1966 WM-300 radiator blowing water out of cap.

[Bob2]

I need a little help here on my 1966 WM-300 251 engine, the radiator has started expelling water / antifreeze from under the radiator cap and over flow tube. This seems to only happen at 20 MPH and above. Ideling is fine, water temp 170-180 degrees. Any help would be appreciated.

[Charles Talbert]

I need a little help here on my 1966 WM-300 251 engine, the radiator has started expelling water / antifreeze from under the radiator cap and over flow tube. This seems to only happen at 20 MPH and above. Ideling is fine, water temp 170-180 degrees. Any help would be appreciated.

A little more investigation is in order; but it sure sounds like the head gasket may have blown into a water port. This pressurizes the cooling system via engine compression and forces water out the over flow tube like turning on a faucet.

[Gordon Maney]

Is it doing this consistently? I just want to make sure it was not a case where it did it once as a result of overfilling.

[Bob2]

Water blowing out of radiator

Charles / Gordon the water is consistently blowing out.

[Gordon Maney]

Charles / Gordon the water is consistently blowing out.

That would indicate it is not simply a case of overfilling. Charles is steering you in the right direction. The upside of all this is that it is not a difficult head to remove.

[gmharris]

Yep, as usual, they are telling you correctly.

You can have the coolant checked to see if there are exhaust gasses present. Most professional mechanics have some sort of a kit to do the test.

But, my money would be on the head gasket.

[MoparFreak69]

A little more investigation is in order; but it sure sounds like the head gasket may have blown into a water port. This pressurizes the cooling system via engine compression and forces water out the over flow tube like turning on a faucet.

My vote is on this ^ as well.
An easy test that may show you is to run the engine with the radiator cap off. Get it warmed up so that the thermostat is open (if you have one that is). Watch the water flowing through the upper tank as the engine is run at varying speeds. If the head gasket is gone you will probably see little bubbles popping up which is really compression lost into the water jacket. You can try to do a cooling system pressure test, but sometimes it is very hard to tell where the coolant goes, especially if it is a small leak.
The parts are cheap enough, and heads are easy enough to pull you may just yank that sucker and give it the good ol visual inspection and you will probably discover a trail from a water passage to a cylinder.

[Bob2]

Thanks for all the info. I guess my next step is to pull the head and replace the gasket. Would it be advisable to have the head magnafluxed and planed while it is off?

[Gordon Maney]

Thanks for all the info. I guess my next step is to pull the head and replace the gasket. Would it be advisable to have the head magnafluxed and planed while it is off?

I am conservative with castings. Having it checked for cracks is a good idea. As for surfacing, I would only do it if necessary. No need to grind material away if the head is flat and not abraded. Also look at the block deck and check it for flatness.

[Charles Talbert]

Definitely have it magnafluxed; I always surface the head in cases such as I expect you will find. There is usually a reason why the gasket failed, and that is usually a flaw in the head surface. I wouldn't think of running the risk of putting it back on and blowing another one. A good machinist will only take what material is necessary to get flat again, seldom when you check one closely do you find it perfectly flat. The deck is generally not an issue, but by all means check it to be sure. I wouldn't be surprised if you can see a flaw in yours without looking too hard.

[Gordon Maney]

Definitely have it magnafluxed; I always surface the head in cases such as I expect you will find. There is usually a reason why the gasket failed, and that is usually a flaw in the head surface. I wouldn't think of running the risk of putting it back on and blowing another one. A good machinist will only take what material is necessary to get flat again, seldom when you check one closely do you find it perfectly flat. The deck is generally not an issue, but by all means check it to be sure. I wouldn't be surprised if you can see a flaw in yours without looking too hard.

There is a key word in your language, good machinist.

[Bob2]

Thanks a lot. Well, now to roll up the sleeves and get to work.

The head gasket in question would I be able to get it from NAPA, or from Vintage Power Wagons.

If I get it from NAPA would any one know the part number?

[Joe Flo]

Bob,

Napa should have the head gasket. I don't know the part # but if you give them the specs they can look it up. Bring the head gasket with you.

I would drain down the system as soon as possible because a blown head gasket will let water drip into the cylinder which can cause corrosion or worse yet, a hydro lock, if enough water gets into the cylinder. This could lead to a bent connecting rod if you try to start the engine. Good luck and let us know what you find.

Joe

[QuantumJo]

Avoid using any type of abrasive wheel / disk on the block or head. I use an old school razor blade to scrape all mating surfaces.

[Gordon Maney]

Avoid using any type of abrasive wheel / disk on the block or head. I use an old school razor blade to scrape all mating surfaces.

Single edged razor blades are always in my toolbox for final checks of gasket surfaces. They are a great, yet simple, tool.