I'll bet dollars to donuts he's got a 14mm 1.25 thread. They look perzactly like the ones off of our Raleigh Colt which have that size. One thing I've found working with old bikes is having a couple of bolt or thread sizing plates and a pitch gauge. Those two tools have saved me a lot of guessing and potential costly mistakes. As a test, try screwing in a standard automotive/ small engine spark plug. If it threads correctly in the crank arm, even a tad loose, it's 14mm.
Last edited by Greg R; 10-25-21 at 11:09 AM.