Hi guys about to fabricate a new gas tank out of stainless for my pony motor but I'm having trouble figuring out what size nipple to use as the filler neck. I took the cap to a complete bin set of all size pipe nipples and 1 inch pipe is too small and 1-1/4 pipe is too big so did cat make an odd ball nipple for this or am I just being the odd ball that can't see the obvious.....when measured I'm coming up with an O.D. of about 1-1/2

Trying to edit the subject but it won't seem to let me this is on a D4....if an admin could help it would be appreciated (working off my cell phone here in the shop)

It's kind of an odd ball, 1.4380" thread diameter, 12 threads per inch.

Thank you for the reply OM glad someone can confirm I'm not nuts. Any idea on a direction I could go before just buying a piece of stainless pipe and machining/ turning my own threads to make a nipple?

it may be a bolt thread not a pipe thread some bungs and caps are bolt threads on some machines i have seen this on other brands of machinery. dont ask me why but it is what it is.

and really does it matter if its little larger the original as the larger the hole the easier to pour into. trust me 99 percent of people wouldnt notice or even care. and in the big scheme as long as it works is all that really matters.

i would just select a pipe fitting that is close to original size with a cap that looks the part and be done with it. that way if its ever lost or damaged you can go to any hardware or fitting supply and buy a replacement.

Was just trying to keep the original alum cap on it to prevent someone from spotting the difference from a mile away

If you have access to a lathe or know someone who can machine one up for you it should be quite easy to duplicate the original.

 I have made tanks out of stainless steel and made the necks out of a piece of pipe but turned the neck down and threaded the neck 1 1/2 X 12 TPI.  The thread on the neck is a bolt thread, not pipe.  The stainless-steel tank will not rust and keeps the gas clean.  Brian Boaz
 

I appreciate it all guys, I do have a lathe and Brian confirming that a 1-1/2x12 will work I'll just do that. And no need to ruin a good tank that someone else could use.

I think the pony fuel cap was a carryover from the compression fitting connectors. 1-7/16 by 12 straight thread.