Afternoon, y'all!
I thought I might share with you our experience this year regarding piston
to head clearances. The Chevy power book recommends a minimum
of .035" for small block chevies, and we run a 355 c.i. small block
in our Late Model Stock car.
We had a "famous" engine builder build us an engine, and he gave the
dyno sheets on our engine, which indicate 352 HP. This is VERY
competitive, and we thought "great! we won't be giving up anything to
the other racers." I asked the engine builder how he made that kind
of horsepower, considering the very restrictive rules for NASCAR
late model stock engines. He told me that he left only a .022"
piston to head clearance. Now, I figured that this was WAY too
small of a clearance, but he replied "if the pistons ain't hitting the head,
you ain't gonna make horsepower."
To make a long story short, the motor blew up on the 11th lap of
testing one afternoon, with the #1 rod end exiting the block and
ruining one perfectly good motor, and test day. Upon inspection,
the tops of the pistons all had a perfectly legible, .004-.006" deep
impression of the cylinder head combustion chambers. My theory
was this "banging" into the cylinder head hammered the rod
bearings and caused the resulting failure, but the engine builder
basically told me that it wasn't his fault and he wasn't going to
fix it for free....
we spent $10,000 on the motor and it will cost us another
$2500 to build another. The heads were Ok, 7 pistons, 6 rods were
OK, but will need new crank, cam, lifters, block (of course - there
were holes big enough to put your fist in..), oil pan, pump, etc.
IT just isn't worth it to cut it too close on the piston to head
clearance, IMHO. Give up the few HP for longevity, that's the
lesson learned here. As a result, we didn't get to race at all
in '96, and have to wait until '97.
Cya,
Mark Coffin
#31 Overton's NASCAR LMSC Monte Carlo - crew chief (but not engine builder)
|