PREPARATION FOR SPRING RUN - Excessive Coolant Pressure

[Graham Dix]

This problem became apparent last year but I stored LAF717F planning to look at it at some point in the future. Almost as soon as I started her up I remembered this being an issue as water from the overflow pipe ran past my feet. When the engine was up to temperature the thermostat must have opened as both top and bottom hoses were hot. The next day I replaced the original 4 psi radiator cap with a 10 psi cap I had spare. The result was quite dramatic as water squirted out of the top hose join with the radiator all over the inner wing. Ummm. I consulted a couple of professional mechanic friends and the consensus was that the head gasket must be leaking between a cylinder and a water chamber thus pressurising the system. As this is a time consuming and fairly expensive job, to cheer myself up, I thought I would fit a new oil and temperature gauge I had purchased from MGB Hive as the temperature gauge had not worked for a few years now and time and money had put it to the back of the que. The old temperature sensor would not budge from it’s housing below the thermostat so I removed the thermostat to get at the sensor. With the new gauge successfully fitted I looked at the thermostat, comparing it to a new spare I have. I noted that there was no small bypass hole in it. Thermostats as far as know all have a small hole with a ‘giggle pin’ through it. The hole allows a small amount of water to bypass the thermostat when it is closed. The giggle pin prevents the hole from becoming blocked with deposits. (see photo) To end this story, fitting the new thermostat completely cured the excessive pressure. Has anybody else come across this? Is this another story about poor quality replacement parts?
Regards
Graham Dix
Chairman and Technical Advisor

[Dave Linkson]

I checked 2 used thermostats on my shelf today and both were without the jiggle pin hole. Are these holes nessacary for the B engine? have read only for the V8, and in a certain position in the housing. Heaven knows what's in there at the moment, I didn't install it in the new engine..