On the subject of piston to head clearance. From the knowledge I have work
in a machine shop part time and blueprinting older race engines, I know
there are a lot of things to consider. First what is the compression ratio
that the engine is going to be able to run without detonation, because
you'll end up with holes in your piston, or worse. There is an equation to
figure out the compression ratio. I can dig up the formula if you would
like just e-mail me. If yous are running unleaded fuel it should be no
higher than 10.5 to 1, which is the most safley runable on pump gas. The
cumbustion chamber should be polished along with the valves to achieve this
ratio. If you are running race fuel you can bump it up to 12-12.5 to 1 is
is also with polished valves, chambers, and pistons. The octane of this gas
would be about 104. If you want any high you have to get 110 or higher
octane fuel or some form of alcohol, methonal or ethanol.
On the actual clearences, the fact that the piston should be close to the
head in the quench, or squeeze, area should be kept close to make more
power is true. When the piston travels up to meet the intake mixture it
pushes it out of the quench area toward the valve causing the fuel mixture
to mix around atomizing it beter causing a more complete burn. On some
heads this area is not flush with the head and on other it is. but
absolutly don't let the pistone hit the head. a beter way to check this is
to place some clay on the piston, bolt the head on and turn over the motor
by hand so the valve indent the clay along with the head. Then take the
head off and use a razor blade to section the clay so you can measure the
clearance in different places. I recomend that the quench area in an all
out race engine should be no closer than 0.020". This will leave romm for
the piston to grow as it heats up. In the intake valve relief the clearance
should be no closer than 0.040" to take in account piston growth and vlave
float. And the most important, the exhaust valve relief must have a
clearance of 0.100", due to piston growth, valve growth and valve float.
These specs may differ from other source but are pretty close and is what I
run. Now if you happen to be running hypereutietic piston the clearances
could be a little closer, but the thing to remember with clearances is that
a little extra is good insurance, one HP or two vrs. a whole motor, thick
about it!
Now if it is a killer engine you want I recommend polishing the chambers,
polishing the tops of the pistons making sure to leave a flame tunnel on
pop-up pistons, to get the most compression alowable. Also if you have the
head milled after checking clearances, check it again to be sure the is
enough room. Also its not a bad idea to check every cylinder!!!!
If anyone has any questions please email me.
Reuben Gosewisch
Automotive Engineering Technology major at MSU.
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