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The sump nut's a bronze, 14mm allen key in an alloy sump and hasn't been out in at least 15 years. I've stopped short of rounding off the allen, but I was swinging on it and it wasn't going ANYWHERE.

Do I try heat?
Or a cold chisel?
Or drilling it out?

Obviously penetrating fluid won't penetrate because of the o-ring.

Thanks for any help in advance.
John

heat

or give it a sharp tap in the way and then try a spanner on it again

Try an impact driver with a allan key head.

Can you get a pair of mole clamps on the outside of the nut? If so, use them along with the allen key to give extra leverage.

If you're going to tap it, be very careful how much force you give it...don't want to crack the castings