[olddog]

Aluminum expands 1.33 x 10^-5 in/in/F
Brass expands 1.11 x 10^-5 in/in/F
steel & iron expands 6.5 x 10^-6 in/in/F

Therefore using a 180 F temp range (32 to 212) and calculating a 1.5 inch hole, the aluminum hole will grow by 0.004 inch.

A brass plug will grow by 0.003 inch.
A iron plug will grow by 0.002 inch

The plug will deginately loosen when the engine heats up. Brass would cut that in half. Tightening it hot would help. I like the o-ring best.

My metal expert wasn't in, but I doubt brass and aluminum have any problems.