TR2/3/3A TR3 Cooling...Blocking the Bypass

I'm not using the skirted thermostat but the newer type. I'm driving in hot summer weather here so am going with the blocked bypass mod. I recall something about drilling a 1/8" hole in either the plug that blocks the bypass or in the thermostat itself. Anyone know which? Karl

The mod is to purchase a copper 3/4" pipe cap and drill a 3/16" hole in it. Push it into the waterpump end of the bypass hose and reconnect the hose.

I did this on my 3A and see absolutely zero difference in running temp and warmup temp.

Aloha Karl,

I used an old tubeless tire valve stem as a restricter. I removed the air valve and then sawed off about 1/2 inch back from the threaded end with a hack saw. There may still be a short piece of brass tube in rubber body, if so remove it with needle nose pliers. The remaining rubber valve stem has about a 1/8 to 3/16 inch bore and is then inserted in the by pass connection of the water pump. This should need to be pressed in with a little force. The fat end is slightly larger in diameter than the ID of the bypass hose.

I'm not sure how much this helps with engine cooling, but it won't hurt it either. Although I have an electric cooling fan mounted with a thermostatic switch (140 deg F at water pump inlet) and a manual over ride switch I have not had to use it yet. The thermostatic switch has never turned on the fan.

IMO, keeping the cooling system clean and free of sludge/rust sediment build up, water pump operating, radiator shroud in place and lots and lots of air moving through the radiator the stock set up will work fine. It has for me in Hawaii where the coolest temperature my car feels is in the low 60 deg F.

Open the engine block drain petcock and if there only a dribble of coolant it is most likely partially block with sediment. Remove the pet cock and poke around with a stiff rod (ice pick or Phillips head screw driver), this should disturb the sediment enough to flow out. Flush the block with water until it runs freely and clear. The internal bits inside the pet cock are the cast iron block and cylinder sleeves so don't be afraid to vigorously poke around.

The stock fan seems to be the weak link in the cooling system because it just doesn't move a lot of air at low engine RPM. I would wager that if you could alway keep the car moving at least 25 MPH, you could remove the fan and never have the engine over heat.

I'm almost embarassed to admit I drove for 20 years with a 1" piece of broomstick jammed in there. Finally went with the copper cap w/hole when I replaced the bypass hose a couple of years ago.

No idea if it helps but I drive up mountains in Tucson in the summer so I'd probably hang a shrunken head from the rearview mirror if I heard it would make the engine run cooler.

Geo Hahn said:

I'm almost embarassed to admit I drove for 20 years with a 1" piece of broomstick jammed in there. Finally went with the copper cap w/hole when I replaced the bypass hose a couple of years ago.

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Interestin', Geo.

For the racecar, I threaded a brass pipe cap into the hole on the water pump side and put the hose over it for looks. But the hose leaked from the thermostat side, so I cut the hose and plugged it with...

Well just let's say that, like the Edwards Air Force Base Bell X-1 Hangar broom in The Right Stuff, my shop broom handle is a little short.

But I'm short, too.

karls59tr said:

I recall something about drilling a 1/8" hole in either the plug that blocks the bypass or in the thermostat itself. Anyone know which? Karl

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It actually works either way. Drilling it in the thermostat gives you maximum cooling, but slower warmups. I would also suggest a somewhat larger hole, 3/16" at least, so there is a reasonably amount of water movement with the Tstat closed.