69gaugeman

This is possibly because the 'length' of the bearings were too long. There should be no 'lift' of the rod cap at all. The bearings are prevented from rotating by the lock notches and simply cannot move.

The only way to check clearance is to assemble and torque the rod bearing bolts and the measure the bore. It must be in several places around the diameter and at , at least two levels using an inside mike (carefully) or an air gauge (4k+ in cost). Using a dial bore gauge can work, but the spring pressure may actually cause erroneous readings. It has to be done very carefully. Remember we are talking about microns here . This will give you a 3d geometrical view of the rod end. Then you must take a micrometer and do the same to the crankshaft journal. Now you have a picture of how the bore and the journal will interact with each other. If you think that this takes time, you are correct. As much as 1/2 hour or more per journal. This adds up quickly. Then if you need to use a different bearing thickness, you start all over again.

For a street engine though, plastigauge actually works quite well. That is why it is in the workshop manual. Hell, the crank journal bearings in my GT were worse than that. If you do not have oil starvation (and of course no knocking), you will have long living bearings.

Tom. M

Here's hoping the crank journal was ok, since the engine has been back together for a while. No real run time other than a few starts (oil pressure great right away). Took the electrics apart to paint the interior and that is how it sits now... hopefully this fall, it will all be back together in time for next years SCCA season...

GregBBRD

Virtually all rods will separate if you loosen one side while the other side is at full torque (without any bearing in place), so I'm not sure how valid of a test this is.

And I hope that Glyco has high enough quality control to get the length of the bearing correct....I don't need something else to have to check/modify.

GregBBRD

928 cranks have a very tough surface and wear to the journals is virtually nonexistent, unless something very catastrophic occurs.

The bearings you removed will not have damaged the crank or caused any wear.

Actually, if the 928 cranks did wear some, the replacement Glyco bearings (which run on the "tight side" of specification) would be much "happier". (Many "rebuilt" 928 engines have failed from rod bearing failures after the rebuilding process...due to the bearing clearance being too tight.)

I buy Porsche bearings, so that I can select the "red" and the "yellow" sized bearings.....not because they are constructed any differently.

worf928

I've been following this thread. Relative to the stuff I know about 928s I know crap about bearings.

I need to learn (a lot) more so I'm interested in the whole topic, but at the moment I'm focused on evidence-based failure modes diagnosis. I'd like to throw out a couple of questions to The Collective if you (all) don't mind.

What symptoms might be evident if a 928 motor was chewing up both main and rod bearings? Lots of metal in the oil (obviously). Low oil pressure? Inability to 'keep' front main and rear main seals?

What about just mains? Or just rod bearings? Do they produce different evidence of failure?

What diagnostics - short of pulling and tearing down the motor - are useful? Oil analysis obviously. Crank-end-play? Oil pressure?

Based upon those questions one might assume that I have an oil analysis coming back in the next few days. One might further assume that I have a good idea of what that analysis is going to have in it and that I'm wondering if there's anything useful that can be done short of tearing down the motor (or tossing it in the Atlantic.) Rod bearings can be done (relatively) inexpensively. Mains? Not so much.

GregBBRD

The only 928 main bearing issues I've seen have been on engines with cracks in the main webs. And those failures have been limited to the individual journals that have cracked webs.

I've never seen a complete set of main bearings that have been damaged. Even when an engine is destroyed by silicon beads from a bead blasted manifold, the main bearings are protected by the oil filter and do not get severe damage.

GlenL

Get the oil analyzed and listen to the results.

If I was having problems with rod and crank bearings I'd have the crankshaft examined. Or just get a different one.

jcorenman

I would also cut open the filter, if things are coming apart then that's where the bits will be. That won't tell you anything about mains-vs-rods, but as Greg pointed out we don't hear much about main bearings on these engines (except for the thrust bearing, with auto's).

Post up the analysis when you get it. I've got a bunch that I can offer for comparison, as do others I'm sure.

Fingers crossed!

worf928

First, this isn't one of my 928s.

Even if the engine has been run for a long while after a PO cracked the oil cooler (like in a certain '94 GTS)?

So, your experience indicates that in virtually all situations the motor explodes, seizes, or otherwise stops before the mains get damaged?

Will the entrails of the oil filter tell me anything the oil analysis won't?

Yup. We don't. And we also don't hear too many 'metally' oil stories either.

This is a case of trying to figure out how the previous tech eff'd up this engine (and so far I've found about 50 things the monkey screwed-up.) I don't think this is a case of 'normal failure modes.' Somebody did something (else) dumb to this motor. The question is how to find it before this engine dies.

The motor in question has crank-end-play that is well within spec. It has clacking lifters and too-low warm oil pressure. I've done almost all of the low-hanging-fruit-tasks for chasing the oil pressure and lifters. But, I am pretty sure there's metal in the nearly new oil. I'm not touching anything else until I get the analysis.

It could be two different and distinct issues and that metally oil has nothing to do with low oil pressure.

On the other hand, if the rod (or main) bearings were wafer thin wouldn't one expect both low oil pressure and metal?

If the main bearings were wafer thin would one expect leakage from the main seals? Or not? (Leaky main seals due to main bearing clearances is something I have heard in other contexts.)

On the other other hand, could a seriously screwed-up oil pump seal replacement cause both low oil pressure and metal in the oil? Sounds plausible in my mind. If one wanted to screw up an oil pump seal replacement and cause low oil pressure and do some 'mobile machining' could it be done? How?

And, I now wonder if the bearing caps on the cams are swapped (as evidence suggests that Mr. Monkey Tech had the cams out for some reason.) It would be a miracle if a cam turned with the caps swapped around, but if it did I think the result would be metal in the oil... and low pressure?

If any of these question are just stupid. Say so. I don't mind.

If all signs point to needing to have the crank in my hand then this story will not, in all likelihood, turn out well for the Shark in question.

GregBBRD

Cut open the filter. If you have access to it, cut open the previous oil filter.

While the current filter might not have enough time on it to tell you much....the previous filter should tell you quite a bit....especially if it was on the engine, while all this stuff was happening.

Oil testing has its limits, while everything I need to know is inside the oil filter.

The oil filter is like an x-ray picture of what Is happening inside an engine.

I once had a 911SC that had an entire filter full of babbit and copper, from the rod bearings. Had a terrible knock. The oil test came back relatively normal....the pieces that were coming off of the rod bearings were so large, that they caught in the filter, but were not small enough to show up (with any significance) in the oil test.

worf928

10-4. I will cut open the filter. As long as you think it's going to tell me more than this analysis result:

Tony

Off tangent a bit..
Who would you recommend do these analysis and how much are they.

worf928

You are apologizing for hijacking a thread I hijacked?
The above was from Blackstone Labs. They send the kit for free. You send it back full of oil and a $25 check. I mailed it COB last Friday and got the result in e-mail this afternoon. Pretty speedy.

I also got a voice-mail message with someone from Blackstone who wants to talk about the results...

jcorenman

That's who we use also. Those results are all kinds of ugly, lots of different wear-metals. It looks like cylinder bores (alum and chrome) as well as bearings (copper/lead). ZDDP is good, however.

The oil analysis only shows the stuff that was too fine to get through the filter, there is probably lots of larger stuff in the filter. It the filter loaded up and started bypassing, then the crap got pumped everywhere and likely everything is toast.

Was there much metal on the drain-plug magnet?

Cutting a filter open is easy, 5 min's with a pair of left/right metal shears. Certainly worth doing even at this point, just for the info.

GregBBRD

I'd love to know what they have to say and what they suggest.

The guys at Blackstone are really great and really try to give people good information!