Correct depth of rad cap?

[TimDart]

I'm not convinced that my rad cap is the correct depth and therefore not sealing correctly. I'm losing fluid but cant find where, head gaskets seem to be ok, no symptoms as such.
Caps seem to come in two approx depths 1" and 1-1/4". Any idea which i should fit.
Stock smallblock rad.

 

[RedFish]

Given how a pressure relief cap operates a 1/4 inch difference might make few pounds difference. Either would seal to a mating surface around 3/4 inch deep. My question is year model / coolant recovery system ?
Without coolant recovery loosing an inch of water for the top of tank is normal.

 

[TimDart]

Red, its a 360 model in a '74 Dart. I have the rad connected to a return tank.
Also, its not consistant. Sometimes i'll find the top of the rad apears to have partially emptied, other times not.
Cannot find a leak anywhere else.
I drove for about 20 miles the other day, stoped and looked under the bonnet and saw steam hissing past the rad cap.
Drove it further and harder today. No problem..

 

[RedFish]

OK the seal just under the cap should direct spew to the recovery tank. The spring that holds the system pressure closes is pushing the upper seal open. You might ty rolling the locking ears on the tank down just a tad.

 

[TimDart]

Cheers, i'll give that a try