Thoughts on what would cause a headgasket to fail like this?

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BrounHownd
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Thoughts on what would cause a headgasket to fail like this?

Post by BrounHownd »

Motor died on a recent trip--lost power quickly from cruise speed then stopped and wouldn't restart. Got it to run for a bit with some with throttle on muffs in the driveway, and noticed exhaust out of the tell tale. Did a compression test and top hole (2 cylinder 2 stroke) was down to 30psi. Pulled the head and found this. Compression was lower but reasonable for a 40 year old motor (in the low 100s on both holes) when I bought it earlier this year.

No real signs the motor has been overheated, but thinking I should have the head machined anyway? What would have lead to burn-through in that one spot? It's the bottom of the top cylinder. Have new gaskets on the way for the head and the coolant cover. Want to fix it completely while I'm in here.

Thanks for any thoughts in advance.

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MrGiggles
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Thoughts on what would cause a headgasket to fail like this?

Post by MrGiggles »

BrounHownd wrote: 18 Jul 2021, 17:56 Motor died on a recent trip--lost power quickly from cruise speed then stopped and wouldn't restart. Got it to run for a bit with some with throttle on muffs in the driveway, and noticed exhaust out of the tell tale. Did a compression test and top hole (2 cylinder 2 stroke) was down to 30psi. Pulled the head and found this. Compression was lower but reasonable for a 40 year old motor (in the low 100s on both holes) when I bought it earlier this year.

No real signs the motor has been overheated, but thinking I should have the head machined anyway? What would have lead to burn-through in that one spot? It's the bottom of the top cylinder. Have new gaskets on the way for the head and the coolant cover. Want to fix it completely while I'm in here.

Thanks for any thoughts in advance.

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I would definitely lay a straight edge over that area of the head and make sure there isn't a low spot.

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Pappy
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Thoughts on what would cause a headgasket to fail like this?

Post by Pappy »

Right off the bat do what Mr Giggles said.
The culprit is staring you in the face though. Think about what a head gasket has to do and take another look at the gasket. See if you see something That should not be showing.
If you find it we will go from there.

BrounHownd
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Thoughts on what would cause a headgasket to fail like this?

Post by BrounHownd »

Pappy wrote: 18 Jul 2021, 19:01 Right off the bat do what Mr Giggles said.
The culprit is staring you in the face though. Think about what a head gasket has to do and take another look at the gasket. See if you see something That should not be showing.
If you find it we will go from there.
Pappy, thanks for the reply.

I mean, the obvious answer is that the gasket material outside the fire ring is missing and eaten away by combustion gas (and the paper towel is showing, where it shouldn't be). I guess what I was asking was what was the order of events to this failure--gases eating away the gasket by blowing around the ring, then pressure breaking the fire ring, or the fire ring eroding a little bit, the gasket degrading, then the ring falling catastrophically?

The other obvious sign that you could be referring to is that there's oil on the gasket where the coolant galleries are, but that's explained by combustion gases (and unburnt oil) getting into the coolant (and out through the tell tale).

I guess I could have been more clear in my question--is there anything other than a warped head that could have caused this? Based on what I've found, the previous owner liked to half ass his repairs, and I don't think this is an OEM gasket, as everything came apart a bit too easily for a 40 year old motor, and the head bolts had paint chipped off them on the corners.

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Pappy
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Thoughts on what would cause a headgasket to fail like this?

Post by Pappy »

Close enough.
A head gasket is primarily made up of different materials to accomplish different jobs.
The fibrous material is there to seal the water jackets.
The copper fire ring is there to seal the cylinder sleeve to the cylinder head and contain the flame front and combustion pressures.

In looking at your gasket I see that there is carbon printing in areas that should look like crushed copper only. This means that the cylinders were leaking combustion pressure in several areas and in your case a flame front at the area of the failure. The flame front burned through the fire ring.
Normally there are two causes of failure in cases like these and it would be difficult to tell you which one it is/was.
A previous overheat will expand both the crankcase and the cylinder head. As these two expand it will crush the head gasket. Once cooled and back to normal dimensions the compression on the gasket may no longer be enough to maintain the seal. The head can warp as a result as well hence the request to be checked in an earlier post.
The second possibility is that the previous owner did not torque the head gasket properly and probably never re-torqued after a heat cycle or two.
Both can create the exact same failure. A warped cylinder head would be the only tell-tale of a previous overheat being the culprit.

BrounHownd
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Thoughts on what would cause a headgasket to fail like this?

Post by BrounHownd »

Pappy wrote: 18 Jul 2021, 21:36 Close enough.
A head gasket is primarily made up of different materials to accomplish different jobs.
The fibrous material is there to seal the water jackets.
The copper fire ring is there to seal the cylinder sleeve to the cylinder head and contain the flame front and combustion pressures.

In looking at your gasket I see that there is carbon printing in areas that should look like crushed copper only. This means that the cylinders were leaking combustion pressure in several areas and in your case a flame front at the area of the failure. The flame front burned through the fire ring.
Normally there are two causes of failure in cases like these and it would be difficult to tell you which one it is/was.
A previous overheat will expand both the crankcase and the cylinder head. As these two expand it will crush the head gasket. Once cooled and back to normal dimensions the compression on the gasket may no longer be enough to maintain the seal. The head can warp as a result as well hence the request to be checked in an earlier post.
The second possibility is that the previous owner did not torque the head gasket properly and probably never re-torqued after a heat cycle or two.
Both can create the exact same failure. A warped cylinder head would be the only tell-tale of a previous overheat being the culprit.
Makes sense. Thanks for that!

Had a few minutes to put a straight edge on it last night, but the lighting was crappy and I was in a hurry. It does look like there's some space between the edge and the cylinder mating surface on the head, especially in that area. It also looks like the cylinder mating surface on the head almost slopes away from the block ("up"? into the head?) on the inner edge. Said another way, the straight edge touches the far outer edges of the head, the outer edge of the ring on the head, but there's a gap on the inner edge of that ring.

Gonna take it to an engine machine shop and see if they can do anything about it.

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Pappy
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Thoughts on what would cause a headgasket to fail like this?

Post by Pappy »

By taking the water cover off the back side of the head a machinist can deck the head and bring it back to straight and true.