Glenn,
I remember two different types of soft plugs on that engine, a cup type and a slightly dished type. When I put them in I make sure the block opening is spotless and the I smear a coating of the Permatex brown poop, the type that hardens, around the edge of the plug. Use a driver to bang it flush. I wouldn't bang it from the middle, just from the edges of the cup.
On the dish type, I put them in with the same Permatex goop, and I do it with the dish OUT...after they come up on the little lip in the block, I give them one stout smack right in the middle of the dish. This seems to expand them and lock them in place.
It has always worked for me. If yours popped out with a four pound cap on the radiator, it was either the wrong size, in the wrong place or put in wrong...4 lbs isn't very much pressure.
Stuart
