In article ,
Gunner Asch wrote:

now that the engine was warm and had circulated the oil a bit...at least
15 yrs or more..old...it was time to change the oil. I located the oil
drain on the starboard side of the engine..and attempted to remove it.
It is Whitworth.... the closest match was a 15/16 wrench..which
promptly started to round the head of the plug off. A good crescent
didnt work, a 23 mm didn work....Knippex 'channel locks" didnt work...so
i quite while there were still flats on the plug......this is gonna be
interesting to get out.

There are 'channel pliers' made for hex nuts, even better are locking
pliers made for hex nuts. Try a farm store.

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