so from reading another thread on here, people were talking about parts needing to be at 68 degrees for 12 hours to have the same expansion. well, during the winter nothing around me be it my house or garage stays at 68 degrees for more than 12 hours straight. lets say everything was measured at 50 degrees, how much of a change in tolerances can be expected compared to if it were say 68 degrees. id imagine that the cold measurements would be at least a bit looser...

 

I don't think I would worry much about it. This can all be figured out with math. Just need the coefficient of expansion for steel and aluminum and run the numbers. I would bet the difference in size is less than 0.001mm for such small diameters. Just make sure that all parts to be measured are at the same temperature. If we were talking bearing shells that are 2-3 feet in diameter then YES that would be noticeable.

 

I disagree.

I measured my bored at 70 F and I measured the pistons at 80F. Only 10 degree difference, by the measurements, the pistons don't fit but they came out of that motor so I know they do fit and have to remeasure the pistons at 68-70 to get accurate piston to wall clearance.

Needless to say, on a 3" diameter the difference is in the .01mm or .0001" resolution, not .001mm or .00001" resolution.

Also of note, I'm pretty sure it's an aluminum alloy, not straight aluminum. Not sure where you can get the exact makeup of the alloy to get the exact thermal expansion factors.

And lastly, the reason for the specific temperature is to match it up to factory specs. If you don't have the 68 F temp, the factory specs are useless.

 

Well yeah, you measured at 2 different temperatures. If everything is the SAME temperature the difference should not be a factor.

 

Aluminum expands at 0.0000123 in/in*F
Steel expands at 0.0000067 in/in*F

At 70*
Aluminum main bearing bore is 2.32283"
Steel main bearing bore is 2.16535"


At 50*
Aluminum main bearing bore is 2.32225"
Steel main bearing bore is 2.16505"

Did they shrink? Yes! How much did they shrink relative to each other?

0.00028"

For someone chasing the last little bit that is probably a significant number assuming their tools are capable of measuring to that degree of accuracy repeatedly. When the spec for acceptable bearing clearance is 0.0008" wide being 0.00028" off is probably not the end of the world.