Engine Rebuild Horror Story

[Wayne 962]

Originally posted by NevenM
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

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

[ischmitz]

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

[snevman]

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

[William Miller]

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

[NevenM]

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

[snevman]

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

[NevenM]

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

[snevman]

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

[NevenM]

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

[SeriousFunk104]

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

[ChrisBennet]

Originally posted by NevenM
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, ...
Neven

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

[NevenM]

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

[ChrisBennet]

Originally posted by NevenM
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

[aturboman]

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!

[NevenM]

Originally posted by SeriousFunk104
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?

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

[1fastredsc]

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?

[William Miller]

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!

[Doug Zielke]

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

[davidl]

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

[snowman]

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.