Its threaded into a water bottle type of braze-on. It has an M4 stud in the center on the back side and just screws into the braze-on. This link will take you to some photos the last photo shows how the cap attaches. http://groups.msn.com/Feltmotoredbike/shoebox.msnw
Butch , The Gas Cap You Have Asked About On The Felt Fike Is Not For Gas But Appears To Be A Working Cap On A Tank It Does Screw Into The Welded On Bung On The Aluminum Tank Of The Frame .
You Could Probably Modify It Some Way, Are You Considering Using The Frame Of A Felt Bike For Your Gas Tank?
If So There Are Many Obstacles You Will Have To Confront I Would Be More Than Happy To Explain How I Sealed The Tank On My Black 1903 Model I Have Have Done Many Of The Felt Bikes .