OldIHC

I am finding it impossible to find a radiator cap for my 1936 C30 with a GRD 233 engine. Is the only option to cut the fill neck and replace it with a more modern cap? If so, what cap should I consider that looks close to the original? Thanks in advance. Mike

Those big caps are out there. Don't give up, yet. I bought an assortment on ebay that had one.
The cutting off and substituting idea works. The radiator shop should know if you can still find one open. I then install a very low pressure cap.

I didn't know those old cooling systems were pressurized?

The early IHC's take a "B" size cap, which was STD cap on heavy trucks until recently. Smaller trucks wern't a pressure cap, they just kept stones and small boys from falling into the radiator. The larger K's (and I assume c's and D's had low pressure 4 or at most 7 psi "B" caps. You will not find one at an auto parts store, check with a radiator shop that handles big trucks.
If the orignal was not a pressure cap, and doesn't have the lower "seat" in the filler neck that a pressure cap needs to seal, you can still use the pressure cap to close off the neck, it just will not hold pressure in the radiator.

OK thanks I will try a radiator shop for large trucks and go from there. Appreciate the input from everyone.

Take a look at Ebay item #272873238874. It looks like a good match.
Dave

https://www.ebay.com/itm/Antique-car-ra ... 0005.m1851
Looks like a B non pressure cap but is a gas cap. The radiator cap would have the tangs on the outside.
https://www.ebay.com/itm/VINTAGE-STAINL ... G0&vxp=mtr
Looks like a modern car cap spot-welded to a large chrome cover. Not for early cars as advertised. this the cap mentioned above.