Cracked Piston - Smoking Gun.

[McRat]

Dave at BMS Machine showed me a piston from an engine (LLY core engine) that burned a hole through another piston that was severely cracked.

The piston he showed me had not failed outright yet, it had just started to crack. The crack started at the lip between the squish band and bowl. No doubt.

This is the most damning evidence so far that EGT's are to blame. This lip has the most exposure to the heat of combustion. If it were purely cylinder pressure doing it, it should have cracked down in the bowl where they burn through into the oiling racetrack, and the material is the thinnest.

 

[mick]

Welcome back Pat. hope things going better for you. Good to hear your voice again. So whats the answer - besides Supercharging?

 

[McRat]

I am building a backup engine with the "lip" machined off to retard crack formation, as well as Ceramic coating the crown. Guy at SoCalDiesel did the pistons.

We are starting to play with water injection to hold down EGT's as well.

 

[mick]

How about large water/air core sitting between turbo and inlet?

 

[McRat]

Pics:

 

[McRat]

How about large water/air core sitting between turbo and inlet?

Cooling the inlet air and reducing exhaust backpressure should help, but not sure how much. Certainly can't hurt!

 

[Mike L.]

Cooling the inlet air and reducing exhaust backpressure should help, but not sure how much. Certainly can't hurt!

Sorry for off topic but change one of the digits in your avatar. :cool2:

 

[sweetdiesel]

thanks for the info Pat

 

[super diesel]

Solid info Pat. Thanks. I too play with the water injection but for different reasons over the years (as well as lower EGTs I use it for cleaning coked injector tips after running heavy concentrations of oil in the fuel). Good to know it will be good for preventative maintenance too.

 

[dynarex]

wondering if all the cracks are in line with the wrist pin?

 

[JOHNBOY]

Wow

Awesome pics!


That pretty much proves it IMHO! The Lip is the root of evil! I am glad I choose to remove the lip in my engine!

 

[ecc_33]

thanks for the info pat

 

[turbo lcc]

Do the aftermarket turbos help keep EGTs down over the stock VVT at the same powerlevels?

 

[McRat]

wondering if all the cracks are in line with the wrist pin?

Yes, all cracking is parallel to wrist pin.

Aftermarket chargers will normally reduce EGT's at the same HP levels.

 

[dynarex]

wouldnt cylinder pressure be the cause of the cracks?
the wrist pin is obviously the fulcrum so maybe more material in the wrist pin area would stop it?

 

[turbo-max]

i see those (pistons) also have the "star" pattern on them aswell.

 

[super diesel]

I had some 02 pistons that were run stock for 105K before removal. Thay had a way big star pattern on them as well (larger than the pistons that had run a big tune for 47K). As every one knows, the aluminum does get much weaker as EGTs goes up. This is the main reason for the oil gally in the piston (wick the heat away). Now, how do we attract more heat wicking oil? Maybe run slightly larger oil tubes on the oil sprayers. I'm sure this isn't the cure all though (may help however). No matter what you do, cylinder presures is what makes power. How do we know if the rod, piston isn't compressing a explosion before top dead center (to much advance)?

 

[Trippin]

IMHO, we have a few things going on here:

1) The lip has been shown in many SAE papers to be a stress concentration. It is an emission crutch and has also been shown to not be necessary with the higher injection pressures we are now using.

2) The enitire system, bowl diameter, nozzle hole angle, timing etc is designed to spray fuel just below the lip so that it is directed along the wall down to the floor and back up the center of the piston where the hump causes it to tumble out on to the crown.

3) With increased pulse width and injection timing, we start the spray above the lip. Unfortunately the spray is actually ignited fuel, i.e. a cutting torch, and the lip travels through it once on the way up and again on the way down, super heating it to unknown temps. Add a some cylinder pressure to a super heated area that is also a stress concentration and you have a recipe for disaster.

 

[bajatoy]

Just on a side note, there is a new Bulletin from GM not to warranty these holes and cracks. GM is placing all the blame for piston failures on programers. From what I am hearing, they are not even looking for programs on the tech 2, the are just flat out denying any claim, and they are sticking to their guns and not covering them at all.

 

[sweetdiesel]

Just on a side note, there is a new Bulletin from GM not to warranty these holes and cracks. GM is placing all the blame for piston failures on programers. From what I am hearing, they are not even looking for programs on the tech 2, the are just flat out denying any claim, and they are sticking to their guns and not covering them at all.

interesting but there was a court case involving a member on the diesel place he sued them and won
i think the saying is "innocent untill proven guilty"
i think GM is useing this as a scare tactic but i might be wrong