I've been rebuilding my old '74 TdF Gitane. Everything is clean, greased and running fine now. The bike still has its old Ti clip pedals and I want to replace them with Speedplay X3 taken from another bike. The French thread on the cranks is 14 mm x 1.25 while the Sp pedals are 9/16 x 20 tpi. I can get the cranks rethreaded for the Sp, no problem.
My question is : after rethreading, will it still be possible, if need arises, to put back the old pedals without rethreading to french thread ? I fear the new ISO thread will be too big and the old pedals come loose... Or am I just drinking too much Porto ?