As been posted, with the "cupped" freeze plugs I put a little coat of aviation permatex and/or Super 300, or similar around the outer cup, use a socket that just fit's inside the cup and drive it in as evenly as humanly possible. As also stated drive it in until the outer edge of the cup is even with the casting it's being driven in to.
On the slightly dished type freeze plugs that aren't cupped, I still like to put a little coating on the outer edge, then gently and evenly start the plug into the hole with the dished part (convex) facing out. I also use a socket that almost matches the size of the plug but will fit into the casting. Most of these type of plugs are driven in until they bottom out on a race or shoulder. Once fully driven in, take a ball peen hammer and place the peen end in the center of the plug. Using another hammer strike the ball peen hammer so it will indent the center of the dished plug inwards. This has the effect of expanding the plug outwards, making for a tighter leak proof seal. Don't go gorilla on it tho, try to match the dish to the same dimension as the one you removed.
Dave
