The Stovebolt Site Forums The Shop Area Engine Shop 235 thermostat question

I’ve got a 1955 Chevy first series truck with a 235 engine. There was no thermostat in the truck and I installed a 180 degree thermostat but the truck is running hot when at an idle. Did the 235 engine originally come with a 160 degree thermostat? Seems like I’ve read about some people drilling a 1/8” hole in the thermostat- is there any merit in that? Thanks in advance.

Here is the thing with the thermostat. If your cooling system is not able to keep the appropriate temperature, including at idle, a change from 160 to 180 will not help you. It just means that the thermostat will open a little earlier from when a 180 thermostat will open. So you might delay getting "hot" for a few extra minutes, in the end, the motor will get "hot" because the cooling system is not up to snuff.

What you probably want to do as low hanging fruit is to kick the bushes and see if you can get your hands on a GM five blade fan that will fit the truck, or measure what you have and see if Derale offers a 6 blade fan that will fit. You will be surprised the difference that the extra blade makes in moving air, especially at idle.

It is also possible crud has built up on the back side of your number 6 cylinder. This is pretty common. It can be helped, but it takes some work and you'll probably have to remove your water pump. Sadly you can't buy the chemicals that used to dissolve this stuff very well any longer. I would suggest opening the drain cock at the far rear of your engine (manifold side) to see if anything runs out there. If it doesn't, you're plugged up. If you have the original mechanical temperature gauge, it will be located at the rear part of the head and this is where the heat from that sort of situation will be located (at the rear of the engine). Occasionally you can remove the drain cock at the rear of the engine and work a straightened coat hanger with a small hook bent into it up and behind the number 6 cylinder to get the engine to drain. Sometimes you just won't be able to solve a lot of this problem with, though. I have managed to get all the coolant out of the block and then remove the water pump and fill the block up to the water pump opening bottom line with CLR and let it sit overnight. You can also make a strong solution of citric acid. You would drain either of these out the rear hole where the drain cock was and repeat every day for about a week. The fact you had no thermostat suggests to me a prior owner was trying to keep the engine cooler because he had the same problem. In a perfect situation, you'd remove the engine, remove the large access piece at the back and clean it that way, but this isn't always convenient. Good luck.

Edit...also the 1/8 inch hole you've heard about (actually most folks use a slightly smaller drill bit) is a "burp" hole. It allows air to escape preventing a trapped pocket which will also make your engine run hot. But most thermostats are now built this way or they have a "jiggle pin" which does the same thing.

Thanks so much for the replies. I’ll first open up the rear drain cock and see if any coolant comes out and go from there.

The plugged up ones that I have dealt with had hard to dig out black tar that was as hard as a carp. It was likely some kind of stop leak added several times by Farmer Bob, Otto, Norville and the like from back in the day to get another season out of an expensive to replace leaky radiator.
I cannot imagine any solvent that would break that stuff down to be able to flush it out. Engine removal and complete teardown is the only way that I could get it out.

Here are a couple of views of a "jiggle pin" in an old but still good 160 degree thermostat. Hopefully a trapped air bubble is part of your problem. Also, granulated citric acid can be purchased on Amazon. I get it to use as toilet bowl cleaners as it will dissolve minerals. I'd need to look but as I recall 6 tablespoons in a 32 oz spray bottle will give you about a 6% strength mixture. You'd need more than that to dissolve heavy rust. As I said, I've used CLR full strength and a another fellow last night told me he had used a product called "Evapo-Rust" but said it works more slowly so you have to be patient.

Thank you

Define "hot"- - - - -is the coolant boiling? A 7 pound pressure cap which was standard on your truck will keep the coolant liquid up to 232 degrees before it starts to boil over. Unless it's boiling at idle, it's not "hot". Does the coolant temp drop at road speed and/or if the RPM is raised a little? You might need a fan shroud, or a good pressure wash of the radiator core, from the engine side of the radiator.
Jerry

I’ve had good luck drilling a 1/8” hole on thermostats to help rid the system of air.

As for flushing:

If/when you flush….here’s a helpful tip I learned here and can attest that it works. Rig up a npt to barb fitting in the block drain hole. Attach a section of tight fitting hose. As you are flushing/ draining (would be good to wait until you have any caustic stuff out of the system)….give an occasional blast of compressed air through the hose and into the block.

You need to have a place to relieve the pressure. I like to leave the rad petcock slightly open. Having a little shutoff valve somewhere in your drain hose is helpful. But you can just as easily get pinch it off to direct when/where you want to drain.

You’ll be amazed how much crap the air loosens up, long after the water has been flowing clear.

I’m going to add a suggestion for you. If you don’t have fan with more blades nor a shroud consider installing an electric radiator cooling fan. It can be set up with a thermostat probe that slides into the radiator or like I did & use a toggle switch.
That takes care of my 53’s issue.