Davis,
I am not an engineer or chemist, but consider the following (guys, pitch in!):
Maybe it isn't the loctite as much as it is a super tight fit, or a standard old stuck bolt. If that is the case, then applying heat will be COUNTERproductive if only the fastener is heated, and none of the heat transmits to the piston. I tried running the numbers on the expansion of alloy steel (the fastener) and came up with this- if you assume that the fastener and the piston are at 70 degrees F, and you apply localized heat to the fastener and none of it is transferred to the piston (it remains at 70 degrees) then the 8mm fastener will increase in diameter by .000185 inches (sorry for the mixed units) in a hole that has not enlarged, resulting in an even tighter fit! If, OTOH, the fastener is cooled to 0 degrees F (use dry ice or liquid nitrogen, freon, etc) then the fastener will reduce its diameter by the same .000185 inches, making for a (slightly) looser fit.
Is this correct, guys?
If this flies with the wiser members of the forum, I might be inclined to warm the piston as much as possible and then hit the fastener with the cooling agent. With luck you might be able to get a 140 degree difference, resulting in a "whopping" .00037 inches additional clearance.
JMHO...
Bluegrass