Radiator pressure cap.

[jonnyb]

My car has a 4psi pressure cap and and runs fine and doesn't seem to lose any coolant. I have an official BMC WSM which suggests a 4 psi cap and also an Owners Workshop Manual by JRS Hall which suggests 7 psi. I think the MGA and early MGB used 7 psi caps. The club sells 4psi and 7psi caps. Interesting too that the Riley cooling system has a 13 pint capacity system while MGA/B were between 9.5 and 11.5 pints.

What do all your cars have?

[vue from the north]

i recall a conversation with someone about this that went along the lines of the later 5 bearing crank engines had a higher pressure cap than the earlier 3 bearing cranked engines,

[canuck]

Hi John,

The Riley does indeed call up a 4 psi radiator cap. 

These cap pressures are intended to provide a higher operating temperature in case of mega hot weather.  If we use a 50/50 glycol to water mix in the cooling system then the boil over temperature is 220⁰ F? 
The math is 3⁰  F  per psi? So with the standard cap  we will see a higher range to +12⁰  F, or boil over at 232⁰ F 

My MK1 Riley has a 4 psi cap and never had an issue. My MGB has a 10 psi cap and no problems.  
I can't speak to the larger coolant capacity in the Riley.  

FWIW I did make and install  a sheet metal 'deflector' at the front and between the top of the radiator tank and the grille.  My thinking is I wanted to direct the flow of air through the radiator and prevent flowing above it.  I was thinking this also may reduce air pressure in the engine bay.   No science to prove this out.   

Bruce