davek9

Good point and great suggestion, thanks!

mark kibort

good point.


Now, greg, wouldnt you agree those bearings look very sad???

slate blue

But it doesn't ride on film of oil or you wouldn't get the irregular wear patterns you see.

I am reducing the grooving from 220 degrees to 140 degrees which is the optimum. So I am increasing the surface area.

slate blue

Where in fact the Porsche crank drilling is more favourable to the use of these updated bearings due to the fact that factory cranks are cross drilled and oil differently from the " straight shot" or Chevy style that is commonly employed today whether it be in stroker cranks for 928s, the LSX engine,NASCAR engines to high end European engines. Even Porsche now uses my adopted grooving design.

To the other posters who state that this method leads to higher friction, it is in fact the complete opposite as the reduction of the groove allows for better load carrying capability so the oil layer doesn't shear.

In modern engine development this has allowed the oil to become thinner, i.e 5w-30 for example to again further lower friction due to two factors, smaller bearing journals which I first did on the rod journals back in 2007 and now smaller main bearings hopefully by the end of the year. (The crank maker has the block now for other crankshaft work currently under way.)

The other factor leading to lower friction in mine and other engines that have similar development is the tighter oil clearances. The tighter clearances allow for better "hydrodynamic lubrication" this essentially means that the shaft in these engines "surfs" the oil film better.

Imo000

The lubrication of an engine is an system therefore, if you change one parameter (different bearings) will change the way the entire system operates. To know exactly if this change is positive, negative or neutral, massive amount of testing is required. If you are willing to take a chance with your engine that's fine but, to suggest everyone follow suit is wreckless.

FredR

Whereas I am sure replacing all the shell bearings whilst the motor is stripped is a good thing I struggle to see anything that looks obviously wrong with the wear pattern shown on the photos- am I missing something here? All shell bearings invariably "polish" to some extent as part of the running in process but those shells do not look in anyway "wiped" to me.

This motor has not covered anything out of the ordinary mileage wise and I am sure many motors are still going strong having covered way more mileage [I think Bill's motor is close to 300k miles now?].

I can understand a need to improve a weak design but is the stock design really in need of improvement? We know there is a perceived weakness in the 2/6 big end when high revs and high cornering forces are conjointly applied but that is not the fault of the bearing design per se or so I have been led to believe.

It is one thing if there has been a lubrication failure but I would be very disappointed if a motor used sensibly could not make 300k miles without being stripped and in need of rebuild if not a lot more.

Regards

Fred

slate blue

I am not suggesting everyone should do what I am doing, I am however pointing out that the design is dated and it has more friction and wear than modern engine designs.

mark kibort

that top surface should be a mat finish, with only smoothing due to wear. when its shiney, it doesnt hold and distribute oil as it should and causes further wear.
all those rod bearings should be replaced. im the cheapest guy here with regards to repairs, and those bearings would never go back into an engine of mine or friends.

Imo000

The bearing design is not dated. The 928 lubrication system is appropriate for the engine. It was designed as a system that functions well for what it was intended for.

GregBBRD

Just adding points of order, so "regular" 928 people don't start thinking this is something they should be attempting.

Try to keep in mind that the OP is asking questions about what he should do with his stock bearings in his stock engine.

Adamant1971

And they are all getting replaced. Working with Roger on another order.

I should have the block and heads back in a couple weeks. Will post pics of the motor build up.

GregBBRD

I was hoping....

Bearings, rings, gaskets are like oil filters...consumables that should be replaced each time they are removed.

You do need to be careful....and have your machinist measure the bearing clearance very carefully.

"Standard" was actually three separate distinct sizes, when these engines were assembled, at the factory.

More than just a couple of these engines have had crankshaft and bearing failures immediately after a "rebuild", because of improper bearing clearance.

I've written volumes about this subject on this Forum.

Because I'm a total gearhead and can barely plug in a computer without help, I have no idea how you find this information....but there are some brilliant people here that might be able to tell you how to find these threads.

BC

Probably rhetorical, bit you can search by the author Instead of the subject.

FredR

For those playing along at home, the OP asked a question about the perceived condition of his shells based on the photo evidence proffered. He did not ask whether he should replace the shells- that is another matter.

Personally speaking, I would be interested to know how much life is left in those shells irrespective of what happens to them. The only photo I was a bit suspicious of was the front crank bearing and wondered if the wear pattern shown there might possibly reflect a harmonic damper that is not working as intended due to age hardening?

As Greg wisely says shells are a [long term] consumable item, they wear [as these have] and as I intimated in my earlier post in most circumstances an engine strip should consider replacement of such on an opportunity basis rather than [necessarily] a need basis.

It would be interesting to know how much [if any] life those shells have left in them irrespective of whether they were to be reused or not.

I have no idea why the OP stripped this motor or if he had any untoward signs of distress- perhaps he is just doing a systematic refresh as a matter of course. I have followed Greg's posts about bearing clearances and given there are three [?] different clearance grades, how on earth does one go about replacing these things without having three sets of each grade to start with?

My expectation is that the shells should cover at least twice what the OP has got out of them to date- do these shells have such incremental life in them or not? Are they visually scrap with no need to measure or would they need measurement to determine residual life?

Expert inputs truly appreciated given most of us long term owners are going to arrive at this juncture sooner or later.

Rgds

Fred

slate blue

I think that you are partly stating the obvious given the oiling system was of course designed for the 928 but is that the right question to ask when looking at issues and problems with various designs. I am not at any point saying that the average Joe 928 owner should change their style of bearing but merely pointing out that the bearing wear the 928 suffers from is most likely due to the grooving employed on the main bearings.

So you asked for research, here is Mahle Clevite's paper on the topic.

http://www.mahle-aftermarket.com/med...dfs/tb2051.pdf

Below is excepts of that paper.

"In an effort to develop the best possible main bearing designs for High Performance engines, we’ve investigated the effects of main bearing grooving on bearing performance. The graphs on the next page illustrate that a simple 180° groove in the upper main shell is still the best overall design.


While a slightly shorter groove of 140° provides a marginal gain, most of the benefit is to the upper shell, which doesn’t need improvement. On the other hand, extending the groove into the lower half, even as little as 20° at each parting line (220° in total), takes away from upper bearing performance without providing any benefit to the lower half. It’s also interesting to note that as groove length increases so do Horsepower Loss and Peak Oil Film Pressure which is transmitted directly to the bearing."

You may also be interested in this comment which is directly relevant to yours.

"Notes: The aftermarket bearing business being a look back in time, (bearings generally represent the technology of the time the engine was developed)means you will still find some full-grooved main sets offered for older engines where demand is low and the engineering cost to bring the sets to 2010 standards is not warranted."

It should be noted that a 5% HP saving over what the 928 uses (220 degree) and if owners or mechanics used the 360 degree grooving the HP loss over the optimum 140 degree grooving is 13%. Those losses or gains refer to friction in the main bearings.