Roland,
This is kind of a hard question to answer. I would think that you need
.160" minimum crown thickness for forged pistons & .200 for cast
pistons. So much depends on piston design & how much detonation & heat
the engine is subjected to & how hard it is run.
Rod stretch due to piston inertia as it goes over top dead center is the
major factor controlling piston to head clearance. This obviously
becomes greater at higher rpm or with heavier pistons. I would be
uncomfortable with less than about .055" clearance between piston top &
head. If the engine is not exceeding 5,000 rpm by much, .045" clearance
would likely be ok.
Assuming .045" desired clearance with a .060" gasket thickness, you
would only need to take .015" off the pistons. However with an unknown
"dish" volume the compression ratio could be unacceptably high. You will
need to calculate the compression ratio for the combination that you select.
The very safest & easiest route may be to leave the pistons as they are
& find a .075" to .090" thick head gasket as you suggest.
This whole subject is so full of variables in theory, materials, &
actual experience that you will likely get differing recommendations.
Dave Russell
BN2
Roland Wilhelmy wrote:
> A friend of mine is building a 100 4-cylinder engine to M specs. He
> has new pistons, offered for that purpose, concave top. Installed in
> the block they project .030" above the block. There is no way of
> knowing the past history of the block and how much it has been shaved.
>
> Question: is it safe to take as much as .030" off the top of the
> pistons? If not, is getting a thick gasket from someone like Mordy
> Dunst a viable solution? If not, what then?
>
> I will relay replies to my friend.
>
> -Roland
|