Piston failure

[Walt Fricke]

A friend has a 3.4 in his SCCA GT2/PCA GT3 911 race car. He's using 98mm J&E pistons. Last weekend at a race he had a piston break at the wrist pin (as best one can tell by looking up through the missing underside base of the cylinder, but it wasn't rod failure). This is the second time this year this has happened to him. He has noted that there is not a whole lot of material under the wrist pin in this area of this piston.

Anyone else had this misfortune? Theories?

I once had a 90mm Mahle racing piston break through the wrist pin area, but that was preceeded by the driver missing the 4th to 5th shift and getting it into 3d. I could imagine that increasing the yanking forces considerably. But there is no such incident in the history of these 98s, at least that he owns up to - and I believe him.

Walt Fricke (whose interest in this includes the 95mm J&Es he has for his short stroke 2.8 project)

[jluetjen]

Have you shown the results to J&E and asked them???

[cnavarro]

I would most certainly call JE and tell them.

Were the pistons cut with forced pin oilers or run with lightweight pins coupled with the lighten combo option? I'm assuming he was running them in Mahle cylinders, but with what piston to cylinder clearance?

[MBruns]

Walt, we have seen some of the problems you describe, some have alot of hours on them and have some cracks developing in them at the skirt/pin boss area, depending on what type of wrist pin they have some of the lighter pins ie.. tapered wall design or thin wall JE has some thoughts of pin flex having an effect on the piston, but we also have seen cracks coming out of the pin oiling holes in the pin bore, and have changed ours to broached style oiling, also the type of forging, the M has a better strength at the pin boss than the P forging and if it has an offset pin it can get thin on one side as well. the type of forging can be seen under the dome unless its had under crown milling. The forging is seen as example on a 95mm bore with a P forging "96P" or "96M"
under the dome.
Hope this helps, Mike Bruns JBRacing.com

[Steve@Rennsport]

Hi Walt:

We've experienced several JE 98mm wrist pin boss failures and we'll not use those anymore. Some had the standard weight pin and some had their light ones. Some engines had less than 10 hours on them.

IMHO, this particular design is not their best work and that why we went to another piston manufacturer and had them make a 98mm one to our own design. So far, so good.

[TimT]

I'm guessing the end results looked something like this? We have btdt

[Steve@Rennsport]

Looks very familair,.........

[JohnJL]

Yikes, I have 98MM JE pistons about to go into my new cylinders for my supercharging project...

[cnavarro]

You have to remember, this is not the norm. I've used hundreds and hundreds of sets over the years, and I have never had a single customer to date have a failure, and many are running very heavy pistons with huge bores in type 4's, doing the same rpms with roughly the same hp/cyl. Some even have 100,000 mi. These failures have occured over the years with many brands of pistons too, not just limited to JEs. Keep that in mind too.

[sww914]

I've seen 2 of those, 1 JE, 1 Mahle. Each had quite a few miles on it and was converted to a street/track day/time trial car.

[Mysterytrain]

Makes me glad that I listened to Steve and installed a set of Mahle's for my 3.2 ss.

[Walko]

I had JE 98mm in my 3.2SS with MFI and it has logged some serious hours with no failure.

After seeing that glad I sold the motor.

Michael

[MBruns]

This is the problem with damage pics that start a panic regarding a component failure without all the information. I build more than 25 911 engines per year give or take a few, less that 5 are stock street type and most use JE pistons, among others including Mahle, I run a 3.8 in HSR and we run it hard and have a look at it around 50 hrs. if we dont make any mistakes. the pistons and valves etc.. are a timed out parts that are the cost of having fun, but can not be ignored or improperly setup to begin with. Of all the engines we see and build that type of failure is rare. Most 911 engine failures start with a missed shift that if it doesn't do imediate valve trane damage it hurts the rod brgs. and then a short time later spins a bearing then a rod comes out the side, usually they blame the rod bolts or what ever "because I was just going down the back straight and boom" Most of the pistons will go 2 build cycles and then get replaced, but get looked at closely. We still use JE and if we see an issue they are the first to correct the problem, Jerry Roche is the Porsche piston contact at JE and takes it seriously, he races a 911 himself on the West Coast that we have built several engines for.

Thanks, Mike Bruns JBRacing.com

[dbanazek]

I have actually had 4 sets of pistons with failures. The first was a set I ordered for a friend and Al Johnson in Grand Junction, Colorado built the engine. They failed after a couple of hours. 98mm The next set was not a catatrophy. I took the motor apart after 2 years. These were 100mm with the pin oiler holes. Every piston had cracks in the oiler holes. Jerry offered a reduced price on replacements for those. They came without oiler holes, just a broach. They ripped apart after one PCA weekend at Topeka. Rod & bolts were intact. The last set was a 98mm set that failed after one weekend in Sebring and one in Pueblo Colorado. That motor is not appart but seems to be the same failure looking through the hole in the cylinder. Jerry did not seem to have any good answers or remedies so I have sought pistons elsewhere. I will give him another call and send him the debris when he gets back from vacation in October.

My thought is that all of these failures were in cars with 12 or 13" rear tires and tilton double disc no slip clutches. I think this puts stress on the pistons beyond what they are capable of, especially when the revs are not matched perfectly on a downshift.

[cnavarro]

Dave, did the pistons have the light pins (.120" wall) or the standard, thicker ones? Did the pistons have the lighten combo done to them as well? What kind of piston to cylinder clearances were being used - I'm assuming on mahle cylinders, right? Just trying to get the complete picture here. I plan on calling Jerry when he gets back from vacation.

[dbanazek]

The 100mm 101 P forging did have taper wall pins. Did not have lightening. Using the 3.0 type wrist pin. 1.5 mm piston to wall, using Perfect bore cylinders supplied by J&E, no oiling holes

The 98 mm was a straight wall pin 3.2l type, piston was not lightened ( I don't see it on the invoice I will check when I get it apart). 1.5mm clearance, bored and plated Mahle cylinders by US chrome. with oiling holes for pin

Pretty much the same as I ordered for my friend who lost one about 1.5 years ago. with oiling holes

[cnavarro]

Thanks Dave for sharing that info.

It seems as if most every failure is with an unlightened piston, either with regular pins and pin oilers (cross-sectional weakness of wrist pin boss) or no oilers and taper pins (flex?).

Anyone else want to share their combos that failed?

I typically don't use pin oilers at all and use the lightweight .120" straight wall pins coupled with the lighten combo AND thermal barrier coating on the crown. Maybe when the piston is not lightened, there is too much extra mass under the crown. Correct me if I'm wrong, but typically a Mahle piston is lighter than a normal JE?

[TimT]

The picture of the failed piston I posted wasn't meant to scare people from using JE pistons.. Its just a scary pic of a failure.

Its also a picture of the only failed piston we have experienced, and we have used JE pistons in many many engines.

Also the driver of the car that piston came from was notoriously hard on equipment, regularly over-revved the engine, shifted almost violently. Broke transmisssions regularly as well

We couldn't with any certainty say why the piston failed, only that it did. Was it a result of the abuse the driver imposed on it? did the valves smack the top of the piston a few times before the boss failed and the rod started sawing through the case? Or did the rod just pull out? We dont know.

[MBruns]

One other item to ponder , are the majority of the failures here with stock length rods ? All of the pistons that we design and use are of the long rod and moved pin location which may have an effect on skirt/pin area loads. I have seen some of the high time parts crack in the oiling holes in the pin boss that have been suspected pin flex wih the taper wall pins (hard to believe but...) Just a thought that has not been tossed around.

Mike Bruns

[dbanazek]

I would like to say that I think the J&E pistons used in most street/track applications are probably fine. These pistons have been used in very high performance race engines with big tires and no slip clutches. I also think J&E could build a piston strong enough to handle these loads. I have always asked that they supply the best all out race piston they can, no expense spared. Lightening of course would help, but there does not seem to be too much material to be removed except under the dome. I certainly would not want any removed from the pin column.

There must be a solution to this problem. The long rod might help, pushing the pin up into an area with more material and making a lighter piston. We did build one motor like that with the 2" diameter rod bearing and 1/4" longer rods. I think that engine did get the lightening done to the piston. That engine is still together, although it has not seen much use.

I have talked to a few people who have said they still use J&E enthusiastically or reluctantly and say they have come up with their own designs. I can maybe understand J&E keeping a combustion chamber design secret (although I don't think you have a lot of choices for a high compession engine). But, I do not understand why the consumer has to do the structural engineering work to design a piston that will stay together though.