mine is 3/8 unc at the carb end and 1/4 unc at the top end.
 
I needed a much longer one to accommodate a taller filter so I drilled the top of a 3/8 unc bolt then cut the head off a long 1/4 unc bolt pushed the two together and brazed them. I am useless at starting a die square hence the long bolt instead of a piece of 1/4 rod.
stuey 

Have an Ford Autolite 4100 carb here, in good condition except the air cleaner hold down stud is missing. Looking into the recess, it appears the threads are stripped out? The recess hole appears to be about 3/8" in diameter, although the upper hold down needs to be about 1/4" dia. Was it threaded originally? Can anybody shed light on this? .  Thanks.


Paul