1992 C4 Rebuild – Unusual wear pattern on mains?
 

The car has ~65K miles on it, of those only ~10K are mine. The rest are from the three previous owners. This project originally began as what was supposed to be a top end/reseal.

It had the standard leaks… power steering pump, valve covers, oil temp, cam towers, chain covers, etc…smoked like a WW II battleship… plus it was time for a new clutch.

During disassembly, I opened the oil filter and found a few small magnetic particles….the only thing I’ve found is some pitting on the #4 exhaust lobe…so I decided to go all the way and split the case just to be safe.

When I got inside the case I found an interesting wear pattern on the mains. The amount of wear tapers from 2 -> 4, with no wear on 5, 6, or 7. None of the rod bearings show any wear spots. It almost looks as if the crank is running ever so slightly downhill towards the flywheel, there’s also a complementary opposite wear pattern on the inside of the number eight bearing it approximately 10-11 o’clock.

Anyone seen this type of wear before or know what may have caused it? Something possibly out of balance?

I plan to document the rebuild process here, but it’s going to take a while… stay tuned.

Thanks for looking!

Chris

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Here's the other side of the case...

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Zoomed in for a better look...

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I have heard that intermediate shaft bearings wear due to the tension on the chain. Could it be something like that tilting the crank?

Looks like your crank ran dry for a few seconds repeatedly. That's my opinion.

I'd need to make some measurements and check the crank in the lathe, but it appears that either the crank is slightly bent or the main bearing saddles are out of alignment.

There is no evidence of a lubrication failure.

The crank can be checked in a lathe for straightness using dial indicators, but the main bearing saddles need to be checked with a line boring/honing machine.

Thanks guys for the replies.

The case and crank go to the machine shop tomorrow for a full checkup.

Just for fun, I measured the runout myself by placing the crank in one case half supported on #1 & 8 bearings.

First time I tried it, I installed one flywheel bolt in the crank to use as a handle to turn it. When I did this I got almost 0.001" deflection...quit alarming.

But I noticed that my eccentric cranking motion was actually causing the deflection. Makes sense since the crank isnt bolted down.

Then I installed the pulley bolt and setup a 22mm socket in my cordless drill.

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Running the drill at slow speeds I see essentially zero runout. I did this for bearings 3, 4, 5, & 6 and used both case halves with the same result at all bearings.

The video clip is of the #4 bearing. Its a little hard to see b/c I didnt hold the camera very still but you can see just the slighest amount of deflection on the gauge. Night and day difference over turning it by hand.

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Steve,

Could that type of wear be caused from the 2,3,4 end maybe not being torqued down as much as the 5,6,7 end? Just curious.

We'll see what the machine shop finds. Will post results when I get them.

Hi Chris,

Precision is whats required to accurately measure crank runout and that means the crank must be properly supported to see meaningful results. This is why we use the lathe and its live center. :) :)

Its mostly likely that the crank is OK, but you need to check and make sure.

Assuming your machine shop is well versed in 911 engine cases, they should reassemble the case, torque it all up, and measure the alignment of the main bearing saddles.

Because the wear points are at the split line (or close to) I suspect the case needs to be line bored.
The holes are out of round and "pinching" the crank at the 12 and 6 O'clock position.
Bob

Anyone have the spec for how "in-line" the main saddles should be?

Only spec I can find is for the main saddle bores.

Thanks!!

And the verdict is...
 

The case...anybody got some bacon to go with these eggs??

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The crank came back fine which I guess is some consolation.

So now, does it make more cent$ to line bore to +0.25mm or resize to std?

Line boring appears to be about half the price of resizing, but the first over bearings (PAG) are so expensive, resizing for standard bearings appears to be more economical. Is there a source for first over Glycos?

Thanks!!