Adding Coolant Overflow Reservoir to 1967 305E

[jgpnashville]

My 67 K15 with 305E is stock as it came from the factory, without an overflow or expansion tank for the radiator. Almost every time I drive it, I check the coolant and notice it is about 3 inches down in the radiator from the top. I top it off with coolant/water and drive. Next time, it's down about the same amount. No head gasket issues, so I can only presume that it's expanding and going out the expansion tube onto the pavement when the engine warms up.
Is this cause for concern? Should I fashion some sort of coolant overflow tank and drop the rubber overflow hose from the radiator into the tank or leave well enough alone?
Is it normal to have the coolant run about 3 inches down from the top? My truck is missing the owner's manual so I don't know what GMC had to say about the coolant, but to my modern mind, coolant 3 inches down from the filler seems a bit low. I know the water pumps move a lot of coolant through the engine and even though my truck only has the idiot TEMP light and no temp gauge, it never seems too hot and the TEMP light has never come on while driving.
Thoughts?

[LordNatedawg]

I haven't seen any kind of agreement on whether these trucks came with an overflow or not. My 65-66 service manual mentions that only "L" models have an overflow tank. Not sure about the 67s. However, it seems to be agreed that one is not needed. Running the coolant a little low in the radiator is fine if it isn't overheating and you don't mind the occasional drop of coolant on the driveway. As a general rule of thumb, you would need to be missing about 12% of your coolant before the water pump starts sucking in air. For the 305E with a 32 quart capacity, that means you'd have to lose almost a gallon of coolant before problems arise. That rule is a "one size fits all", so take it with a grain of salt.

Based on my 65-66 manual, the overtemp light should come on at 225 and the factory thermostats should be 180. I wouldn't expect those values to change much in a new model year, but who knows. If you want to make sure your overtemp light works, all you have to do is ground the wire to the temp sensor. If the light comes on, the wiring is good.

I personally like using overflows to ensure my radiator stays completely full and it keeps coolant off the driveway. The reservoir is also nice to have because I don't have to pop my radiator cap to check and top off coolant. I can just top off the reservoir and I'm good to go.

Tldr: It's up to you whether you want a reservoir. Filling the radiator a few inches below the top is not an uncommon practice when you don't have a reservoir. I do the same on my 1938 Buick.

If you want a reservoir, I've attached a picture of my 1 gallon Canton reservoir for your reference. I fashioned my own brackets for it by bending 1/8" flat steel in a bench vice. Also note that I have a 350 V8. Operating temps, coolant capacities, and expansion rates vary.

[Prowbar]

Coolant being low is normal for these engines in my opinion. With 32 quarts (8 gallons) of coolant there's going to be a lot of contraction/expansion in the liquid. When the 478 is cold I cannot see any coolant in the radiator, but when the engine is up to temp it is nicely above the coolant passages in the radiator.
Never have any problems with overheating, period. If I add more it is blown out through the overflow hose.

One could add an overflow tank, but it one knows the proper level of the coolant I see no need to.