If the head was set-up to run "X" octane and the head was decked .020 There is a HIGH
likely hood of problems. You need to verify your squish clearance. Here's some info I on how
to do it, courtesy of Howard Richards of RED.
"The "squish" band is a somewhat narrow angle cut into the outer periphery of the
combustion chamber that comes into a close clearance proximity with the piston crown outer
edges at TDC (Top Dead Center) of crankshaft rotation. Measuring it accurately so as to
make decisions about what modification(s) should be made to an engine's compression ratio
and combustion chamber is a critical reference step in planning a high performance motor.
To determine your actual squish clearance...
You will need about a 10" length of solder of a diameter that is thicker than your current
squish clearance. Common available diameters are approximately .038", .065", .090" and
about .140". Start with the .065" (in tightly set-up race engines of smaller displacement you
may need only the .038") and if it is not thick enough because your squish clearance turns
out to be greater, just go up to the next size.
Cut the solder tips squarely with a single edge razor blade so each tip is cut perfectly
perpendicular to the solder length. This is VERY IMPORTANT to get accurate indications from
the very edge of the combustion chamber limits! Bend the solder into an upside down "U"
shape with the "legs" about 1/2" apart and parallel to eachother. Now cross the "legs" over
eachother (in the shape of one of those ever popular politically correct symbolistic ribbons
that we're instructed to wear by our fearless leader everytime we have a national
tragedy...). You should now have an inverted "U" of solder with "legs" crossed in an "X"
pattern.
Remove spark plug from your engine. Bring the engine up to approximate top dead center
(follow the piston up the bore with another piece of solder stuck into the plug hole.......). Now
back the motor up opposite normal direction of rotation until the piston has dropped about
an inch back down the bore. Insert your two "legs" of solder into the plug hole while holding
the solder by its' "U". Push and allow the "legs" to splay left and right across the piston
dome INLINE WITH THE WRIST PIN AXIS. Do NOT push the "legs" fore and aft (front to back)
in the bore or the potential for the piston to "tilt" on the wrist pin in its' cylinder wall
clearance could give you false readings.
You'll be able to "feel" the tips of the solder as they move left and right out to the cylinder
wall limits. When you feel them touch the cylinder wall, stop pushing and hold what you've
got with one hand while rotating the motor up to TDC with the other hand by its' flywheel or
similar. If the solder is thicker than your squish clearance you will feel resistance as you
reach TDC. If the resistance is too great to roll past TDC, select another length of solder of
the next thinner diameter.
Roll the engine through TDC and allow the piston to descend enough down the bore to easily
retract the legs of solder back out through the plug hole. Keep track of which tip was at
which side of the bore so you'll know which is which if there's a difference in squish clearance
from one side of the bore to the other! Differences in clearance from one side to the other
are common and can be caused by a "tilted" squish band in the head, "tilted" piston crown,
bent rod, cylinder bore not cut perpendicular to the head mating surface, etc.. Noticing that
there is a problem and which direction the "tilt" is will alert you to track down the culprit.
Now measure the compressed tip ENDS (it is fairly common for the squish band angle in the
head and the radius on the piston crown to diverge slightly, thus your "tightest" clearance
will be right at the very ends of your solder) and record them. This is your actual squish
clearance at TDC."
The minimum squish I personally would run in that bore with fresh parts would be .038"
Measure the compressed pieces of solder with a micrometer, the thickness is the squish
clearance.
If .020" has been decked off your cylinder and you had a .040" squish clearance before, and
if the squish was not re-cut, then it's entirely possible that the squish clearance is .020"!!!!!!!
Too tight!!! That's why I asked about the piston, pre-ignition and detonation will make the
crown on the piston in those areas look like it has been sand blasted. If the squish was
.020" you would have most likely heard it already!!! The piston may/will contact the cylinder
head and cause a knocking sound.
Might be worth your time and a box of pistons to determine how much compression you have
and your squish clearance!!!
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--Backcountry
"Ride hard, ride often"