While attempting to sort out the cooling problems with my Stag, I found a potential problem with some of the thermostats that are now being supplied to TR7 owners. The Stag has a bypass port behind the thermostat that is supposed to be blocked off when the thermostat is fully open. I found that I didn't have the correct type of "reverse poppet" thermostat in the Stag, which was leaving the bypass port wide open. This allows hot coolant to enter the water pump suction line instead of being forced through the radiator. In my case, this led to overheating and boilovers within 25-30 miles of startup.
In my search for the correct type of thermostat, I found that the Robertshaw type 412-180 thermostat was listed for the Stag. This thermostat has the reverse poppet disk that closes off the bypass port when the thermostat is fully open. The same thermostat was also listed for use in the TR7, as well as several other vehicles. You can see the reverse poppet type thermostat (GTS 108) on the lower left frame on page 9 of TRF's TR7/8 catalog. GTS 108 is a 88 deg C thermostat. The picture of GTS 101, the 82 deg C thermostat used in early TR7s, does not show the reverse poppet disk as clearly.
How does this affect the TR7 owner? I received the latest flyer from TRF yesterday with a listing of a variety of thermostats with varying temperature ratings. The whole range is listed as common for the TR250-TR8 range of cars. I know that my TR6 and TR8 do not use reverse poppet thermostats, so if TRF is selling TR7 owners a normal thermostat instead of the reverse poppet type, there will be a bunch of overheated TR7 engines next summer.
If your TR7 is having overheating problems, try looking at your thermostat. If you don't have the blanking disk ("reverse poppet") on the bottom of the thermostat to cover the bypass port when hot, the chances are that the improper thermostat may be contributing to your overheating problem even though it may have the correct temperature rating.
Originally posted to the World Wide Wedge TR7/8 Internet mailing list, and reprinted in The Triumph Tribune by permission of the author.