Strange, mysterious coolant leak

I have a newly restored 1955 Chevy BelAir that I had a new 330 HP 350 c.i. crate motor installed in. Everything is new, radiator, hoses, thermostat, etc. There is a small leak around the lower radiator hose at the water pump. I have tightened and even put a second worm gear clamp around the hose at the water pump and it still leaks. I have it as tight as my 220 lbs. of torque can do.

The leak itself is mysterious, since it never leaks when the car is running, or when it is hot. I have had it running for hours and the water temp around 200 degrees and no leaks. It doesn’t leak even 4 hours later when the water is back to 70 degrees. But before morning, maybe in the middle of the night (I have snuck up on it to see it leaking around 2 AM after shutting it down at 5PM the day before), it leaks. Why would it leak so long after it being hot? It has a 16 lb. pressure cap and coolant recovery system too, but still leaks even if I release the pressure after it cools down. Anyone have any ideas?

While it may not be the problem, I would suggest that most anything that gets tightened can be too loose, and it can be too tight. You may now have it too tight, you might even have damaged the fitting or hose.

As for why when I can only guess. Maybe a part is being sealed by the heat and or pressure and when the pressure has been off for a while it is deforming back to a shape that is allowing the leak. Soft materials like plastics (including “hard” plastics) and rubber can do this.

If you have a hose clamp that tight, you have probably destroyed the hose. I would replace the hose first, if that didn’t work I would remove and reinstall the water pump with a new gasket.

Well, the leak was worse before I tightened it up and added the second worm gear clamp. I tightened it up with a screwdriver, so I doubt it is too tight and I have checked the hose and replaced it too. But I could try backing off on it and see if that works. The water pump gasket is not leaking, it’s leaking out of the area between the hose and the pump spout that the clamp is supposed to seal.

Is it possible you have a manufacturing burr or something on the pump connection that is preventing a good seal? As a last resort you could use some gasket sealant around the hose connection (a less than elegant solution).

It’s not the hose that’s leaking. From what you’re saying it sounds more like there’s a leak where the pump mounts, then the coolant drips down to look like the hose is leaking.

Check that all water pump bolts are tight, and if they are, take it off and inspect/replace the gasket.

True, that could be it, and I guess I could put some Bars Leak or something like that (not at all elegant!)in the radiator too. The thing that bothers my logic is why it leaks with no pressure or temperature on it and is tightly sealed when the pressure is high and it is hot. Maybe a burr could explain that.

Personally, I wouldn’t put Bars Leak in anything. I would probably take off the hose and make sure the pump connection surface was smooth and look for any casting defects. I suppose it’s possible that there is a pinhole leak through the pump casing itself that only opens up when it’s cold (long shot).

Look from the underside of the water pump at the week hole just behind the pulley. My first guess is the pump.