Matt wrote:
If the hole is not 60mm, what size other than 60mm would be about 60mm?

A 2" NPT pipe thread is approximately 60mm in diameter.

See, the NOMINAL diameter of the inside of a 2" iron pipe is 2". The
wall thickness is approximately 3/16" thick. 60mm is approximately
2-3/8". You do the math.

You say the new plug is plastic? You won't hurt the cast threads a bit
if with a plastic plug if you happen to cross-thread it. The plastic
threads will simply strip out.

I'll bet if you compare the two plugs closely you will see they aren't
quite the same...