Cracked Piston

[Fingers]

Well, it finally happened. Although it was preventable. Two Cast Oval Bowls cracked in a full on race build running well over 1000 HP. The cracks are on the outside of the piston and run from the pin boss up through the steel ring land. Most likely caused by ring stress from loss or reduction of cooling oil on these two pistons. Owner was totally unaware they were cracked while the engine was in service.

Heating just the center of the piston visibly opens up the cracks. The oil gallery in the piston acts as a thermal break if it is not kept full of oil. So when empty, it allows the center and outside of the piston to be very different temperatures. Something like double pane glass windows.

 

[IOWA LLY]

So I'm not quite following you on the cause/possible prevention.....

Are you saying that you believe oil pressure may have dropped from heat or something like that? Or that piston squirters weren't enlarged when they should have been or???

Very curious to hear more of your thoughts. Obviously this piston design has more then proved itself. Everything has its limits and this is nothing to be ashamed of or even upset about in my opinion.


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[Fingers]

There could be several causes of the oil not cooling the pistons. Considering it was a drag truck, it could have starved the pickup at launch or even overloaded the top end with oil. Internal wind in the crank case could have blown the squirters off their mark. (not likely)

All of the pistons show about the same wear marks, but the two cracked ones have metal transfer from the rod to the pin. Indicating an oil issue. I've heard that the marine applications have a running history with inadequate oil supply to the pistons and the pin journal.

 

[IOWA LLY]

Funny you mention the material transfer from the rod bushing to the wrist pin. I've seen that a couple times, always in engines that had bearing issues..... I always thought the bearing issues caused the oil pressure to drop which caused the issues with the wrist pin. But maybe oil pressure from temperature or something else caused those as well.


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[TheBac]

Could the clearances between rod and pin have been just-a-bit too tight on those two cylinders, Jon?

What did the bowl areas look like on them? No indications of cracks starting?

Might be you've just found the next area that needs to be addressed is all. Could've been overlooked on this build. I doubt it was in the piston design or material.

 

[95 z71]

Thanks for sharing your findings Jon.

 

[Harbin_22]

FWIW Jon, the current engine in my truck has around 13-14k miles, been around 1,200hp for quite a while and beat on this thing constantly. Still going strong with your pistons!

 

[rcr1978]

FWIW, this engine has around 13-14k miles, been around 1,200hp for quite a while and beat on this thing constantly. Still going strong with your pistons!

This one yours then? I would like to see the tops, kinda weird only the sides split and they didn't blow through.

 

[Harbin_22]

This one yours then? I would like to see the tops, kinda weird only the sides split and they didn't blow through.

No it's not mine, sorry I worded that a little wrong lol.

 

[Fingers]

Pin clearances are good. Hard pack indications are only on the top of the pin bore from the obviously significant chamber pressures. Pattern is even. Bottom half of pin bore shows normal/minor wear. No indication of binding. As you can see in the picture, the crack is only half way across the top of the pin hole. Aligns with the oil gallery location as does the crack going up the side.

Top side of the piston is almost perfect.

 

[Fingers]

Video

https://youtu.be/o1lloyhPJFI

https://youtu.be/tq5Le6laeyQ

 

[bcdeutsch731]

Thanks Jon for going into detail on the cracked pistons! No reason to hide anything with these pistons. They have held up to everything that has been thrown at them.

 

[malibu795]

Impressive on all accounts.. really!