Piston failure
 

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)

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

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?

Piston issue
 

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

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. :)

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

http://forums.pelicanparts.com/uploa...1190315853.jpg

Looks very familair,......... :(

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

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.

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.

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

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

JE pistons
 

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

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.

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.

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

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?

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.

Piston question
 

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

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.

One way to move the pin up, thus allowing the benefits of a longer rod, is to run only two rings. Porsche did this on some of its racing pistons. Should allow a bit more room for beefier pin bosses, if that has anything to do with this kind of failure.

1) Which ring gets deleted? 2d or oil?

2) By how much does this shorten component (ring, piston, cylinder) life? Shorter ring life might be OK for 40-80 hour amateur race motors. Bit costly if the more expensive components need more frequent replacement.

3) How does having two rings save weight (beyond the weight of the missing ring)? Add back in the weight of the missing land, plus the extra weight of the longer rod.

You don't hear much about 2 vs 3 rings - what's the drawback for racing?

I don't know about 98s, but I weighed a 95mm J&E bare at 470g. While perhaps not a good comparison, a used Alusil Kolbenschmidt stock 9.3/1 95 weighed 530g. (knew there was a reason to keep those pistons).

Walt Fricke

Usually you delete the second ring, and the oil control is very small, 2mm or smaller if I remember, since JE does have a line of two ring piston rings. I'm not sure if the weight savings is that monumental. I'd be willing to bet you'd save more just doing a lighten combo on any given piston.

The majority of the flat top pistons I make run a 1.112" pin height with a 22mm pin. On the 356, it is a 1.065" pin height, again with a 22mm pin unless they use the rods I make, then I set it up with a 19mm wrist pin - lighter pin, more material around the boss, lighter pin end on the rod. Any weight savings on a 356 translates directly to longer rod bearing life.

Walt, I found an interesting article on this, cite below:

http://www.autospeed.com/cms/A_108913/article.html

I have seen some SINGLE-ring pistons but I don't know how you would get away with that unless you are using a two-stroke. Where it says F1 engines only have a single compression ring, they MUST mean one compression and one oil ring, because oil has an octane rating of zero!

Just for fun, here's a photo from www.elevenparts.com of an old Carrera GS piston! Count those rings!

http://forums.pelicanparts.com/uploa...1190661291.jpg

A vacuum pump would also help with the oil control on a two ring piston engine. Not exactly ideal, but that's what many do to get away with low tension rings on drag engines.

Good discussion.

Dave, sorry for your misfortune.

Something not mentioned here are operating conditions (other than over-rev). Detonation can be an issue if the piston is already ‘fragile’ in this area. Even a slight ‘hydraulic lock’ only once can damage the pin or piston. Of course temperature is always an issue.

Another clue is finding pistons with cracks at the pin oil hole. That tells me there is some issue with the piston.

Best,
Grady

With the forced pin oilers, I'm wondering if there are stress risers introduced, pre-disposing the pistons to fail at the oilers. Same idea goes for connecting rods - when they edm the oiling hole down the middle of the rod, if you don't extrude hone the passage, the rods crack and fail, starting at the hole. Short of extrude honing the pin oilers, it may be best to leave them out and look at other solutions for pin oil retention and longevity. Calico offers coatings for wrist pins as does Anatech, Ltd with their thin film diamond like carbon coatings, typically reserved for Ti valves. Another thing I have been looking at for some time is micro dimple formation. WPC Treatment does this (www.wpctreatment.com).

I'll revive this thread to let everyone know that JE has come out with a radically different forging specifically for the 95-105mm bore sizes for Porsches, espcially beefed up around the wrist pin bosses but should not add more weight to the piston overall from what I was told. Very similar to what CP sells now. Better yet, no added price for the new forgings.

I've also spent the last two months researching the modes of failure for 2618 forgings and have come up with some ways to improve the high temperature/high stress micro-fracturing of pistons under extreme loads. Weight reduction, i.e. JE's "lighten combo" can help save some weight. Also, a thermal barrier coating can reduce the crown temperature about 100F, which is pretty substantial considering the temperature at the top ring lands do not typically exceed 200C.

Additionally, one could always have the rods bushed for smaller wrist pins (or made up to use smaller ones), which would increase the cross sectional strength of the wrist pin bosses. That's what I have been doing as of late, making rods with 19 and 20mm wrist pins.

Just to add some information to this thread.

J&E 3.4L Pistons have gone through 3 revisions and the new design is not bad but pretty much anything pre-2009 is going to fail. The originals will fail in short order and usually at high RPM meaning catastrophic engine damage. If I had these in my car I would spend the money now to tear it apart and remove them. If you do not know what you have in your car then tear it apart and replace the pistons. The fail rate of the early pistons is 100% I have seen atleast 10 failures. All of them have caused serious block damage as a result. I know they attempted to fix the problem on the piston by moving the oil galley machining on the original forging away from the thin spot at the wrist pin. It did not work so even the 2nd generation pistons fail that being said they might be alright for normal street use. The 3rd generation of this piston is an entirely different forging and they are strong as hell completely redesigned and if they fail it is the result of a poor build/tuning.

I'm sorry, but this is just not true. You're statement that all of them are going to fail is just ludicrous. I can't even begin to imagine how many thousands of JE pistons we've sold and I can only recall one failure of the wrist pin boss reported back to us when fitted to our cylinders. It was on a 105mm bore, 12.5:1, with a piston over 600 grams in a full race engine and had 235 hours on it when it failed. They should have been houred out long before that.

I have fielded plenty of phone calls with piston failures from other brands including Mahle Motorsports, Wossner (I think at least 3 with failures in our cylinders), and others, so this problem isn't something isolated to one brand and certainly it's not epidemic. One reason you see less of the Mahle ones fail is that those tend to be houred out earlier. I know of several builders who hour them out at 100 hours. Technically most pistons should be houred out no later than that in race use, but no one does. They wait until their engines go boom instead.

Charles, is the part where they made a 3rd generation forging accurate to your knowledge? It seems to me the main problem was the thickness of the material below the wrist pin.

I have an '88 Carerra in which I used 98mm JE pistons and Nickies cylinders supplied by Charles Navarro; the car is driven mostly on track around 200 miles a day. I have 26,000 miles on the engine and 201,000 miles on the chassis.

I have had no problems at all, installed them myself in my garage...

I think maybe considering the number of JE pistons out there a few failures would be logical, wouldn't it?

If I remember this issue correctly, JE determined that they were using wrist pins that were flexing (too thin?) and that flexing and extended use fatigue caused the piston failure.
They have responded with piston updates to address the failure but I believe the "real" solution was a better wrist pin and educating customers as too the life expectancy of any racing component.

IR Telemetrics, Inc. - Wrist Pin Sensor

It seems reasoanble to assume that pins that deflect too much will significantly affect the clearance between the pin and the piston and generate high levels of stress

It seems quite difficult to model the behaviour of the pin/rod/piston interface using numerical methods and I have seen FEA solutions that predict maximum defletion at the free ends of the pin and other solutions that show maximum delection in the centre of the pin.

May be the telemetry system may help.

It's likely a combination of using lighter(thin wall), more flexible pins and pin towers that flex with them. Newer forgings and especially the FSR version goes a long way towards solving this issue using shorter pins in conjunction with a considerably more advanced pin tower reinforcement design.

Just to add my $.02
You can run a single compression ring and one oil ring on a piston.
I have done it several times with good results.
BUT...you need to use a Total Seal type ring for the top...and a good quality oil ring.
AND...I used Teflon buttons on the skirts of the pistons to keep it straight in the bore (to prevent piston rock).
In this way the buttons reduce the friction way down (benefit of no skirt scuffing)...and the oiling requirements are less too.
Bob

Here is the answer straight from JE:

"Six or seven years ago we received requests for a more durable Porsche piston for racing applications. We replaced to old 96P forgings with the new 32P forging. The new forgings are internally braced and gained some weight. To address the extra weight and produce a modern racing type forging we produced a Forged Side Relief forging called 932P."

The 32P forgings have been the same for six or seven years and are still used today for the majority of jobs. The FSR is an upgraded piston that is lighter.

5 years in now with at least a thousand hard miles on 12psi supercharged boost and my JE s showed no sign of problems when I did a preventative rebuild last winter. The skirts and domes are coated and all were intact.

Bob - do the Teflon buttons replace the piston pin clips?

If they do that reliably, as well as reducing rocking, and last at least 100 high RPM hours, they sound like a winner all the way around. I've never understood why J&E doesn't cut its pistons for a regular circlip, with ears and holes, so installing and removing the pins is easy.

I got used to the clips Mahle uses. Despite having Stomski's install tool, I still find the J&Es tough to install. And instead of just using a machinist's scribe or the like to remove them, I had to cut a notch in the end of a pin drift, which I insert in the relief and then twist so I can get a scribe in to start teasing the clip out. It all seems so much more difficult than using snap ring pliers.

Great question, I just fought for what seemed hours trying to remove the clips on my JE pistons. Having a normal circlip or at least a little tail on the wire clips to aid in grabbing with a set of needle nose would help tremendously.
Why does JE go this route?

I rotate the clips in the groove to get a free end near the pick hole. Then I coax the clip out of the groove with a pick (of course) and then grab the end of the clip with some small needle nosers. Not saying its easy peasy japanesy. Just saying that's how I do it successfully without too much swearing.