radiator cap (overflow tank) part number R1

[BillyBob]

Suspect my '63 R1 has a bad overflow radiator tank cap.  Who knows the aftermarket part number?  Thanks, Billy Bobs

[Gunslinger]

I assume you mean the expansion tank that the factory installed, not an aftermarket overflow tank.  

The factory shop manual calls for a 12-15 lb. pressure cap.  That's really all you need to know when buying a new one...and make sure it's for an open system, not a closed system!  If you have the stock, factory cooling system it's an open system.  You can buy an oe type or the kind with a pressure release lever depending on how original you want your car or what's available at the parts store.

If you have an aftermarket overflow system added to the car, you need a cap of the same pressure rating but for a closed system.

[BillyBob]

Got it - thanks!  Hard to believe, but I believe I'm simply not running hot enough to build pressure.  On a 75F day at 60mph, I'm running about 170F!  Timing has been retarded a bit to improve driveability around town.  Also have electronic ignition with Edelbrock 4bbl.