Hello Chip,

The standard cap supplied with the engine heat exchanger would normally be 13-15 psi.

The header tank requires a cap with a lower relief pressure than the heat exchanger tank cap. This is because you do not want the heat exchanger cap releasing pressure before the header tank cap releases pressure.

I suggest a 13 psi cap on the header tank and a 14 or 15 psi cap on the engine heat exchanger tank.