The Stovebolt Site Forums The Shop Area Engine Shop 270 Cooling Question

I have a temperature sensor question on a 270 engine that I installed in a 1952 GMC 1/2 ton truck. The engine has an old electric temperature sensor located at the horn above the water pump. I ended up installing a fluid filled sensor since that's what my gauge required in this model of truck. This sensor I added is located in the top of the head near the firewall on the drivers side. My temperture gauge in the truck often reads around 200 degrees even with a 160 thermostat. When I take readings with a thermal sensor, I'm at that 160 degrees or less at the water pump and on the passenger side of the head. The head on the drivers size is closer to the 200 degree temperature. I'm starting to think that this increase in temperature reading is due to its proximity to the exhaust manifold and maybe my fluid filled sensor is not correct for this application. If I used the electric sensor, I would be right at the 160 degree temp. Thoughts? Should I be worried, maybe get an electric gauge? Thanks

That hole at the top rear of the head is the hottest place in the cooling system of the GMC six cylinder engine. Mine typically runs 10-15 degrees hotter there that at the thermostat housing.

Some of the military versions of our engines installed a piece of tubing from there to a fitting on the engine side of the thermostat housing. Solves two problems.
1. Gets the hottest coolant immediately back to the thermostat and into the radiator.
2. Solves the "air pocket" problem. When the coolant gets low, like when you drain and refill it, the back of the head ends up with an air pocket. You can get rid of it by cranking open the fitting back there with the engine running and let the air escape.

Here is a picture from US Army TM9-8025-1 Ordnance Field and Depot Maintenance GMC Model 302 Engine. Item A is the tube I mentioned in the previous post.

My 228 GMC (and most of the others I've seen) have a boss on the head, but it hasn't been drilled and tapped. It is near the hole where you have your temp sensor installed.

The back of the head sensor port is the hottest spot in the cooling system and 200° isn't that uncommon. If you have 50/50 coolant it will raise the boiling point by at least 10°. 200° is till quite a way from the boiling point. And the gauges aren't that accurate. The big thing is to keep the thing from boiling because that is what causes damage. I have been operating over the road trucks for over 60 years. We do things a little differently now than we did back in the gas GMC days. We have master gauges to check critical things such as temp oil pressure. If the temp worries you, use a known correct gauge on it and see what you have. Back in the day if we cracked a head, we just went out and got a different one. GMC heads aren't that plentiful any more. What kind of radiator do you have? if you have the one that cooled the original 228, it will be borderline on a 270. Back in the day we always installed radiators out of trucks that had a 302 to keep engines cool in Texas. They bolt in, but have thicker cores. I like aluminum radiators. They transfer heat better than the old brass ones did. But if you buy one, make sure it is for a GMC. Lots of vendors list radiators to fit Chev or GMC. That is not the case. The mounting brackets on a Chev sit at a different angle and the filler is on top of the tank. You need one with the filler neck like you now have.

It should be pointed out that a 160 thermostat doesn't prevent your engine from operating at temperatures above 160 degrees. It begins to open at 157-165 degrees and doesn't fully open until the engine reaches 185 degrees.

It's purpose is to allow the engine to warm up faster on cold starts by staying closed until the 160 temp is reached.

Your engine is going to operate at whatever is normal for that engine. That's probably somewhere between 180 deg and 200 deg.

I use a 160 T-stat in my 216 even though the factory standard T-stat was 143 deg. Using either one doesn't limit the temperature at which the 216 is designed to run which is 180deg under normal conditions.

Thanks for your insights, and your comments make me feel better. The engine runs great and I haven't had any boil over, but was just worried when I saw the higher temperatures on the gauge.

I have seen something similar to the return hose mentioned on some willys jeep engines. At some point, I may try to use the electric sensor and see about installing a hose from the existing thermostat location to the front of the engine. I'm wondering if I could T off the return line of the water heater or would that cause other problems?

I have two water temp gauges on my 261-powered 51 3800. One, the original gauge is in the standard location between cyl 5 and cyl 6 on the manifold side. The other one is in the water outlet neck below the t-stat. The t-stat location gauge always runs hotter than the one in the head. How much hotter depends on how hard the engine is pulling. That could be because the sending unit is a little off or the gauge head is off or that there is enough resistance in the 3-way switch for the various sending units to throw it off; engine oil temp, water temp, and aux. transmission temp. Or the water at the outlet to the radiator is actually hotter. I mostly water the factory gauge.

Over the years I've seen three Chev sixes with burnt pistons, two number 5 and the other number 6. Maybe the military was on to something when they speced the 270 and 302 engines with the bypass pipe.

1 ton - do you know exactly where the military bypass pipe went to. It seems like I could connect a pipe at the 5/6 temp. sender location on the head and run it into the water pump with a T of some sort.

Use a pipe fitting called a "street tee"- - - -it has one male thread and two female ones. Plumb it so the temp sensor adapter goes into the vertical thread, and the bypass line uses the right-angled thread.
Jerry

They used the water pump bypass

Here is a picture of my GMC military 302 head.

The threaded hole to the left of the cast R is at cylinder 5. That hole is there and threaded for every GMC 228, 248, 270 and 302 I've ever seen. The temp gauge screws in there.

The raised area to the right of the R is at cylinder 6 and is there on every 228, 248, 270 and 302 engine I've ever seen, but is only drilled and tapped on military engines. This is where the bypass tube originates.

I didn't check, but I'm pretty sure the hole size and threads are the same for both holes.

Those holes look like 1/2" NPT. That's an easy drill & tap job, even with the head on the engine. Just put a little heavy grease on the tap flutes to catch most of the shavings.
Jerry

Mr. Crenwelge notes the use of thicker cores in his post above. Sometime before 1982 my original core, which would have been for the original 228, was rotting and leaking. I took it to a radiator shop near where I worked, and the man told me the best fix would be to have a new core installed. When I picked it up he told me that he put a bigger core in. I did no measuring before or after the fix. However, since the 1980's my temperature gage goes to 180 and never moves. Like the original poster I drive a 270 in Virginia (VT CE 1987.)

I took a measurement today with a straight ruler inserted on the side of the core then pushed to the stop. Mine measures 1-1/2 inch from that stop to the visible front of the core. I don't know if this is any help, but if your measurement is less than 1-1/2 inch then that would confirm a thinner core.