1966 Elger bathtub. The valve seat is very long, at least an inch; brass cylinder. Not only is the face cracked & deeply pitted due to acid water, but I can't get it out because the square hole in the back has crumbled into a round hole.

I hesitate to take my hexagonal removal tool and pound on it.

Any trick to gently remove it?

I wish I had a tool which would go into the cylinder and expand.

No room to grab the outside diameter. Might be able to grasp the cylinder wall with long needle nose, but that doesn't give much leverage to get it turning.

Worst case, need to remove 1/4" from the face of the seat and use a much longer stem. Hoping the crack does not go longer than that. Already using a #5 length valve stem (Danco measure).

Anyone know the thread size for this seat?