GregBBRD

I wish you had a "red" and a "blue" bearing measured at this same time....then people would begin to understand what happens when they replace rod bearings without measuring either the clearance or the original bearings.

You get a crank that was originally "red" and stick a set of "blue" bearings on it...there is going to be grief.

GregBBRD

Did I see this picture, before today?

Rob Edwards

I don't think so, I had to upload it to the webserver to post it. Am I going to be doing a warranty re-shelling on the Cobalt car?

mark kibort

when bearings look like this or the upper ones that have the wear marks, i think it would best to replace them.... certainly after checking the clearances as Greg points out, if you do it blindly, you can really have an issue.(if you pic the wrong family of tolerances)

when they look like this, dont you change them? I dont think greg would put those bearings back in a rod cap.

And you guys were giving me a hard time (blasting me!) for reusing a wheel bearing inner race that looked good?
these bearings are shot.... call me old fashion, but they would never be in an engine i built. 160K miles!!! toss them!!!

mark kibort

we finally agree on something. uhhhhhhhh, hey, i reuse oil filters on my street car all the time (at least once) I think reusing rod bearings is more like re-using someone else's used oil because it only has 3000miles on it. nah, that isnt even close.... Man, i cant think of anything that you can reuse that could cause more harm than a used rod bearing!

GregBBRD

Depends....do you want to be able to drive it over the Grapevine?

GregBBRD

Sorry Rob. Did that leave a mark?

Rob Edwards

The truth hurts.

Already offered to replace them to Mark, and will put his name on my AAA Platinum account.

Imo000

You can't inspect the filter but you can absolutely inspect the bearing and then make a call on it. Considering Porsche has 3 different size rod bearing that are all labeled as standard, there is a pretty good chance for a screw up unless you really know these engines. The OP'S project sounds like a repair rather than a rebuild so if the bearings look good, as I said before, leave them alone.

GregBBRD

There's certainly merit to that. There's been more than one 928 engine "rebuilt" with a tight set of rod bearings on a large dimension crankshaft which promptly tossed a connecting rod when ran.

Porsche handles the rod bearing clearance issue pretty well. The bearings come in sets of two. They will always send two yellows (middle size) or one red with one blue (largest clearance with smallest clearance.) They will never send two reds or two blues. (I still have a huge box of blue bearings from building 930 race engines....I routinely bought two sets of bearings and used only the reds.)

Worth noting for the "economy engine builders", Porsche charges a serious premium for this "sorting".

The problem was with Glyco. It was common to get an entire set of very tight (small amount of clearance) bearings. That was a disaster in a small box.

Lately, I've only seen bearings from Glyco with medium to loose sizes. Perhaps they finally figured out that 928 crankshafts do not wear under normal use, eliminating the need for them to supply tight bearings for service use.

Only time will tell....perhaps the next production run will be tight, once again.

However, as you suggest, perhaps the best play might not to change the bearings, if one is not able to measure and figure out the clearances and can not rationalize the Porsche premium for sorting.

GregBBRD

Actually, visible wear is only a small part of the equation. The amount of crush that is required for the shell to fit into the rod is another part.

Because the stock bearings are so soft, any amount of pounding from knocks will quickly reduce this amount of crush. This is the really big problem, as this is the immediate precursor to the bearing spinning in the rod.

Also worth noting is that #2 and #6 are not always the worst bearings in the engine. That statement is "Internet Myth".

I've seen plenty of 928 engines with virtually perfect bearings in #2 and #6 with absolute trash in another cylinder. All it takes is one cylinder with a weak injector that drives the knock count in that hole beyond what the knock sensor system can deal with and the bearing will suffer.

Put this in perspective. Sixteen rod nuts are not much more expensive than four, in the picture of having the oil pan off.

mark kibort

Good information... i cant believe how much trust i put on the glycol packaged products... however, i always used plastigauge on all rods. I guess that is the only insurance, is to check them... dont know what i would have done if they were on the tight side... probably just install them, and that might have been bad.

so, what about the bearings in the above pic... are those considered good? i always thought when there was a different colored strip of wear, that meant, "toss them". No??

Rob Edwards

In retrospect it was a mistake to reuse the bearings, at least the #2 bearings at the top- I know that I measured all 4 shells and all were within 0.0001" of each other, but I don't like the shiny stuff on the uppers. I should have known better.

And Greg's point is well taken on #2 and #6 always being assumed to be worst- Here are the bearings from the '91 GT motor that became my stroker motor. Total mileage unknown.

Cylinders 1-4, then 5-8. Cylinder 5 was clearly unhappy while 2 and 6 are no different from the rest.





5 closeup:

mark kibort

Rob, (edit.. i was not looking at the right order)..... but anyway,... are you guys ok with bearings that look like this? it doesnt look good from my limited experiencing. what ive pulled out of my motors looked almost perfect vs new.
some of those look really bad.. like 5 is horrible.! 1 is bad, 2 is worn, 6 has wear like the others and 8 isnt very good either.

What did the bearings look like on that motor that blew at willow?

GregBBRD

It is somewhat common for some engine builders to Scotch-Brite off that first grey "protective" layer from these bearings.

I'm not going to say if that is my practice....tiny details like this are quite proprietary....I'm just throwing it out for information purposes.

Porsche Motorsports (US) has always done this, on this style bearing, AFAIK.

Worth noting, this practice would also increase the oil clearance a bit.