P/V C
Piston to Valve Clearance

Piston to valve clearance is easy to measure, even with your heads installed. It should be measured at engine assembly, or anytime you make head changes or changes to your valve train.

Contributions by Michael Yount

Q If I bolt these heads onto my block, will I have enough piston to valve clearance.

A There is only one way to find out. You must measure.

Q But my buddy said he did it and they worked great!

AHave your buddy get his car nice and hot, then have him put it in neutral and rev it past 6200 RPM. If everything is still working, he got lucky. You may not be as lucky. You still have to measure.
(Not recommending this 'test procedure'. This is just to illustrate the circumstances that may produce engine failure due to insufficient P/V clearance).

Q Okay, my buddy 'got lucky'. I have the same engine, cam, and heads that he has. So I should be okay, right?

A You cannot make this assumption.

Q Why not?

A You may have the same block, cam and heads as your lucky buddy. But here are just a few things that contribute to P/V clearance:

Block deck height
Head deck thickness
Head gasket thickness
Rocker arm ratio
Cam grind tolerances
Cam timming
Cam bore alignment
Valve seat recession
Valve margin thickness
Main bore alignment
Crankshaft throw tolerances
Crank journal tolerances
Piston wrist pin tolerances
Connecting rod tolerances
Valve spring integraty
Bearing clearances
Push rod length

And some variables:

Engine temp
Oil viscosity
Oil pressure
Engine age
Carbon build-up

Have you measured and accounted for all these items?

Q Okay, I did forget to measure a few items. So do I have to remove my heads to measure P/V clearance?

A There is a way to check the distance from the valve head to the piston top without removing the heads.

There is only a very few degrees of crankshaft rotation where contact is possible between the valves and the piston. This is between about 30 degrees BTDC (Before Top Dead Center) and 30 degrees ATDC (After Top Dead Center), and only on the exhaut/intake stroke (your degrees may vary). This is the danger zone, the end of the exhaust stroke and the beginning of the intake stroke. Anywhere else in the crankshaft rotation piston to valve contact is not possible.

Q How do I know when the crankshaft is in this area of P/V contact danger?

A When the crankshaft is rotated through the 0 degree timing mark, and both rocker arms of the cylinder to be tested are moving, this is the end of the exhaust stroke and the beginning of the intake stroke, the P/V danger zone. If both rocker arms are not moving (or the distributor rotor is pointing to that cylinder), you are on the compression/combustion stroke (the valves do not move on the compression/combustion stroke). Simply rotate the crank 1 revolution. As you pass 0 degrees timing you will be at the end of the exhaust stroke and the beginning of the intake stroke, the P/V danger zone.

When the piston is on the exhaust
stroke, it is moving up the cylin-
der toward the fully opened exhaust
valve. There is no P/V issue at
this time.

As the piston moves closer to the
top of the exhaust stroke, the ex-
haust valve must close before the
piston makes contact with it. The
piston is chasing the exhaust valve
up the cylinder.

When the piston moves past TDC
(Top Dead Center) the intake
valve begins to open. The intake
valve chases the piston down the

The exhaust valve is left open as long as possible to maximize the exhaust stroke. If the exhaust valve spring is weak, the exhaust valve may "float" for an instant after the cam begins to allow it to close, allowing the piston to make contact.
The intake valve is opened as soon as possible after TDC (actually, even before TDC in most cases) to maximize the intake stroke. There is no valve float danger with the intake valve, only improper valve timing or lift rate.
Valve/piston contact usualy occurs with the exhaust valve. A late exhaust valve closure, along with a weak or hot valve spring, and improper P/V clearance are a formula for disaster.

Q Okay, how can P/V C be checked if I already intalled the heads?

A After removing a valve cover, and finding near TDC of the exhaust/intake stroke of the cylinder to be tested (both rocker arms are moving as you crank through 0 degrees timing) , P/V C can be checked using the Drop Valve method. The valve springs of the test cylinder are removed. With the piston at TDC on the exhaust/intake stroke, the valves cannot fall into the cylinder

It is beyond the scope of this article to discribe the exact procedure for checking piston to valve clearance, and removing and installing valve springs. There are some precautions and saftey procedures that should be observed when performing this type of work. Check your service manual for proper saftey precautions for your year and model car.

There are two types of valve spring removers for installed heads, the pickle fork, and the spring compressor. The spring compressor will work on valves that have a single spring and no damper spring.
Valves that have more than a single spring, or a damper, will need the pickle fork, and a way to hold the valves up.

Q What if I only have single valve springs?

This $15.00 compressor from Sears, easily removes single valve springs.
With this tool, it is also possible to reinstall the spring without
having to hold the valve up by other means.
Caution, compressed springs store enormous amounts of energy. A compressed spring can cause serious damage or injury if you lose control of it.

Q Suppose I have more than a single valve spring, or a damper?

A Valves that have more than one spring, or a single spring and damper, require a pickle fork type spring depressor. This also requires that the valve be held in place while removing the spring. One way to hold the valve is by compressed air. This can be dangerous and you should consult your service manual for proper procedures and saftey when attempting to pressurize your cylinders with compressed air.
Another way is to insert some nylon rope into the spark plug hole and bring the piston to TDC. The wad of rope in the combustion chamber supports the valve.

As the crankshaft is rotated through the danger zone, the end of the exhaust stroke and beginning of the intake stroke, valves are dropped down to rest on top of the piston. The distance between the top of the valve stems and rocker arm is checked with feeler gauges, being careful not to compress hydrolic lifters any, if being used.

Always follow proper saftey precautions when hand cranking your engine. Check your service manual for the procedures for your car make and model.

The crank is rotated a few degrees, the valves are set to rest on top of the piston, and checked for clearance. The crank is rotated a few more degrees and the valves rechecked. The crank is rotated a few degrees and valve clearance checked only through the danger zone, about 30 degrees before and after TDC on the exhaust/intake stroke. Measuring the gap between the top of the valve stem and tip of the rocker arm, with the valve resting on top of the piston, translates to your piston to valve clearance.

Tip of the Day Here is a good suggestion from Michael Yount: while feeler gauges can be used to check for sufficient P/V C, a dial gauge is prefered. You can also take this opportunity to "map" your current cam and P/V C with a dial gauge. Then you will have the information available to calculate the effects on P/V C of future valve train enhancements, such as cam changes, or rocker arm changes. A dial gauge and adjustable magnetic base can be found for less than $20.00, on sale, at some tool outlets (

Q How much of a gap should there be between the valve stem and rocker arm using the Drop Valve method?

A For the SB Ford, Ford recommends a P/V C of .100" for the exhaust and .080" for the intake minimum, for RPMs up to 6000. So these figures should be what your gaps should measure. Consult your machinest or mechanic for higher RPM applicatons.

Q So then if my gaps are large enough, there will be no P/V clearance problems?

A Not quite. While the Drop Valve method can indicate static clearance between the head of your valve and the top of the piston, it cannot indicate proper clearance between the edge of the valve head and the side of your valve relief in the piston top (if valve reliefs are present). This can be of greater concern when larger valves are being used.

Q How can I check the gap between the edge of the valve head and the side wall of the piston relief?

A The clay method is the only way I know to do this.

Q What is the clay method?

A The clay method requires the heads to be removed from the engine block. A small amount of clay is placed onto the top of the piston and the head is mounted. The crankshaft is rotated through the exhaust/intake cycle. The valves will make inpressions in the clay, which are then measured for thickness. The thickness of the clay is the valve clearance to the piston. Using the clay method is much more comprehensive, and the prefered method, if done correctly. But, the clay method is also much more time consuming.

Using the clay method also requires a solid valve lifter to be used in place of hydrolic lifters. This is because hydrolic lifters will compress while the valve is being pressed into the clay, leading to inacurrate impressions.

When a clay impression is made, one way to measure the clay impression is to slice off small sections with a razor knife and measure the thickness of the remaining clay. Keep slicing off sections and measure, across the entire clay impression. Your smallest measurement is your P/V clearance.
Using the clay method will give you the clearance between the head of the valve and the top of the piston, and also the clearance between the edge of the valve head and the side of the relief in the piston top (if reliefs are used on your pistons. Not all pistons require reliefs).

Things to keep in mind

Hand cranking your motor and/or using compressed air to hold valves in place can be very dangerous. Please consult your service manual before attempting these procedures. Disable the ignition system. Make sure the car cannot move. Be aware that the crankshaft can turn on its own rather suddenly and with great force.

Before removing valve springs from mounted heads, be sure that the piston is at TDC on the exhaust/intake stroke to prevent the valves from possibly falling into the cylinder. Always keep in mind the possiblity of the valves falling into the cylinder.

When turning the crankshaft with the valves resting on top of the pistons, they can bind against the piston top. Try to lift the valves while turning the crank, or don't force the crank if it stops. Back it up and lift the valves.

When removing valve spring keepers, be aware that they are very small and can quickly disappear inside your engine. Use a magnet to remove them after you have dislodged them from the valve stem.

Also, valve retainers can get really stuck on the keepers. And the keepers can be difficult to remove from the valve stem. Squirt some solvent or WD40 on them before trying to remove to help loosen sticky retainers and keepers.

A special thanks goes out to Michael Yount for all his help and contributions to this article. Thanks Michael!

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