Cracked MHG. Cause?

[grimreaper]

Motor failed a block test (finally!) after quite some time of in-consistent bubbles in the reservoir. There are 1" long cracks following the arch of the combustion ring. The cracks are right in the middle of the combustion ring. I'll get pics up tomorrow. All six cylinders have this. I didn't find to much with a search. Any ideas as to the cause?

piston tops look good but need to clean them up first.. cant see any damage on them at this point
piston bores have some light scoring on all 6 on the intake and exhaust side. Compression is still 150-160 across the board.

 

[Pokey2301]

sounds like it could be a defect in the metal

 

[grimreaper]

machine shop said it looks like fatigue failure. I wont have the pics until i get the head back.

 

[Pokey2301]

machine shop said it looks like fatigue failure. I wont have the pics until i get the head back.

isnt that the same thing as a defect?

 

[IJ.]

isnt that the same thing as a defect?

Only if you're saying it's NOW a defect and wasn't a preexisting defect in the MHG all along

 

[grimreaper]

pics, combustion ring is still intact, just separating like it was cut with a cookie cutter (or 7m head )

Consensus locally is that it was over tq'ed. previous owner may have suspected a small leak and cranked the studs down or simply over tq'ed from the get-go.

Head was very slightly warped. Block still has a mirror finish but with out being able to pull it right now I'm using a new toyo HG.

Side note OEM gaskets have a metal insert in them!?! thick layer of composite with a metal "shim" with a thin layer of composite. Looks promising. We'll see how long 350-400whp holds up.

 

[CyFi6]

Looks interesting. If the block still has a mirror finish why go with a stock head gasket though? I would be concerned about raising the compression with a stock gasket, seeing as how the block and head have both been machined. Ive seen the "steel layer" on the stock gasket that you're talking about but its not very strong compares to a MHG steel shim lol. Im sure the general consensus is you could make around that hp with a good tune, but everytime you step on the throttle you are going to be puckering up and hopeing for the best, which isnt how i would ever want to drive my supra!

 

[IwantMKIII]

Very odd that happened on all six! What type of MHG is that?

I have a used HKS stopper FS used only once for maybe 15 no more than 20 k-ish miles. perfect condition minus the viton of course!

 

[grimreaper]

Thanks for the offer but I'd just pull the block and start from scratch if i was going with any sort of MLS^ HKS hg according to my builder. Has the notorious rivets in the same spot compared to the stopper he had on hand..

A MLS probably would seal, but I'm only hoping for 6 months out of it (although I expect more) and just want to drive it. If it last longer, nice. If not I'll do a full build and get to the suspension after wards. I dont see any signs of detonation at this point and the pistons look perfect. It appears my tune is where i want it, safe. I cant see an OEM hg letting go just because!