Greg:
You are forgetting the head gasket, which is 0.035" or so thick.
Common practice when modifying engines for power is to "0 deck" the
block, i.e. decking the block so that the piston crowns are flush with the
block deck. You then rely on the gasket thickness to provide clearance
between the piston crown and the head.
Obviously, if you are using some exotic gasket (steel, for example)
then you need to make the appropriate changes. The hypothetical example you
cite (assuming 0 head gasket thickness) would not leave enough clearance
for thermal expansion, etc. The piston would contact the head.
Because of squish, decking the block 0.015" will make more power
than shaving the head 0.015", assuming equal compression ratios. Obviously
the limit to raising compression via decking the block is piston to head
clearance, and you can mill the head a great deal more than the block.
But decking the block makes more power than milling the head by the same
amount (assuming you have a squish area on the head).
Obviously, milling the block deck insures a flat mating surface,
but likewise milling the head provides a similar function for the head.
It is equally valid to use these means to raise compression and power, and
so one can kill two birds with one stone via decking/milling.
Vance
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net]On
Behalf Of Greg Dito
Sent: Monday, December 22, 2003 2:15 PM
To: Timothy Holbrook; 6pack@autox.team.net
Subject: Re: RE: Does 0.120" really get 9.5:1 compression?
Whoa! Maybe in theory. But you cannot take as much off of the block as you can
the head for the simple reason that you would be exposing the pistons above
flush to the block.
There may be several reasons for decking a block, but one certainly is to
assure a flat mating surface to the head. To properly deck a block you first
have to measure all pistons and find the highest at top dead center before
taking the engine apart. The difference between the top of the block and the
highest piston, less a clearance height (.005"?), is the max amount to be cut
from the block. Realize a typical human hair is about .004" thick. Cut below
the highest piston height and it will crash into the head the first time you
crank it.
Another purpose for decking the block is to assure near identical compression
in all cylinders. This must be done, though, by also insuring your connecting
rods and pistons are the same length. A practice more for the competition
engine than a street engine.
Greg Dito
CD6250L
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