The Stovebolt Site Forums The Shop Area Engine Shop Drill Thermostat ?

why drill the thermostat and what size hole?

1/8 inch and it's so as You're filling the rad and engine with water the air comes out of the top of the engine and there is no trapped air in the engine when You warm it up the frist time.

Pete

In addition to what Pete said, there's the ongoing advantage of having the thermostat exposed to a small amount of circulating water as the engine warms up rather than heating stagnant water under the thermostat by convection only. On my 235, I had a problem of the temp. guage swinging almost into the red zone before the thermostat would open, after I drilled the 1/8" hole in the flange, as soon as the engine reaches operating temperature, it stays right there.

thank's for the info, it's may help.

I never drill thermostats unless the OE was drilled. Never had a problem with air pockets or overheating.

You are lucky.

I used to only drill replacement thermostats for my 261s (because the original thermostats were drilled). However, I now drill all thermostats that I put in a 216/235/261 engine. Why not (it won't hurt anything, it allows a little low-temp circulation, and it might help clearing air from the system)?

I have never drilled a stat myself. I figured if getting airbound was a hugh deal the stat manufacturers would drill it when they build it. Not saying its a bad idea but I have personally never had a issue with any older (pre 1990) engines getting airbound. I have on the otherhand gotten plenty of junk cheap stats out of the box. I started using Stant Superstats exclusively and never had a issue. I have noticed some rice burner stats have a hole with a checkball staked into it and I assume its probably a air bleed of some sort.

My original thermostat stuck open last summer and I replaced it with a modern one. I didn't drill it and so far no problems.

When I bought my first 261 in 1971 (a 1960 engine out of a large school bus), the thermostat had a hole in it and the replacement thermostat (from NAPA) also had a hole in it. The 261 thermostat and housing (and water pump) was significantly different from those items on a 235.

ongoing advantage of having the thermostat exposed to a small amount of circulating water as the engine warms up

What's the advantage?

panic; Keep reading, the advantage is explained in the rest of that post, at least in my demented mind. During warmup (thermo. closed) there can be uneven temps in the head (for example where the temp guage sender is). The thermostat is heated only be convection as no water is yet circulating possibly leading to a faulty reading by the thermostat. With the 1/8" hole, a small amount of water will circulate, evening out any hot spots and bringing the head temperature to a more even heating that the thermostat can react to. In addition, if the old girl needs a good burp, she doesn't have to wait until the thermo opens.

Also, I'm thinkin' it can't be a good thing to have a 55-year-old cast-iron head a lot hotter at one end than at the other end, even if it's just for a few minutes.

Actually, coolant does circulate even with the thermostat closed. All the liquid-cooled engines I'm aware of have internal passages designed to keep the coolant moving through the water pump, block, and cylinder head even with the thermostat in the closed position. Drilling the thermostat might prevent an air bubble from forming in the immediate area of the stat, but it won't have a significant influence on coolant circulation. Some thermostats have a positive blockoff feature to close off the bypass port once they open, but that's not so on a stovebolt engine.
Jerry

The coolant/water by-pass from the block to the head (that Jerry refers-to above) is described in the right column of this page from the 1954 Shop Manual.

Thanks guys, that's good info there. I love this site, good place to learn!!

If you want to avoid the belching of trapped air in the system just fill the block with coolant before installing the thermo.

I usually drill a tiny 1/16" or 3/32" hole in a thermostat if it doesn't have one already, many do.
Don't expect it to do anything but help burp the air out, and even if turns out it's not necessary I'm sure it can't hurt.

Grigg

Don't see the reference to drilling the thermostat - it says that the front of the block is already vented? To where?

I use the top heater hose connection to burp the system when I refill it. I find that works for me.

The bypass passageway routes part of the coolant flow from the area of the thermostat back to the water pump inlet. It keeps the coolant from forming stagnant spots that would result in steam pockets if it was not in motion. On some engines like the heavy-duty 261's and 292's the bypass passageway is much larger to flow more coolant when the thermostat is closed. Drilling the thermostat valve is not a factory-reccomended procedure but it seems to work to avoid an air pocket forming near the thermostat.
Jerry

I drill 3 3/16" holes on the T-stats in the boat because the Mercury ones are $20 and a stant is $4 The car has a jiggle pin installed already.

While you all are talking thermostats what temp stat should I put back in my '57, 235, 1/2 ton PU?

185 was standard with perm AF.

A 160 degree thermostat was commonly used in standard 6-cylinder trucks.

http://chevy.oldcarmanualproject.com/shop/1955truck2/55ctsm06s09.htm

http://chevy.oldcarmanualproject.com/chevyresto/56160.htm

http://chevy.oldcarmanualproject.com/chevyresto/58t105.htm