Originally Posted by
TejanoTrackie
Regarding the use of heat to loosen a steel threaded cog on an aluminum hub:
1) The coefficient of thermal expansion of aluminum is approximately twice that of steel.
2) Both the hub and cog will expand radially outward when heated.
3) The good heat transfer properties of aluminum will ensure that the heat applied to the cog will transfer to the hub.
4) Therefore, the hub will expand more than the cog, the cog will become tighter on the hub, and this technique will be ineffective.
5) Brother 65er's recommendation to use heat in post #27 was made in the context of breaking down a red threadlocker compound, and not as a method to loosen a tight cog.
6) IME the most effective method is to use some sort of impact technique such as in post #6.
The thing you fail to account for in number 4 is that once the Al hub cools it'll shrink twice as much as the steel one and may be looser after a few heat cycles. This is exactly the reason why lightbulbs sometimes get loose in light fixtures and why Al an Cu wires connected with wire connectors not listed for Al/Cu will get loose as well -causing a fire possibly because of a loose connection.
Heat cycling MIGHT work to help loosen it up -but probably not until after it cools or goes through a few heat cycles. It might also work to break up some threadlock that was put in there too.