Over flowing radiator

[Shug4]

When I first start up my m38a1 antifreeze pours out the overflow tube it also flows out the top of my radiator when I stop engine and undo the cap. This happens when antifreeze is cold. What causes this?

[Ben Dover]

Is it a 4# CAP Does it have a rubber seal on the valve or is it the early type that is used with the fiber seal in the filler neck?

[artificer]

First the coolant should not be filled right to the top but about 2 knuckles under the bottom of the filler neck.
Leave the cap off & startup if there are bubbles it is indicative of a blown head gasket.
If so, check by doing a compression test 'properly'.

[Ben Dover]

M-38A1 is usually not that touchy with correct cap.
Does your engine still have the bypass elbow between the head and the waterpump?
Again which cap do you have?

[Shug4]

Will check on the bypass elbow.

[Shug4]

It looks like I might have a head gasket blown because when I start it up it blows grey smoke out the exhaust. Also I have the cap with the rubber seat in it. I think it is smaller than the neck.

[artificer]

To work as intended radiator caps must be the correct length for the rubber or composite material to seat in the radiator neck.
Blown head gasket will show on a properly conducted compression test or bubbles will show up in the radiator on a leak down test isolating 1 or more cylinders.
Smoke can be indicative of many things & grey is not usually associated with head gasket, white is.

[Ben Dover]

Can you post a pic of the type radiator cap you are using? If the valve on the cap has the rubber seal, there is no ned for the fibre seal in the neck. Steam upon start up is not a good sign. Does it stop after a few seconds?
Evidence of coolant in a cylinder is indicated by a spark plug with little or no deposits when compared to the others, also, rust may be on the tip of the spark plug in the affected cylinder.
Removing the spark plugs will give you a quick answer.