In a message dated 97-11-30 12:31:10 EST, spawn@net-link.net writes:
> I'll jump in here. This is from a materials class I had... Ok
> here goes,
> the piston, when it gets cold shrinks/contracts... the cylinder, on the
> other hand, expands (or, to state it better, the walls contract). If you
> think about it, for the cylinder to get smaller would require that the
> block expand. Thus, bigger clearances between piston/cylinder.
Chris:
You may be right - it's been many years since I had a materials class, and I
didn't pay a whole lot of attention then - but I have always thought about it
a litttle differently.
By way of explaination, consider an engine block that has had one cylinder
sleeved. Assume further that the sleeve is made from the same material as the
block, such that it would expand and contract in the same manner as the block
itself. Remove the sleeve, and turn it down in a lathe till it is only a thin
shell. Slice a very small piece from the end, such that you have a thin ring
of material. Cut the ring in two, and straighten it out to form a small,
straight piece of "wire." If you now freeze this piece, it will shrink in
length. If you then rejoin the ends such that you again have a ring, the
diameter of the ring will be smaller than before. Working your way back to
the original engine block sleeve, you will now see that the cylinder is
smaller than before, as a result of the shrinking from cooling.
I read this in an automotive magazine a long time ago, and it seemed to make
sense, but who knows? This would perhaps explain why things tend to crack
more from cold than from heat. Maybe there is something missing. The ring is
also now thinner, for example, which would make the cylinder larger? I think
maybe I will go bore a hole in a piece of metal, toss it in the freezer, and
see if the fit to a bolt which has been kept at room temperature is looser or
tighter (hopefully, someone else will beat me to it and report back to the
group so I won't have to!).
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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