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piston to head clearance - LMSC results!

To: "'vintage-race@autox.team.net'" <vintage-race@autox.team.net>
Subject: piston to head clearance - LMSC results!
From: Mark Coffin <mac.overtons@coastalnet.com>
Date: Wed, 11 Sep 1996 15:39:46 -0400
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)


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