View Full Version : Thermostats

06-01-2011, 07:49 PM
Hi All,

I recently installed a new modern thermostat in my car. There was a modern one in there already, but it looked kind of ratty so I decided to replace it.

My question is this. The design of the bypass hose presumes that we're using a Bellows/Sleeve-style thermostat. Obviously, I am not. I'd rather not use the old style thermostat because they are prone to failure. So, I should block off the bypass, correct?

What should I do this with? I tried the auto parts store to see if they had rubber radiator hose caps in 1", but they didn't. I ordered some caps from McMaster Carr, so we'll see if those work. If not, what would someone suggest? I want to make it look nice, and as clean as possible.

All ideas welcome! Thank You!

06-01-2011, 08:27 PM
I believe Geo wrote that he found a copper pipe cap that would slide snugly inside the bypass hose. Drill whatever size hole you think is appropriate.

I went a different route, and tapped the water pump housing for a pipe plug (1/4" IIRC). Then, since some water circulation is essential even with the engine cold, I drilled a 1/4" hole through the thermostat backing plate (so a small amount of water can always circulate through the radiator. That might not work for you if you drive in the winter, but I've not had a problem even in temperatures below 40F.

PS, the factory did not use a sleeved thermostat in the TR4A. The bypass is already somewhat restricted by thermostat housing (unless someone has swapped your housing for an earlier one).

06-01-2011, 09:45 PM
I believe I have the correct housing, although, there is some amount of bypass allowed with this housing. I read over on M**s Motors website that if using a modern thermostat, the bypass should be blocked. But according to what you're saying, maybe it's not necessary at all to do this.

I thought of tapping the pump housing, actually, but I figured the caps would be easier and still look clean.

I'll test it out with the bypass hose and without. I may never even see a difference. Thank You!

06-02-2011, 06:25 AM
I may never even see a difference.
That's pretty much been the result of all my experiments.

Not very good, but maybe you can see the difference in the housings :


06-02-2011, 07:42 AM
I see what you're talking about now. 2, 3, 3B, 4 on the left, 4A on right.

I'll see what happens. It won't hurt. :smile: