Piston to valve clearance deals with the space between
the valve and the piston during a critical point in the
cam timing knows as the "overlap period". At
no other point in the four stroke cycle does the valve
get closer to the piston. Thus it is imperative that enough
clearance exist in order to avoid a collision between
the piston and the valve.
The overlap period occurs near the end of the exhaust
stroke and at the beginning of the intake stroke, when
both valves are open for a short
period of time. Overlap is a
critical period in the cam cycle, as the movement of exhaust
gases out of the combustion chamber "draws"
in a fresh charge through the intake valve.
During Overlap both valves
are open as the piston rises.
Many people have even erroneously measured clearance at
the point of maximum valve lift, thinking that is the
most likely point at which interference would occur. However
when a valve reaches its maximum lift, the piston is actually
the furthest away since this occurs during the intake
stroke, when the piston is moving down the cylinder bore.
clearances between the valve and piston should be 0.080"
on the intake valve, and 0.100" on the exhaust valve.
If you run aluminum rods, the clearances should be even
greater. In this article we'll show you the two methods
most commonly used to measure piston to valve clearance.
There are a couple ways to check for clearance.
One way is to place a 1/4" thick strip of clay
on the piston, and turn the engine over through
one full cycle (two rotations past TDC) allowing
the valves to make an impression in the clay.
The downside to clay is that the heads must come
off to do the check. Clay is also cumbersome, it
tends to stick to the piston, and requires a lot
of practice to get reliable measurements.
The clay is then carefully peeled off the piston,
cut in various locations, and the thickness is measured
to determine how much clearance exists. This method
works well in that it gives you a three dimensional
view of not only how much clearance there is, but
also where the interference is occurring. This helps
determine if the problem is too shallow of a valve
pocket in the piston, or if the pocket is not wide
enough for the valve head.
This involves using a light
weight valve spring and a feeler gauge. The procedure
is to replace the intake and exhaust valve springs
for one cylinder with light tension "checking"
springs. They can be bought at any hardware store
for under a dollar. They must fit squarely between
the seat and retainer and be just stiff enough to
hold the valve closed.
Adjust the rockers to zero lash, irregardless of
whether it is a hydraulic or a solid lifter. It
is extremely important if you are using a hydraulic
lifter, to AVOID preloading the lifter. Instead
you want to remove all the slack in the pushrod,
without forcing the plunger in the lifter downward.
Be sure to make the lash adjustments when the cam
lobes for that cylinder are on the base circle.
(Piston is at Top Dead Center of the compression
stroke.) A quick way to set the lash is to adjust
the intake rocker when the exhaust valve just starts
to open. Then adjust the exhaust rocker when the
intake valve is just beginning to close (coming
back up after full lift.)
The benefit to this approach
is you don't have to take the heads off, but you
do need to remove the valve springs with some sort
of spring removal tool.
With the test cylinder
ready, set your feeler gauges to 0.100". Starting
with the piston at top dead center of the compression
stroke, rotate the crank one full cycle in direction
of normal rotation.
As the piston travels back up the bore the exhaust
valve opens and the exhaust stroke begins. Keep
an eye on the valve train, and you will notice the
exhaust valve spring slowly compress as the valve
fully opens. As you approach TDC you will notice
the intake will begin to open before the exhaust
valve has fully closed. This period, roughly 10-15
degrees before and after TDC is the overlap period.
Both valves are slightly open and piston is near
It is precisely during this overlap period where
you'll want to use the feeler gauge to measure the
distance between the valve stem and the rocker arm
tip or roller. The piston is so close to the valves
at this point that you can push down on the checking
spring and feel the valve contact the piston. It
is this distance which is critical, and needs to
be a minimum of 0.080" on the intake side,
and 0.100" on the exhaust. It may take several
cycles to get the hang of where the overlap period
is and how to take the measurements.
You will notice that the overlap period does not last
very long, and in fact it is very easy to miss. In terms
of degrees of crankshaft rotation, the total overlap
period may be as little as 40 degrees for a mild cam,
in other words 20 degrees before and 20 degrees after
TDC. At some point within this range the piston and
valve will be the closest. Therefore it is imperative
to check the clearance at least every two degrees during
the overlap period.
Take a few measurements, then rotate
the crank a few times and check again until you get
the same number each time you measure.
If you determine that you have
at least .080" on the intake and .100" on the
exhaust side then you have sufficient clearance to run
A helpful trick is to push down on the retainer with
your thumb so that the valve a contacts the piston.
Use your index finger to keep the rocker arm taught
against the pushrod. As you rotate the crank the valve
will "ride" the piston, and you will be able
to see the space between the valve stem and rocker tip
get smaller and smaller during the overlap period. Use
your feeler gauge to measure the point where the gap
is the smallest.
In the event you don't have enough clearance, there are
several options. The easiest option is to not use that
cam. Most people don't want to hear that, especially since
they've already paid for the cam. This leaves a couple
options in order to achieve the necessary clearance.
If your clearances are relatively close, within 0.010",
you can attempt to retard or advance the camshaft. Retarding
the cam effectively opens the exhaust valve earlier and
the intake valve later. This translates to a few thousandths
of an inch more clearance on the intake side, and roughly
the same amount less on the exhaust side. In the case
where your intake valve clearance was running close, and
your exhaust side had plenty to spare, you could retard
the camshaft 2 to 6 degrees and gain some clearance. Conversely,
advancing a cam will bring the intake closer and the exhaust
slightly further from the piston during overlap.
Realize, however, that when you advance or retard a cam
you change the performance characteristics and power-band
that was designed by the manufacturer.
The other option is to replace the pistons with aftermarket
types which accommodate larger valves. Replacing pistons
means a significant amount of work and money in tearing
down and rebuilding the motor. However in the end an aftermarket
piston with a deep valve pocket will ensure proper clearance.
The final solution is to notch the existing pistons. The
following steps detail how to accomplish this.
Put a dab of white latex paint on the valve face,
and then with the head on the block and the piston
at TDC, force the valve down until it contacts the
This results in the paint transferring on to the
piston in precisely the location which needs clearancing.
In this case, to the right of each intake valve
With all the pistons marked, put a grinding stone
on the Dremel® tool or die-grinder and carefully
extend the valve relief's. Stop often to install
the head and to remeasure the clearance. Try not
to over do it, as removing weight results in upsetting
the balance of the motor, and also weakens the piston.
Be sure to take all the necessary precautions to
keep the grindings out of the motor. We covered
up all the adjacent cylinders and used a shop vac
to suck the grindings up before we moved on.
The "professional" way to do this is to
buy a piston notching tool, such as the Isky Cams
product shown here. The tool is simply a valve head
with "teeth" on the face. This head attaches
to an arbor, and is then installed in place of the
valve in the cylinder head. A drill is attached
to the arbor and used to turn the tool, and as a
result the teeth grind the piston exactly where
the valve would contact. The tool comes in various
valve and guide diameters to work with basically
any popular engine and head combo.