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Thermostat for a Tr4a

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ric350 Avatar
ric350 Jim Feeney
Holmdel, NJ, USA   USA
Folks, I want to replace the thermostat on my TR4a, so I went to the TRF site and was stunned to see the original “bellows” style thermostat is $120!!!! They also list modern thermostats for $5, but say the bypass hose must be blocked off. The plus side of going with a modern one is saving $115, but what’s the downside of blocking off the bypass?

Thanks

Jim



Edited 1 time(s). Last edit at 2019-01-14 12:49 PM by ric350.

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Geo Hahn Avatar
Mt Lemmon, AZ, USA   USA
Not blocking the bypass will allow some coolant to bypass the radiator - engine might run hotter.

Blocking the bypass may delay warm up a bit - mostly an issue if you are trying to get the heater working on a cold morning in Michigan.

Actually, most feel that you should not totally block the bypass but reduce the opening - this may avoid having cold coolant suddenly dumped on a hot head when the thermostat opens. At least that is my understanding of the logic.

For many years I had the bypass totally blocked (a piece of broomstick) with no ill effect - but now I use a ¾" copper cap with a 3/16" hole in it so reduce the flow there:



This is more important on a TR3 where the bypass opening is large. Somewhere long the way the thermostat housing casting was changed and the opening was reduced a bit.

trrdster Avatar
trrdster Wayne Tate
Spencer, NC, USA   USA
I put a cut off switch on the by pass hose and am able to regulate the amount of flow. I used it for driving to shows in the Roadster on over 90 degree days. Worked like a charm to keep the temperature under 200, even in stop and go traffic.
The Roadster is set up like the TR3, so a little larger by pass than the TR4, but it should work fine with the plug idea of Geo.
I think I remember someone using metal spacer to place a standard thermostat in place, it was a smaller one from a different car, just can't remember, it's only been 25 or 30 years. Plus sometimes you hear of people doing things that sound like good ideas and never get any feedback on how it worked. Folks are busy, I guess.



Wayne
1970 TR6
2000 Jaguar XK8
1949 Triumph Roadster 2000
1978 Spitfire (rust victim)
1971 GT6 (tarp covered for 12 years, rusted inside out)
1980 Spitfire (getting all the good GT6 parts, all poly suspension and Spax shocks)

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ric350 Avatar
ric350 Jim Feeney
Holmdel, NJ, USA   USA
Thanks Geo! That’s a clever solution.

Jim

Peter-K Peter K
Central, ME, USA   USA
Your TR4A already has a reduced bypass in the casting so a modern $5 thermo is fine to use, no mods required.

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
In fact, the original thermostat for a 4A did not have the 'skirt'.

After extensive experiments on both my previous TR3A and my current TR3, I found no difference at all in cooling with the bypass open, closed, or partially blocked. I currently run a modern "wax pellet" type thermostat, which IMO works much better than the original bellows thermostat, and the car doesn't overheat even when the driver does (115F ambient is about my limit, time to pull over and find a cool spot to hide).



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

ric350 Avatar
ric350 Jim Feeney
Holmdel, NJ, USA   USA
Peter, Randall, thanks for the info! A $5 thermostat it is!

Jim

jerryvv Avatar
jerryvv Jerry Van Vlack
Hudson, USA   USA
My TR4A had the skirt 1966 and I used it until 2016 when it failed.

TrippingTR Avatar
TrippingTR Edward O
Phoenix, AZ, USA   USA
I'd like to add a question to this discussion for you thermostat experts, I too have a 1966 TR4A that will have an all new/rebuilt cooling system soon and I was contemplating the bypass situation as well as whether to run a 160 degree F waxstat or a 180 degree F? I'm planning on using Evans waterless coolant which many people say runs warmer than regular coolant mix. And I have Weber 40 DCOE carbs which like it a little warmer too. Any thoughts on which temp thermostat to use and whether blocking the bypass would be appreciated.

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
IMO you're better off with the 185 thermostat. The early aneroid type thermostats started to open sooner, because they took so long to move to full open (full opening was supposed to be at 92C, about 200F).

Blocking the bypass in some fashion (but still allowing for circulation with the thermostat closed) will improve cooling slightly under certain conditions. In Phoenix, my suggestion would be to block the tube entirely, and drill several holes in the thermostat backing plate so it passes some water all the time. You'll get slightly slower warmup (which doesn't matter much when it's warm outside), and maximum cooling because no water bypasses the radiator. But don't expect to see much difference beyond the slower warmup.

FWIW, here's the illustration from the factory 4A parts catalog



And the factory TR4 parts catalog





Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

TrippingTR Avatar
TrippingTR Edward O
Phoenix, AZ, USA   USA
Thanks Randall, that makes sense. Drilling the holes in the thermostat backing plate and plugging the bypass route is a great idea. Maybe a couple 3/16" holes?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Sounds reasonable to me. I used a single 5/16" hole.

It probably doesn't matter much, lots of folks run without and report no problems. But without some sort of flow, the cylinder head can get very hot before the thermostat opens (especially around the exhaust valve seats) and get a big thermal shock when the thermostat opens and cold water from the radiator suddenly flows through the head.



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

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