Bob, if there is no air in your coolant system, you may be observing an hydraulic lock type phenomena. My water pressure will immediately zoom to 21 psi on engine start up. As the engine warms up the pressure drops back down. My understanding is that with no air and the incompressible nature of a liquid - that even small perturbations anywhere in the system can cause the sensor to signal pressure. As the engine warms and a very small amount of expansion in block, rad cores, lines etc. happens that is enough increase in volume of the "containment" for the pressure to lower.
This is why some folks recommend leaving a small amount of air under the cap. If you are not seeing oil in your coolant or bubbles then its unlikely you have a busted O ring.
Ed
----- Original Message -----
Sent: Sunday, February 24, 2008 11:52 PM
Subject: [FlyRotary] Water pressure
I ran my 20B at idle, till about 160 degrees. My water pressure rose up to 23 lbs. That seems excessive to me, but I've never watched water pressure before. I've bled the block a couple of times and I dont feel like I have an air lock. Heat is even throughout the radiator. I was a bit worried about compression leaking into the cooling chamber, but I'm not loosing any water, the plugs aren't wet, and it starts flawlessly each time. Hot or cold. Recon I'm all worried about nothing?
Bob Mears
Supermarine Spitfire