I'm looking at a 40 Olds with a 6 cylinder and needing some advice. The owner says the car runs great up to 50mph but at 55 to 60 begins to overheat. The motor has been gone through and a new radiator was installed. It also is stick instead of auto. Did these cars have these problems normally and can it be corrected with different running gear? Is there an overdrive unit that can be fit? The trans and rearend are supposedly original and shift fine and make no noise. I was thinking check radiator cap and thermostat assuming they had those then. If they work, is this what to expect from a stock 40? Thanks Pat

OK, anyone with a similar era car that this sounds normal? I figure most GM of the time would act the same.

Pat:
There are a number of things that can affect the temperature of the engine in old cars. At 50-55 MPH you are getting good air flow into the radiator. The radiator is filled with coolant and gives up this heat to the inrushing air. In this situation, a few things come to mind. The lower radiator hose that goes from the lower (cooler) part of the radiator to the pump input is under significant suction at high engine speeds. To prevent the collapse of this hose inward on itself, thereby blocking the coolant flow into the pump, there should be a spring inside this hose that you can feel when the engine is cool. If the lower hose feels mushy or soft then there is likely no internal spring.
Next, you need to be sure the radiator is not filled with crud on the inside limiting coolant flow. These two things seem to be the most important at this point.
Next, I would be sure there is a good radiator cap that is sealing the system effectively. The cap keeps the system under pressure to prevent boiling when really hot and when not that hot will suppress local hot spots in the engine from turning the liquid coolant to a vapor, thereby reducing cooling capablity.
Next, check the ignition timing. This should be done with the vacuum advance disconnected and the line to the carburetor plugged. The engine idle needs to be reduced so that the centrifugal advance is also not operative. Set this to the recommended setting by Oldsmobile shop manual. Late timing can cause an engine to run hot.
There are thermostats and there are thermostats. Generally for this vintage car I would reccomend a 160 degree F unit. The next consideration is the size of the opening when the thermostat is fully open. I have seen some that have an opening diameter of only 1 inch while other more heavy duty types have openings of 1 and 1/2 inches. Select one with the larger opening if possible. Consider a Prestone Power Performance type.
Finally, remember that when this car was new it did not overheat at 50 MPH, so unless the engine block has a fair amount of crud in the water passages, the considerations discussed above should resolve your problem.
Good luck.
Joe, BCA 33493

Thanks for the info. I figured those to be the places to start if the deal goes through. The car supposedly starts fine and will run at 50 all day so I thought check the small stuff first and if all looks good, determine the rear gears. Wouldn't a 4. and up cause the motor to over work at higher speeds? Thanks Pat

You should be able to determine the exact rear end ratio for this car. I suspect that the ratio is around 4.1 to 1. Buick Specials with the "small" straight eight commonly had a ratio of 4.45 to 1. And at 55 MPH the engine was really working, but not necessarily overheating. My guess is that if your ratio is 4.1 or 4.45 you could get a pumpkin from another maodel, with say the straight eight or the Hydramatic, that might be a little taller to give this car some legs.
Joe, BCA 33493