Does anyone know the difference between the 41a-8100-a and 41a-8100-c radiator caps? They are both 4lb caps.

 

Do you mean 1T-8100-A and 41A-8100-C? Somewhere in an old thread on here, there is a vendor drawing that shows the depths of radiator necks. NAPA is as good a source as any for a cap, just measure your neck so you know what you've got, your radiator may not be original.

From a previous thread: for a 3/4" deep neck, Stant #10208
My truck uses a Motorad T-4 (from NAPA)

 

Ross, in the 1950 parts book, July edition, the radiator cap for the C series was 41a-8100-a. I have been looking for one of those but can't seem to find one, one just popped up on ebay but all I did was push the bid up above $50....


I only ask because until recently my radiator leaked ever so slightly at the front solder joint. I picked up a NOS Eaton 4#, and the leaking picked up steam and has turned into drips, it even looks like the top might have bulged a little on me. Cant remember what it looked like before I changed the cap......


Anyway, I don't know if that Eaton 4# is a 4lb cap or stiffer, I know the 41a-8100-c is a 4lb as well as the 1T-8100-a, but the 1T appears to be for 52-53 215 trucks.


I may just try the 41a-8100-c and see what happens. Its important for me that cap is correct since its so visible.

 

Autozone et al "loan" a pressure cap tester. Easy (and worthwhile) to test caps regularly, even a brand new one.

But note that if the radiator neck isn't stock, especially if it's shorter, it will compress the cap's spring more and raise the pressure. Also if the overflow/relief tube is plugged, it raises the pressure.