoroughly after you're done. Place it over the head and line up the bolt holes and take a look. Go back and grind on it some more until you're sure you've got lots of clearance. When you've got it right, clean it up one last time and then put the rocker arms back in, with assembly lube on the bushings.



This is what you want to see, making sure the valve is fully closed when you look at it (best to do this check before you put the rockers back in so you know the valve is closed). You want a visible gap all the way around that spring, without causing damage to the wall that captures the rubber gasket. Don't take a chance with this at all. Get plenty of clearance. It's important.


Put your pushrods in place (longer pushrod on the exhaust), and then put the rocker box gasket and rocker box itself onto the head.



Tighten down at least the four main bolts that hold the rocker box down (shown above). No need for pushrod covers at this point.


If your lifters are standard hydraulic units, and they have oil in them, the valves will open as you tighten down the rocker boxes. The pushrods will have a lot of pressure on them and you won't be able to turn them by finger. As the oil escapes from the lifters, i.e. the lifters "bleed down", the valves will close. When you can turn the pushrods by finger you know the valve has closed and released the pressure on the pushrod. Do not turn the motor over until you reach this point! The result could be bent valves.


If your motor has solid lifters in it, extend your pushrods until all the slack is out.


If your motor has Hydrosolids or other travel limited lifters, you need to extend the pushrods until the valves are open and stay open, even after ample bleed down time, and then shorten them until the pushrod just goes loose, i.e. the valve just closes.


This motor had new hydraulic lifters in it, but they had no oil in them. As a result, with the standard non-adjustable pushrods the plungers just pushed down, without the valves opening, and the lobe was going to have to push the lifter up about another .100" before the plunger bottomed and the valve started opening. So even though this motor has standard hydraulic lifters, I still put adjustable pushrods in it for the purpose of making this measurement, so that I could bottom the plungers and get the valves to start opening as soon as the cams told them to. Watch for this if you're using new lifters. If you ignore it, the valves won't open as far during this test as they will in a running motor, and your measurement won't be valid.


Now we're ready to turn the motor over. Watch the valvetrain as you do this.



If you did everything right, you won't have to turn the motor very far at all and the exhaust valve will start opening. Notice how the pushrod end of the exhaust rocker has moved up in this picture. Keep turning, you'll see the exhaust start closing. Then as the exhaust approaches closed ...



... the intake will start opening. It's hard to see in this picture but both valves are slightly open, the exhaust is almost closed and the intake has started opening. This is the overlap period. The piston is passing through TDC as you pass through here and the valves are smashing the clay. Keep right on going, don't pause or back up.



The next thing that happens is the intake valve fully opens. Notice how the pushrod end of the intake rocker is higher than the exhaust. Keep right on turning, let the intake valve fully open, and then close. As soon as the intake valve closes, STOP.



Pull the head back off and hopefully you'll have something that looks like this. Remove the gasket and bring the piston up to TDC for clay measurements.



I usually just stick a machinist's ruler into the clay to do the depth measurement. Some people prefer to remove the clay and measure with a caliper, but it's very difficult to remove it without changing it's size. You need to see at least .060" of vertical clearance on the intake and .090" on the exhaust. The piston chases the exhaust valve closed, where the intake valve chases the piston down, so valve float can cause contact with the exhaust valve, but not the intake. So the exhaust clearance needs more margin. The intake clearance conversely is just to accommodate thermal expansion and piston rock.


As you can see, this pocket has more than .100" of vertical clearance, so it's fine. However, notice the "eyebrow" clearance, i.e. the thickness of the clay against the wall of the pocket. You want a good .050" of eyebrow clearance. In the area near the ruler, it's plenty, but at the edges (top and bottom of the photo) it gets substantially thinner.



I carefully peeled the clay from the pocket and measured the eyebrow clearance with a caliper. I had plenty in the center of the pocket, but like I said, I was concerned about how it lost clearance out away from the center. The pocket diameter wasn't big enough.



For minor adjustments like this, I generally don't remove the piston and fixture it up into the mill for a precise cut. Instead I just tape off or otherwise cover any area where debris could get into the engine, and using a die grinder I modify the pocket. Here Jon is using a coarse grit Scotchbrite pad. It works well because it leaves a smooth finish and also rounds off sharp edges. A sandroll can also be used to remove larger quantities of aluminum and do it faster, but it's a good idea to finish off with a Scotchbrite pad.


Once you're sure you have plenty of clearance, clean everything up and do the final assembly. Here are a few more tips ...



On 2003 and older Sportster motors, heads can be installed first and then the pushrod tubes put into place. On 2004 and newer models, you can't do it this way, the pushrod tubes need to be there when you put the head on. Here Jon is installing the front cylinder pushrod tubes using NRHS billet aluminum pushrod bases.




Here's an easy way to check pushrod length if you're using standard hydraulic lifters. Finger tighten the 2 large rocker box hold down bolts on the spark plug side, while the pushrods are supporting the rocker arms and rocker box on the pushrod side. Now take your caliper and measure how far the rocker box is being held off the head. When you tighten down the bolts, this is how far the lifter plunger will get preloaded. The plunger has about .200" of travel and ideally you'd like it centered, so you'd like this measurement to be near .100". This one is nearly perfect.



Pre-lube everything! I always put a dab of assembly lube on each end of every pushrod, as well as the valve tip, and of course in the rocker arm bushings as well. And before I button it up, I pour oil everywhere. It takes a bit for oil to get pumped up to the rocker box and I don't want it running dry. Be sure to pour some down the pushrod tubes, too.


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