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Help! Damsel in Distress

My pedal has worn out the threading on my crank. I have an enthusiastic friend who is willing to try to fit a new one for me but
a) what bits do i need to measure/ check when buying a new one?
b) what things does he need to know before embarking on the task?

Anything else I/we need to know that isn't mentioned above.

I'd say your crank is probably knackered now. Threads on a pedal = tough steel - threads on a crank = soft alloy.

New crank time I think, in lieu of a better picture.