I put a centre point tool into my hammer drill After soaking the threaded top of the cover
with wd40 , or similar rust remover , I established four grooves around
The edge of the plate and gave it good hit in these four places with the
hammer drill. Worked for me , and the plate had been untouched for more
than 30 years. Faied to mention I aimed rhe centre point in the anti clockwise direction
And took it steady .
Q