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 .