DR,
It is a regualar 3/8" coarse thread. I use a 3/8" bolt with the head cut off with a hole drilled thru it to fill it.

I took the fitting and hose off my hand pump, put a barb fitting on the hose, screwed fitting into a valve with a gauge, put an quick disconnect on the other side of the valve to connect to the air compressor.

Don't go over 10 psi!!!!

I bought a 7 dollar oil pump thing from west marine and hooked it up to a hand tire pump......cheap and easy