Mr. Cab, I have figured out where your instructor messed up. He was using the values for VOLUMETRIC thermal expansion. But unless you are trying to figure out whether the aluminum will float, it has little value to know that. What we're trying to determine is how much the Linear coefficient of thermal expansion differs between steel and aluminum.

Aluminum coefficient of linear thermal expansion is 21 - 24 relative units.
Steel coefficient of linear thermal expansion is 11 - 12.5 relative units.

So the correct rule-of-thumb multiplier is 2.
Aluminum expands in length at about 2 times the rate of steel for a given temperature increase.

I have noticed on new stock replacement, pressed pin type pistons, quite a few pins need a little rap on the pin to get it to come out. I am inclined to believe that millions of such pistons have been installed without any additional pin fitting.
I haven't ever heard of needing 300 degrees F to get the pin to slide out. Usually, hot water from the tap is all that's necessary.

R.