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I am restoring a 1941 GMC 3/4 Ton pickup that "supposedly" had an engine rebuild before I bought it.
The Head Casting number is for a 228 cu in and the Block Casting number is for a 248 cu in.
I probably have 100 miles on the engine and am wondering what is the NORMAL indicated operating temperature as indicated by the dash gauge?
Mine will reach close to 212 degrees F after about 15 miles in 80 degree outside temp. The Radiator Cap is Champion Cooling 0.9 which is 212 degrees.
The temperature gauge is a capillary type that has the bulb installed in an adapter fitting in the head at the left back corner (close to the exhaust manifold) and the bulb is probably held out of good water flow. I do not know the rating of the thermostat as it was installed when I got it; but that rating should not influence the maximum operating temperature.
It does not appear as if the radiator cap has ever allowed fluid to pass to the overflow reservoir and the engine does not "seem" hot after shutdown.
Any comments on:
1. Head vs Block Castings
2. Temperature issue
Thanks

If it's not boiling coolant out past the radiator cap, it's not overheating. I assume the 0.9 cap is a metric measurement, .9 BAR which converts to 13 PSI. A 1941 cooling system should probably have a "zero pressure" cap, but one that gives a maximum pressure in the cooling system of 4 PSI will probably work OK. The cap you have might present a danger to the radiator core, and/or the heater core if the system ever pressurizes far enough to bypass the cap. Every pound of pressure on coolant raises the boiling point 3 degrees, so a 4 lb. cap would allow the coolant to reach 224 degrees before it boils. A .9 BAR cap will allow coolant to reach 251 degrees F. before it boils, and that much pressure will destroy your radiator.
Jerry

GMC Heads and blocks interchange. There are four heads that I know of, the 412. 819, 477 and a large port H head used on the 302 or possibly 270. I dont know enough to tell you that the large port head would benefit a 248 or 228. The experts here can discuss advantages or pros and cons.

Piston type is best matched to the head casting codes. One head best matched for dome top pistons and the other for flat top pistons.

Check this out:

http://www.oldgmctrucks.com/Photos/GMC_Numbers.htm

A couple for data points on my GMC 228:

With 160* tstat the engine running temperature runs consistently at 160* on the road, prolonged idle to slow driving maybe 170*.

With 180* tstat he engine running temperature runs consistently at 180* on the road, prolonged ill de or slow driving 180*.

On the road the temperature never gets too far off tstat temps. My system is zero pressure system ( stock). The overflow pipe is soldered in the top tank.

If yours is actually running 200-212* first suspects would be the radiator, tstat issue, water pump, water flow blockage, compromised air flow.

Can you borrow a gauge or use a heat detecting gun?, like Jerry said, if is not boiling, it is not too hot. Could be a bad gauge or sender.

Ed

Where is the overflow tube attached to the radiator? If it's in the top tank, it won't matter what PSI cap you have- - - - -pressure will never build up at all. If the overflow attaches between the upper and lower flange of the radiator neck, then a spring-loaded cap will allow pressure to build up. Use a laser-pointer heat gun to check various points on the cylinder head, radiator tanks, hoses, etc. to get an idea how well the cooling system is functioning.
Jerry

Thanks for the ideas of what to pursue.
The radiator cap came with the NEW Champion radiator that has the overflow on the neck. Seems as if the higher temps are dropping as I drive it more......possibly just now breaking in?
The heater core is part of the Air Conditioning system I installed, so it it modern day; but still 13 psi seems high and I will change for a 4 psi cap. I will test with a heat gun and report the findings.

The fact you replaced the radiator with an aftermarket unit and have installed A/C would have been good to know in your first post. The more information you provide in the initial post will help us help you. Just a point to remember in future questions.🛠

A good compromise will be a 7 PSI cap. That will allow a 21 degree rise above the atmospheric pressure boiling point, or 233 degrees at sea level. Any normal operating range in the 200-215 range will be perfectly safe, and you can run a 180 degree thermostat that will actually keep you warm if you drive in the winter. if you run a 160, invest in a good goose down vest and some gloves!
Jerry

If you are using the original heater core, that will be your weak pressure point.

Ed

Nope- - - -he's got a new, high pressure heater core, also. I'd guess the head gasket or the water pump seal will be the weak point, so the 7# cap is a good compromise- - - - -original equipment through 1960 0r so.
Jerry

If like you said, the temp bulb is in the back of the head and not quite fully incased in the coolant, then it no surprise the gauge reads hotter then the actual coolant temp is. Flow is likely slower that far from the water pump, and most all engines are hotter at the rear then the front due to the slower coolant flow. Move the probe up under the thermostat and it should read a truer temperature. The exhaust manifold also radiates a ton of heat which could heat up the temp bulb.

The capillary tube on the gauge is not long enough to reach the thermostat position. 'there is an inoperative sensor {electric} at that location that I may replace with an aftermarket temp sensor/gauge leaving the gauge under the hood in order to not clutter the cab. what type of sensor and location was stock in 1941?
The capillary tube/sensor plus the mounting fitting that hold it out of the water flow (as poor as it may be at the back of the head) came with the new instrument cluster the previous owner bought but never installed........looks stock, is beautiful and all works well except temp indication.

To get a pretty good indication of the accuracy of the dash gauge, try using a small camp stove under the hood, maybe setting it on the intake manifold, to boil a small pan of water and put the sensor bulb into the boiling water. At least that will give you a reference for the "no pressure" boiling point the gauge is indicating.
Jerry

Mount an electric gauge up front on the motor and drive for a while to see how hot it truly is, then figure the difference in gauges if there is any and remove the electric. Then you just have to remember how far off the stock gauge is every time you look at it.

HRL's suggestion is probably the most accurate. At least you can observe boiling water and the gauge at the same time. Multiple temperature gauges bring to mind the old saying about two watches:

"A Man with One Watch Knows What Time It Is; a Man with Two Watches Is Never Quite Sure"

SimS