This is driving me nuts. I want to walk into Carquest, NAPA, etc. and buy a radiator cap, but without a part number they are lost. It would be a Ford pass-car radiator 1937- 48 (not pickup) with either a 4 lbs. or 7 lbs. rating. Stant or whomever. Any part numbers? Otherwise, it's ordering through Mac's and waiting.
probably the reason you can't find it at the parts store, is because they don't sell them any more. Rock Auto doesn't list it either. Happy waiting
Ordering is always on option, but walking in and buying one locally is the question. Perhaps the correct part will cross to a newer application, say farm tractor or something. All online part numbers are the old Ford PNs suppliers like to reuse that don't cross to a common generic part number.
The NAPA in your note is BK 7031419 which they cross to a Stant 10281. I have one, but it reaches only 3/4" into the neck and won't provide a tight seal against the lip in the radiator neck. One with about a 1" free seal reach is needed (will compress when installed).
The pickup uses the smaller ID cap, for the 'A' size neck. Pass cars thru 1948 use the 'B' size neck. Just can't find one with a seal deeper than 3/4".
My buddy had the same issue, I'll see if I can track him down, but he put something in the neck so a "modern" cap pressure thingy <<<tech term there, would seal.
you might try a good radiator shop. I just had my '41 chev pickup radiator recored and they gave me a new 7 lb cap.