Big End Bearing Problems on 3.2 Engine

[Keith Ulyatt]

I've just spun the big end bearing on cylinder #6 after only 7000 miles!!! I bought my 3.2L engine, history unknown but believed to have been run low on oil and seized, 3 years ago. Stripped it and found cylinder #5 & 6 had turned their big end bearingslocking the crank. I rebuilt the engine with two new con rods on these cylinders and re-ground the crankshaft. Everything was cleaned thoroughly and measured and found to be well within tolerance on rebuild.
Engine has now run for only 7000 miles. During this period, it has never been run low on oil and this has been changed 3 times, pressure was always above the 1 bar per thousand rev rule. Initial symptoms of trouble were an audible tap above 2500 rpm which quickly developed into a rattle at all rpm, but with no loss in oil pressure!
I am now rebuilding the engine with another crankshaft and replacement rod, but am concerned as to whether I will get further problems. I have also heard that the 3.2/3.3L engines especially are prone to wrecking the #6 bearing. Is this true, and are there any special measures I should be taking to prevent this problem happening yet again?
When tightening the (new) con rod bolts on the 3.2L engine, do you go by the spec book procedure of an initial 20nm torque followed by a 90 degree turn, or use a set torque figure?

[ChrisBennet]

Did you clean the oil galleries and check the oil feed to the bearings?
-Chris

[Doug Zielke]

And did you clean (I mean *really* clean) out the entire oiling system?

[PORSHIN]

And I'll third that, once one of these babies spins a bearing, You have a major cleaning project on your hands or history will repeat itself!

[Early_S_Man]

Keith,

Did you by any chance reuse the old oil cooler? Were the factory plugs taken out of the old crank and galleries thoroughly cleaned prior to installation?

Cleaning engine oil galleries is straightforward and easy compared to cleaning spun bearing debris out of oil coolers! There are those who advocate driving a really big nail thru any oil coolers attached to an engine that has suffered bearing failure!

[Keith Ulyatt]

Because of what I found on the engine when I initially stripped it I was particularly carefull about cleanliness. Crankcases and crankshaft were cleaned very thoroughly - although plugs were not removed. It does not seem neccessary on the crankcases as all galleries are accessable, and Bruce Anderson does not recommend removing crank plugs.
On engine oil cooler was replaced with another one which was known to be good - and also flushed before use. Car lines and loop cooler had not originally been used with the engine I bought - the engine was retrofitted fitted to my early car.
Stripping the engine after my most recent problem only shows bearing on cylinder #6 to have any damage. All other bearing shells are in excellent condition - as you would expect after only 7k miles! Oil pump also checked and found to be OK.
I am now very concerned about my future rebuild and whether this can happen again. As I said in my original post, I have heard, and read that this bearing can be troublesome on 3.2/3.3L engines. Is this a known fault?

Keith

[ChrisBennet]

Keith,
I feel your pain. #6 bearing totally destroyed itself in my 3.2 engine a few years ago. The bearing pieces looked like confetti when I tore it down. Even without any bearing at all, oil pressure was not changed.

While it is well known that the rod bolts on 3.2's can stretch and spin their rod bearings, I have not heard that #6 was special in this regard. I'm not saying that #6 isn't more likely to fail, just that (like yourself) I was looking for a pattern of failure with that bearing and I never came across anything.

Hopefully an expert will have better answers for you but this is what I do/would do:

Before I cleaned anything internally (and destroying the evidence) I would test the #6 bearing oil feed - to see if oil starvation was the culprit. The factory manual tells how to do this. After I cleaned the motor I would test all of of them.

IMO, the oil galleries are really not accessable so I would remove the plugs and clean the galleries. Remember, if bits of bearing went in there, they were carred in with very hot, thin oil in a vibrating engine. When the motor cooled those bits could have been trapped in cold, gummy oil.

After that I would start measuring paying close attention to that rod and rod journal. Is it in spec and round?
good luck,
Chris

[Early_S_Man]

The #6 rod bearing is the most likely to fail in ANY 911 engine, since it is the last one to get oil pressure ... furthest away from the pressure pump in the oil system!