Head failure; cause?

integrale Evo · Post by **integrale Evo** » Tue Jul 25, 2017 9:36 am

First off this isn't one of mine, It had previously suffered from severe det', so bad it had melted the head and the pistons, this was as always on the inlet side and you can see where it has been welded up. It does not look like there was any welding done on the ex side which is where this occurred.
Head was held down by APR M12 studs and sandwiched in there was an MLS gasket. No damage on the gasket and I haven't seen the rest of the engine yet as it's still in the turbocharged rally car.
As you can see there is some not-very-secret passageway from combustion chamber to waterway.
What do you think about a cause? My first thought is that the RA finish is too rough for the MLS gasket and it went from there, but not sure. It did that in a few hundred miles (I think about 600) after a rebuild.

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mag2555 · Post by **mag2555** » Tue Jul 25, 2017 9:47 am

Agreed on the finish and the issue made by it!
I only use a wet milling machine and produce a darn near mirror finish on the decks of any Aluminum head or block!

Frankshaft · Post by **Frankshaft** » Tue Jul 25, 2017 9:58 am

That finish isn't that bad. It couldn't possibly be that the tune up is jacked, and its a turbo rally car. Its possible the heads have been compromised, considering they have been welded on and fixed once before. Even a "mirror" finish isn't a mirror finish. Rub it with some Green scotchbrite or 220 grit sand paper on a flat board, once the "shine"is gone, you will see machining lines like that.

Kenny M · Post by **Kenny M** » Tue Jul 25, 2017 11:22 am

Looks like a 100 RA on that surface. Should be under 50 and more like 20.

Carnut1 · Post by **Carnut1** » Tue Jul 25, 2017 11:55 am

Looks like it was hit with a cutting torch. Tune caused detonation.

integrale Evo · Post by **integrale Evo** » Tue Jul 25, 2017 12:29 pm

Carnut1 wrote:Looks like it was hit with a cutting torch. Tune caused detonation.

As per my original post, det damage on this engine occurs around the inlet side and also the gasket was untouched. If it was det' you would have thought there would be damage to the gasket and pistons.

mag2555 · Post by **mag2555** » Tue Jul 25, 2017 12:46 pm

I am questioning why the outside and the inside imprint of the fire ring is so wide in that failed area as compared to the rest and right now as it looks like the fire ring was moving hence the wider marks.

Racing68 · Post by **Racing68** » Tue Jul 25, 2017 12:46 pm

Last one i saw that looked like that was a mechanical injected 8-71 SBC. The problem after doing the same thing twice was traced back to a fuel inlet restriction. Gasket looked perfect but melted underneath it, no piston or valve damage either.

Carnut1 · Post by **Carnut1** » Tue Jul 25, 2017 1:38 pm

integrale Evo wrote:
Carnut1 wrote:Looks like it was hit with a cutting torch. Tune caused detonation.
As per my original post, det damage on this engine occurs around the inlet side and also the gasket was untouched. If it was det' you would have thought there would be damage to the gasket and pistons.

After it was welded it now has different heat dissipation characteristics. Looks like detonation lifted the head enough that hot gasses burned a path.

mag2555 · Post by **mag2555** » Tue Jul 25, 2017 2:02 pm

Yeah ,that could be it Charlie as the expansion and contraction rate of the weld would be different enough from the base Aluminum to make for that!

Newold1 · Post by **Newold1** » Tue Jul 25, 2017 5:31 pm

I am assuming from the moderate rust stains in the head that the block for this engine is iron? In any event inspite of the block material the sealing of most MLS gaskets call for as others said here an RA finish between 20-50RA and it looks as though those specs. were not held on the resurface of the head.

Also the straightness and evenness of the surface finishes on both the head and block are critical. A straight and even head surface must be accompanied by the same refinish of the top of the block or the best installation of a good MLS head gasket will be destroyed. It takes two good surfaces to achieve a good seal! The former serious failures most likely compromised the block deck surface and it would have to be restored.

Cubic_Cleveland · Post by **Cubic_Cleveland** » Tue Jul 25, 2017 9:40 pm

Frankshaft wrote:That finish isn't that bad. It couldn't possibly be that the tune up is jacked, and its a turbo rally car. Its possible the heads have been compromised, considering they have been welded on and fixed once before. Even a "mirror" finish isn't a mirror finish. Rub it with some Green scotchbrite or 220 grit sand paper on a flat board, once the "shine"is gone, you will see machining lines like that.

X2. Finish isn't that bad, and if it's had bad detonation previously I'd be looking at the tune up first!

Cubic_Cleveland · Post by **Cubic_Cleveland** » Tue Jul 25, 2017 9:45 pm

Racing68 wrote:Last one i saw that looked like that was a mechanical injected 8-71 SBC. The problem after doing the same thing twice was traced back to a fuel inlet restriction. Gasket looked perfect but melted underneath it, no piston or valve damage either.

The last one I saw was similar. Turbo efi SB2.2, some smart cookie decided to make some new braided fuel lines. Didn't realise you had to clean the line out after you cut it, and a piece of swarf blocked the number 1 injector. It leaned out and gas axed the chamber towards the front of the engine. Dug a .060" deep trench in the head, block and gasket was fine...

Cubic_Cleveland · Post by **Cubic_Cleveland** » Tue Jul 25, 2017 9:47 pm

mag2555 wrote:I am questioning why the outside and the inside imprint of the fire ring is so wide in that failed area as compared to the rest and right now as it looks like the fire ring was moving hence the wider marks.

I agree, gasket looks like it was fretting on that chamber.

Frankshaft · Post by **Frankshaft** » Tue Jul 25, 2017 9:50 pm

Sometimes, using a regular composition gasket, is smart. Use the gasket as a fusable link so to speak. Instead of melting a hole in a piston, or doing what this did, the gasket will pop first. If your tune up is good, you usually are ok, and the gasket gives, and the piston/block/head lives if you have an issue. Think about it. By the way, that's better than 50ra.

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