Overview
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
During Overlap both valves are
open as the piston rises. |
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.
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.
Minimum
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.
| Clay
Method |
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. |
| |
| Spring Method |
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 the top.
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. |
Conclusion
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.
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.
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 that cam.
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.
| Gaining Clearance |
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 piston. |
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 relief. |
| |
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.  |
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