Leaking Front Fork

There is a leak in one of my front forks. I pulled the wheel and checked the Allen wrench fitting inside of the bottom of the forks. One was tight (not leaking) the other turned loosely (leaking).

I next pulled the top nut, lifted out the spring and tried to get a socket to fit the nut inside of the tube. If this is the problem (loose nut) what is the size of the socket (SAE, metric)? If not, what is the solution to stop the leak?

 

That is what I thought. Do you know the size of the nut in the fork... I've done this before and had a fit, but cannot seem to get a fit on the nut this time...
TY
77 bonnie

 

Thank you for your reply.... The parts list shows a nut with a parts number but no size indicated. What is UNC or UNF?

 

After hours of screwing around with the nut in the fork tube, I finally figured out that it is not a nut but is the end of a 1/4 inch socket extension. Fits nicely into the hole in the bottom of the tube. Put an Allen wrench (Hex as I think you Brits say) into the slot from under the bottom and 10 seconds later all tight and no more leaks. Thank you all for your suggestions and assistance.