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Triumph TR3 - Sleeved thermostat

I've noticed a few threads about overheating TR3s. Surprisingly, I don't recall anyone mentioning the special "sleeved" thermostat that is referred to in our Moss catalogs, and in the workshop manual. Fot eh fact that Moss offers other less-expensive thermostat certainly implies that non-sleeved ones are okay, but some cooling efficiency has to be lost.

The thermostat housing has a bypass route that circulates water from the engine back through the engine bypassing the radiator. this helps the engine warm up when cold. However, the sleeved thermostat then blocks the bypass when the thermostat opens. As a result, (relatively) cool water from the radiator enters the engine.

Without the sleeve, the hot engine will get a supply of hot water from the engine mixed with some cooler water from the radiator.

This is my understanding of the system. Anyone with different ideas? I suspect there are a lot of TR owners out there who don't know about this secret device.
Chet Harter

Chet, I can't say that I've heard of this yet, but perhaps some who have will reply here. What about a lower temp. thermostat? I have not yet read my shop manual on the cooling system yet, but there has to be a practical solution. Thanks for the thought.
Mike Gambordella '56 Tr3
Mike Gambordella

I installed a new sleeved thermostat in my TR3A when I did my restoration in 1987-1990. It had a sleeved one in before that too. Chet is correct on his description of how it works. But in Bob Shaller's booklet (available from TRF), Bob suggests something that I have done as well. I bought a brass threaded pipe plug (similar to the drain plug in the oil pan - but in brass) and removed the top "bypass" hose that comes out of the rear LHS of the top of the thermostat housing and I tapped some pipe threads into the iron casting where the hose had been attached. I used brass because it won't rust. A stainless pipe plug is OK but harder to find and harder to drill.

Yes, I drilled a "vent hole" about 1/8" diameter from one end of the brass pipe plug through to the other end. This is essential to allow the air to "vent" upwards out of that part of the water circuit when you are filling the rad and the whole system. The brass plug closes off about 90% of the bypass flow and will enable anyone to use a regular thermostat if he can't find a sleeved one. For many years the sleeved one were not available.

Even though I use the sleeved t/stat and the plugged bypass, my TR still overheats when I stop in traffic for longer than a few minutes. I'm sure the above helps a bit but it will not solve the problem.

We all know that water boils at 212 deg. F. at seal level. That's why we want to see the engine running at 185 deg. F. With a 50/50% antifreeze mix, the mix will boil at about 230 deg. F. But we have a 4 psi rad. cap that allows the pressure to build up and the 50/50 mix will not boil until about 243 deg. F. Remember your high school physics (P*V)/T = (p*v)/t where T and t are in degrees Rankine (add 460 to the F degrees). V = v so that drops out. Do the math for yourselves. P = 14.7 psi and p = 18.7 psi. t = (212+460) - solve it for t .

So when you think about the fact that the gas/air inside the cylinder is igniting and as the deflagration or flame front is propagating, that it may reach about 3000 F, why do we worry about the 185 F water temp when it will still conduct heat away until it starts to boil at 243 F ?

Until I have the answer to this, I will continue to turn on my electric fan in traffic. It has done me well over 12 summers and more than 70,000 miles.

The fan may be a "fix" to the problem, but it really solves the worry.

Don Elliott, Original Owner, 1958 TR3A
Don Elliott

Chet-I agree with Don. I plugged the bypass hose and didn't really notice that much difference. It always amazes me that a modern car with a small radiator (and electic fan) can idle almost indefintely without overheating.

This thread was discussed on 26/11/2002

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