Pedal axle/thread size?

how sure are you that the pedals you are putting in 9/16ths? could the cranks have been re threaded? have you tried another set of pedals to eliminate the pedals from the problem?

Thanks - all good advice. I was sure of the pedals but unsure of the crank threading, and had the same issue with two sets of pedals.

But...it's all sorted out now. I gave the threads on the pedals and the cranks a really really good clean, which improved matters. It was still a very tight fit, but the threading was a correct match. No little silver worms.