Engine Rebuild Horror Story
 

Hold on to your seat and read this horror story.

I purchased a ’89 Carerra 3.2l coupe about 1 year ago and love the car and had no problems.

I started having loud valve clacking noise on the right side from cylinder 6. I took it to my mechanic and it appeared to have a bad rocker arm due to being far out of adjustment and it had damaged the cam. He adjusted it down the best he could but another noise was coming from within the case. I had him remove the engine and break it down.

Bad bearing wear on the crank was identified and to make a long story short, I decided to have the engine rebuilt.

I had to have the crank turned and hardened, replace the cam on the right side and the bad rocker arm. All new parts associated with a complete rebuild were installed.

The engine was put back together and my mechanic took it out for a test drive. In less than 1 mile he had to shut it down due to a horrible noise, he thought it came from the right side.

He opened up the valve covers and the rocker arm on cylinder six had broken. The engine turns fine by hand and makes no noise. A possible factory defect in the new rocker was thought to be the cause. After waiting for a week for the new rocker arm, it is finally installed and now the engine won’t turn by hand, it is locking up on no. 6.

He pulls the engine again and removes the cylinder, warning if you have a weak stomach don’t read any further, and the piston is destroyed. It appears that it broke where the wrist pin slides in and the connecting rod kept beating it to death. The inside of the cylinder is scarred and marred and the piston looks like it was attacked by a sledge hammer from below.

Now the questions:

1. What would cause this? The only things I can think of are a mistake was made during the tapping if of the wrist pin or possibly some damage was done to the piston, maybe dropped, during the teardown/rebuild.

2. Now as the rebuild continues, what should I be on the look out for and make sure is done to ensure that I don’t have further problems down the road? I am afraid that once it is rebuilt and I get 10k miles on it, metal particles or something else as a result of this damage will cause further damage.

The mechanic is a solid guy and has over 30 years experience in Porsche engines. He is not willing to state what he thinks happened until he breaks the case. I do believe that he sent some parts out for machine work.
Thanks for reading this long post and I really appreciate any input you are willing to share with me.

Scott

Did you replace the pistons and cylinders when you rebuilt the engine? Buy a copy of Wayne's book, "How to rebuild and modify 911 engines". It'll help you out. Sorry about your motor, that sucks.

No, I did not replace the pistons or cylinders, they were all in A1 shape.

Thanks for the input about Wayne's book, but I already have it. It does not provide much information about how to follow up when things go this bad though!

Thanks again.

Scott

Could a wrist pin have come out when the engine was running, possibly to due a circlip that broke/was not there?

No, I don't have a chapter on how to sue your mechanic :). However, I do have a lot of information in there on the questions that you should ask your mechanic prior to having the rebuild done. You might want to revisit some of this info, and quiz your mechanic to "test" his accumen.

In order to make a diagnosis, I need to see a picture, or get a better description of what happened. Did the wrist pin break (unlikely), or did the piston itself fall apart?

One guess here would be that your mechanic forgot to, or improperly installed the sheet metal around the cylinder (which is required for proper cooling), and/or installed that cylinder upside down with the fins facing the wrong direction? That would cause the piston to overheat, seize and perhaps break in the manner you described.

Need more info though...

-Wayne

Wayne,

Thanks for the response.

The wrist pin did not break and it was still attached to the connecting rod. The piston did not seize, it slid out of the cylinder without a problem.

I appeared that the wrist pin just "broke out" of the bottom end of the piston.

I don't know if the clip failed or was improperly installed and the wrist pin was wiggling and broke the piston or what, but it is quite a mess.

I will try to get a picture and e-mail it to you.

Thanks again,

Scott

Even if the piston seized, it would probably be loose by the time that the engine cooled down, so that wouldn't tell you anything.

There is almost no force present on the piston when pulling the piston back towards the crankshaft. For this part to have broken, the most likely scenario is that the piston siezed and got stuck, and the wrist pin just pulled the piston apart. Then again, I'm just guessing from your description, which doesn't give me too many details to go on...

-Wayne

Wayne,

Is there a connection between the broken rocker arm and the broken piston? Could the broken rocker left the valve down and consequently broken or stressed the piston. Was the cam still in time? A more likely scenario is the piston came apart and hit the valve when it was open, breaking the rocker.
What do you think?

If the piston failed first, the lightly causes can be:
Hydrauliced where the cylinder was full of fuel or oil and the engine had enough inertia when it came up to compression on that cylinder to break the piston.
The circlip failed or wasn’t installed and the wrist pin moved off to one side, stressing the piston.
Did a ring fail first?

Scott,

What do the parts look like?
What is the condition of the rod?
What does the cylinder look like?
Does it look like the pin worked in the piston for some time?
Are all the rings intact? Are the ring lands beat up?

Best,
Grady

Is that rocker arm getting OIL?

Wayne,

I have linked a few photos as you requested.

I spoke with my mechanic and he has assured me that this was not a defect but rather an error in reassembly, either by him or the machine shop he sent the work out to. He has made it clear that the parts and additional labor from this point forward are his.

He doesn't think the wrist pin broke out of the piston because the cylinder isn't scored on the sides where the wrist pin would have hit. He thinks the piston broke at the wrist pin due to damage and then shot up and hit the valve breaking the connecting rod.

His next step is a total disassembly and cleaning, cleaning the entire oil system, replace the bad piston, cylinder, connecting rod, valves, etc.

Can you think of anything else that I should make sure is done?

Thanks again for all your help.

MY REBUILD HORROR

Interesting. The mechanic seems to be very responsible and professional. Pistons generally do not just come apart, although they do go up and down ONE HUNDRED TIMER PER SECOND at 6000 rpm. At times it seems that the best question is not "Why did this come apart." Instead, I sometimes wonder "How does this thing stay together."

Yes, good mechanic, I suppose.

Good thoughts too, Clark Kent.

Show me a pic of the top of the piston, I want to see how much damage is on the top. Pistons just don't self-destruct like that.

More pics please?

-Wayne

Another thought - the original problem may have been a stuck valve. This would have caused the big clearance between the valve and the rocker, and the rocker/cam would have taken a beating. It's possible, once the engine cooled down, that the valve itself closed and the problem was non-diagnosable when cold. This is a problem, and there's really not too many ways to detect this. It's also possible that this open/stuck valve may have hit and damaged the piston previously.

Just taking some guesses...

-Wayne

I added a photo of the top of the piston at the link below. I didn't include it previously becasue it is really hard to see it well due to the flash. There are two "fresh" marks, one from each valve, but very minimal considering the destruction of the rest of the pison.

I will try to get a better picture, but I was really surprised by the lack of damage to the top of the piston.

I don't know about a stuck valve, it seemed to be running well before the rebuild, just noisy.

Thanks again for all your help.

Scott

MY REBUILD HORROR STORY

Hi

From your description I'd suggest that the piston was in partial failure
first (it had cracked) prob due to an over rev, hence the "but another noise was coming from within the case" after he had adjusted the rocker, This had then 'hammered' the rocker (I guess it was the intake), The mechanics mistake was the assumption that the rocker & camshaft wear were due to the excessive play and not something more sinister, I would have gone over every part of cylinder 6 (con rod, piston etc).
If your Mechanic suspected the main bearing wear was causing an audible noise from one cylinder I'd be very wary of his competance, This is very unlikely (a big end maybe but not a main) and it would have been easily identified, also if you had that sort of problem you would have had zilch oil pressure. The lesson is if you can't explain it you probably haven't fixed it

Neven

Don't throw out that piston and cylinder - if you are going to, I'd like to have it to take a closer look - this type of thing doesn't happen too often...

-Wayne

Cam timing and/or piston to valve clearance too small. Was the mark on the piston from the Intake valve. What machine work was done?

That may have been what he mentioned about an "assembly" mistake.
If the rocker broke the valve was closed so it would be no problem to turn it by hand. Maybe the new rocker was just strong enough or the piston suffered enough dammage that the next time the piston broke.

I found valve to piston contact evedent on my rebuild. I did a complete valve job with new valves, guides and springs. The seats were previously replaced so they just needed a new cut.

No other machine work had ben done on this engine. I did measuer the cylinder heights and they were within spec and not cut.
Anyway upon reassembly I timed the cams and checked clearances per Waynes book. The intakes were tight. Using the quick method and the screws turned in I could rotate the engine freely by hand but I could feel a tight spot where the valve was contacting the piston.
I solved the clearance problem by retarding the cam timing.

I suspect that your engine may have had somethin taken off the mating surfaces at some point and were not as lucky as I. I believe another contributer on mine may have been weak valve springs along with overreving the engine. Valve guides sticking are also a real possibility as Wayne mentioned.

Waynes book is very thorough on these clearance checks as he has learned from some very experienced engine rebuilders. The older our cars get the less likely that they are still box stock so these checks really need to be done every time!

Anyway that's my guess.

All that being said, you have to appreciate the mechanic who is taking responsibility for this. Although it may or not not be his fault that this occurred, the fact that he is taking ownership of the issue speaks loads about the ethics of this mechanic.

The more I think about this, the more I think your mechanic knows what went wrong? Why don't you ask him again?

-Wayne

Wayne

I think the cucial point is "but another noise was coming from within the case" and afaik the cause of this noise was not satifactorily explained,
I think it was a cracked piston, If the mechanic is sweating over anything
he has realised that he didn't inspect the cylinder 6 components carefully enough

Neven

I'd have a tendency to agree, but this is still very rare to find. I'm still puzzled by the original problem, and how it related to this disaster.

-Wayne

How about this: Some sort of mistake was made when installing the #6 wrist pin. Maybe it got tapped to hard bending the rod ever so slightly. The bend rod would have caused excessive heat off the wrist pin and thereby weakened the wrist pin bushig area of the piston leading to the crack.

I think the mechanic has some sort of recollection of something having being ackward during the #6 cylinder assembly. That's why he was quick to offer the "assembly mistake" explanation rather than blaming if on material failure alone or other reasons.

In any case be glad it happened so early in the game and that the shop/mechanic is stand up enough to take care of it. Imagine the engine would have grenaded after 2000km when you decided to take it to redline for the first time......

Ingo

I don't think the original problem had anything to do with the end result. All items that were attributed to the original problem were replaced with new parts.

After disassembly, it was apparent that there were a few bad main bearings and the cam had wear so significant that it had to be replaced.

My mechanic believes it was coincidence that this happened in No. 6 cylinder and it was caused by either damage to the piston or another component during rebuild. Possibly there was a problem with the wrist pin bushings (I believe that is what he said, but if it doesn't make sense I may have misunderstood him).

I don't know near as much as he does or any of you that have contributed, but I do appreciate all the input. I have learned a great deal during this incident and I appreciate it.

I would like to point out that patience is something one has to develop and I am way underdeveloped. I am just "jonzing" to get back in the seat and take it for a ride, but I do understand that to get it right it will take some time.

Keep the input coming and if anybody has any additional questions or opinions, I would love to hear them.

Thanks again,

Scott

Lots of speculation, just let us know how it turns out.
Soon I hope!
Good luck!

Scott

Re 'All items that were attributed to the original problem were replaced with new parts', what does this mean? As I interpret your original post the cause of the noise within the engine in cylinder 6 that existed after you first adjusted the rocker has never been satisfactorily explained, it
most certainly was not your main bearings, so what other work was done? Were the pistons cleaned, inspected and re-ringed?

Neven

Neven,

Thanks for your inputs.

Originally, the noise at the rocker arm in No. 6 was attributed to excessive wear due to being out of adjustment, this caused severe damage to the camshaft as well. The rocker arm was replaced and the valves adjusted the best they could be, but it still wasn't correct.

A complete rebuild of the head, new cam shaft, and new rocker arm is what I meant when I said all parts attributed to the original noise were replaced. The pistons were cleaned, inspected and re-ringed.

After diagnosing this problem originally, my mechanic found that this did not take care of all the noise, there was additional noise coming from the case. After tear down severe main bearing wear was discovered and the noises were attributed to this.

I hope that this provides enough information for further diagnosis.

Thanks again,

Scott

Scott

This is what concerns me, I cannot see how 'severe main bearing wear'
would have caused such a noise, for the main bearing wear to be that severe you would have had no oil pressure, I stand by my theory thats the piston was cracked and the mechanic didn't see it during the rebuild, The crack flexing broke the rocker after the the rebuild and and second rocker did the piston in, either way I applaud the mechanic for his warranty but I still think he now realises this and is not telling....

Neven

Neven,

Thanks. Your explanation sounds very likely to have been what happened.

Is there any way to tell if this is what happened? Is there a type of forensics for engines that can be performed?

Thanks again,

Scott

Scott

Not as far as know, but I also don't see the point, We all are wiser now and so there has been some gain. If I was you i'd pay for the new piston
if your mechanic installed it an paid for the rocker (also check the valve)

Neven

Just a curious question,

Would an intake valve that is failing to open fully cause this problem? Creating a vacuum that was putting stress on the piston during intake?


Jason

Neven,
Are you sure there would be a noticable oil pressure loss? I know from person experience that losing a rod bearing completely doesn't change the oil pressure. I don't know what it would have been like at idle but on the track there was no noticable oil pressure loss when it happened.
-Chris

Chris

I did suggest a rod bearing but Scott only mentions 'severe main bearing wear', nothing about the big ends, Think about it, a big end
can make a percussive noise (as the rod smacks up an down) and can be isolated to a single cylinder pair but main bearings tend to 'rumble' as it requires the crankshaft to flex to create the noise.

Neven

I was just curious as to whether a bad main bearing would cause a big enough pressure loss to be noticable.
thanks,
Chris

Piston failure
 

Just a note no one seems to have mentioned. Porsche has used pistons that have a top and bottom orintation. I'm not sure about your application but all the Mahle pistons I have ever installed have a mark indicating flywheel to piston orientation. Perhaps they were installed "upside down" causing valve contact. This would be more likely to happen with harder used: ie higher revs. Perhaps this is why your new rocker broke and caused damage to your piston. Cast pistons will often suffer damage before the rod bends. I took an early 911 engine apart years ago where it ran ok with a loud ticking. Upon tear down no fault could be seen until I took off the barrel and the piston fell out in 2 pieces. Cut right in half! This may also explain the rod bearing damage. Also make sure your oiling spray bar is working to that rocker. If it is restricted or blocked the rocker,valve, and cam will suffer in short order, and possibly sieze the valve, causing a collision. Good luck!

No, AFAIK, and someone will prob correct me on this, the biggest
stress on a piston is just after the top of the exhaust stroke, The combination of changing of direction of the piston mass and the
'suction' of the exhaust is greater than the force of combustion.

Neven

If you notice the crack is following a solid line. You think that would give away a hint at it hitting one of the valves at an angle?

Just a note and not that it applies:
The top of the SC pistons are shaped to aide Fuel and Air mixing on CIS.
The high side of the dome is actually the closest point to the intake valve. The critcice clearance is just before top dead center where the intake valve begins to open before the piston is at TDC. A weak spring or a sticking valve can result in contact. When cold the clearance at this point in the cycle is only like 1.5 mm. (Plus the valve adjustment gap.)
If the valves are adjusted tight the clearance it's even smaller mostly because the valve opens earlier. The valve chases the piston down.

I believe this is the correct sequence can someone back me up or correct me.
Thanks!

Scott....
Just had a look at your "horror" page. Very horrible indeed, no...actually wonderful! Thank you for sharing.

There is almost nothing familiar in this story, yo me at least.

Why did the bearings go, not explained?
This is not a common fault on the 911, unless it has catastophic oil loss...

Were the heads fully stripped, and the valves and seats recon?

The way I see it, I'd be strongly suspicious of a fundamental problem, and most likley is that the engine had been previously damaged, and poorly rebuilt..maybe with used and damaged parts...a bent rod will definitely break a piston...dont ask how I know this..

Very very odd..

Look forward to hearing your mech's analysisi when he's sorted it all.

Kind regards
David

Tis a very simple problem. Mechanic, you just rebuilt my engine, you inspected everything, your took responsibliity to build a good engine. It broke, FIX it.

If a mechanic changes a spark plug or the oil and the engine fails shortly after, then a full diagnoisis may be proper. But anytime a mechanic takes an engine ALL the way down for a full rebuild, he owns it. If it breaks shortly after, no explanation is necessary, he is obligated to fix it.