I recently noticed one of the frost plugs under the exhaust was weeping and decided to replace it. I purchased a 45mm brass frost plug and pulled the old plug by drilling a hole in it and screwing in a self tapping screw and levering it out with a claw hammer. It was then that I found that the original frost plug is a very shallow dish that goes up against a shoulder. The recess is only 5mm deep - whereas the new brass plug is 9mm deep. My fix has been to carefully hacksaw 4mm off the the new brass plug and tap it in with a film of permatex gasket goo to help seal it. It sits flush and looks sound but I won't know until I have some miles on it.

The original frost plug was convex (dished outwards from the block) and it looks like they were designed to be tightened into the hole by striking the centre with a hammer to flatten them - thus forcing the diameter out a little. The new frost plug has been installed concave (dished in towards the block) and was hammered in using a large socket. The old plug was definitely past it's best - corrosion on the inside was almost through in places.

Has anyone else encountered this problem when replacing frost plugs? What is the consensus - go with original type frost plugs, or are generic brass cups OK/better? I know some will say I should replace all of them at the same time but I am more of the opinion with these that "if it ain't broke - don't fix it".