radiator cap

[1940 lasalle man]

what type of radiator cap do I need for a 1940 Cadillac lasalle 322ci motor. All responses are greatly appreciated. Thank You M McDonnell

[Alan Harris CLC#1513]

I believe that the correct cap is an AC RC-3, which is a seven pound pressure cap. Stant or somebody makes a modern replacement which will work, but won't look right. The key thing is the depth of the filler neck. 1940, I recall was deeper than other years.

[Richardonly]

If it is a deeper neck, DO NOT use a standard cap.  No seal will be formed and it will continue to overflow until there is only steam left.

Trust me, I know 

When my car was bought from the previous owner, it came with a standard cap, and out the coolant came.

[R Sotardi #11719]

Indeed the 39 & 40s used a RC-3 7lb, as did the 40 -47 Buick and 50-53 Hudson. These have a 1" deep neck, where as, I believe the RC-4 , is 7 lb but with a 3/4" neck. BTW, the original AC rad caps did not use the  designation RC-3( part # 850797) until the early -mid fifties. Until that time, the caps were listed with the last two numbers of the GM part, ie, 846713. So the original 40s will have AC  embossed on one of the ears, with 13 under the AC and beneath the 13, 7# . Ron

[R Sotardi #11719]

Does any one know exactly what year the AC Division switched  to the RC designation on there rad caps? I heard it was '54 or '55. ALSO, when did Cadillac start installing AC GAS CAPS on new cars? I believed before 52 they bought them from Stant.

[pauldridge]

what type of radiator cap do I need for a 1940 Cadillac lasalle 322ci motor. All responses are greatly appreciated. Thank You M McDonnell

Yes, those radiators used the deeper cap.  When I had my radiator rodded out, I asked the shop to install a boss on top for a standard depth cap, since those deeper caps can be somewhat hard to come by, and most sellers are pretty proud of them.  Additionally, it gives me the option of going with a slightly higher pressure cap to deal with Texas summers.