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Thank you gossie yeah I know not to do them up tight it was in a mechanic for it’s last service and they are the ones who massively over tightened it. I was the one who broke it trying to get it off to change my oil filter. Unfortunately the the gentle taping with a chisel and a screwdriver has not loosened it and it’s turning into a total pain in the a..You should be able to buy a new cap.
You will need to tap the busted one off carefully with chisel or sharp screwdriver and tap tap tap with care.
They only need to be put down as they touch down never screwed down hard.
What suggestions can I try now.