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Has the 928 rod bearing inconsistency been fixed?

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Has the 928 rod bearing inconsistency been fixed?

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Old 02-13-2018, 05:54 PM
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The Forgotten On
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Default Has the 928 rod bearing inconsistency been fixed?

Since I'm about to get more parts I just want to know if Glyco has fixed the issue with bearing sizes being all over the place.

From looking over the forum it looks like it has been an issue for a while but I never could find out if it has been resolved or not.

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Old 02-13-2018, 06:32 PM
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I put a full set of new Glyco rod bearings in the Red Witch's engine last fall. Bearing clearances were quite consistently in the middle of the range.

See post #58 here:

https://rennlist.com/forums/928-foru...45-btdc-4.html
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Old 02-14-2018, 04:12 AM
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A step up is Clevite racing rod bearings. A must for high performance engines.
https://rennlist.com/forums/924-931-...aring-fix.html
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Old 02-14-2018, 06:08 AM
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Originally Posted by Strosek Ultra View Post
A step up is Clevite racing rod bearings. A must for high performance engines.
https://rennlist.com/forums/924-931-...aring-fix.html
I've seen that along with a similar thread where the rods were machined to accept Toyota Supra bearings.

If it wasn't in excess of $700 to get it done it would be tempting, but seeing as this will be a street engine with horsepower under 400 I think the stock bearings will be sufficient.

I don't plan on keeping it above 5,000 RPM for extended periods of time with large RPM changes found in racing, it's going to be a daily driven cruiser with mild camshafts and timing.

Albeit with 951 internal parts for when the time comes to boost it.
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Old 02-14-2018, 01:26 PM
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Did the 928/944 Glyco bearings ever have the same kinds of manufacturing issues as the Glyco bearings for 911 (and variants) had?
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Old 02-14-2018, 04:54 PM
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Originally Posted by ptuomov View Post
Did the 928/944 Glyco bearings ever have the same kinds of manufacturing issues as the Glyco bearings for 911 (and variants) had?
They changed from 3 metal based bearings to the bimetal ones, same as 911s did. They also had variances in size that could cause issues, usually they were too tight.
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Old 02-14-2018, 05:17 PM
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Originally Posted by The Forgotten On View Post
They changed from 3 metal based bearings to the bimetal ones, same as 911s did. They also had variances in size that could cause issues, usually they were too tight.
Were either Porsche-branded "factory stock" bearings for 928 or Glyco aftermarket bearings for 928 ever anything else than bi-metal aluminum-steel bearings? I didn't know that. If that's true, then I seem to learn something new every day.
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Old 02-14-2018, 10:15 PM
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Originally Posted by ptuomov View Post
Were either Porsche-branded "factory stock" bearings for 928 or Glyco aftermarket bearings for 928 ever anything else than bi-metal aluminum-steel bearings? I didn't know that. If that's true, then I seem to learn something new every day.

I'm not aware that Glyco changed the bearing design. From my testing, the bearings from Porsche are the same as the ones from Glyco....with the exception of sizing. Current bearing are the same as original bearings....and, for what it is worth, when they wear/fail, there is a layer of copper/bronze that shows through.

I'm going to tell people the rod bearing dilemma one more time....someone needs to save this somewhere.....I'm not going to clarify the problem again....it is boring and time consuming to write.

When the engines were assembled at the factory, each crankshaft was marked with either a blue, yellow, or red "dot" on each journal ....main and rod journals . The difference in color was made when the crankshafts were ground and were a reflection of the size of the journal. The people that assembled the engines simply install the corresponding bearing (also marked blue, yellow, and red.) The bearings varied .0008" from one size to the next. Blue journals were the smallest, yellow journals were the middle of the tolerance, and red journals were the largest. Correspondingly, the crankshaft bearings were just the opposite. The blue bearings were the tightest, yellow bearings were .0008" bigger, and red bearings were .0008" bigger than the yellows. Idiot proof....each engine got the exact proper bearings for the proper bearing clearance.

Here's the really important part....the huge majority of the crankshafts were ground to the "red" specification (biggest journal, bearing with the most clearance.)

For a few years, one could order the "blue, yellow, or red" bearings to match up to the color of the journals on the crankshaft. However, the paint markings on the crankshaft washed off after a few thousands of miles, and since no one outside the factory knew the differences in the colors (or paid any attention to them), Porsche changed both the rod bearing and main bearing part numbers to one number only. Bearings then (and to this day) came in a one journal set, with either a blue with a red, or two yellows. You will never get two blues or two reds.

What does this mean? If one were to not measure the crankshaft and the bearings, it would be very possible to mix a "red" journal with a "blue" bearing and end up with virtually no oil clearance! If one were to mix a "red" crankshaft with two "yellow" bearings, bearing clearance could be minimal. And not enough bearing clearance results in fresh engines spinning bearings and throwing rods....which happened/happens way too often. In the past, I frequently bought multiple sets of factory rod bearings and use two reds from two sets to get the proper clearance. The 'extra" two blue bearings were useless and after I had enough of them pile up to last me a lifetime, I simply threw them away. This was fine, when bearing were relatively cheap from the factory, but that boat sailed years ago.

Glyco, although they made these different bearings for the factory and marked them for the factory, never made the distinction of size. And, for years, they supplied very "tight" bearings (they would have been colored "blue", from the factory.) The result was immediate engine failure from insufficient bearing clearance.....as the vast majority of the crankshafts had large journals and needed "red" bearings.....and literally dozen and dozens of engines have failed from this problem.

Why did Glyco not realize they were supplying bearings that would be too tight in the vast majority of engines? I assume that they assumed that the crankshafts would wear a tiny bit, over the years of use, and that "tight" bearings would compensate for this wear. The problem....928 crankshafts are made from some incredibly good material, with the perfect heat treating, and do not wear....unless something catastrophic happens to the engine.

Moving forward to current day. The Glyco bearings I've seen in the past few years are now "loose"....they would be "red" bearings if they came from the factory. And this works really well....unless you happen to have a crankshaft the was originally yellow or blue.

So what is the answer?

Start by buying a set of current manufacture Glyco rod bearings. Measure a journal on the crankshaft. Install a pair of bearings in a rod and measure the bore. If you have .002" (+.0005"/-.00025") clearance, you are fine....on that one rod. (928 engine builder's secret......you will want .0025" (+.0005"/-.00025") clearance on #2 and #6.) Now do this for every journal and every rod (hopefully you marked the position of each rod on disassembly and can keep track of where each rod is going to fit back on each journal.) If you can't get the proper clearance, order factory bearings and start mixing/matching to get the proper clearance.

Now, stop and realize that the exact same color codes apply to the main bearings......and this is not as simple to solve!!!

You do this and maintain the clearances above, you will be "golden".

If not, your engine may not run for very many minutes.
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Old 02-15-2018, 12:55 AM
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Per a conversation with the bearing manufacturer about a decade ago, the Glyco and "Porsche branded" bearings were, at least at the time, bi-metal bearings. The bonding layer coloration is an interesting detail about these bi-metal aluminum-steel bearings.

I agree with Greg that the rod bearing clearance being correct is critical.

Factory tolerance range mid point is likely to work well for most cases that even remotely resemble the factory stock setup. Bottom end of the range probably not so much.

If you use higher viscosity oil, you may want to have a clearance a little higher than that to keep the oil flow up to cool the bearing.

If the engine sees very high compression gas loads, then increasing the bearing clearance is not beneficial. Larger clearance will concentrate the load on a smaller area of the bearing, and that will wear out the bearing quicker. For high compression loads, you want the smallest clearance that you can get away with while still flowing enough oil to cool the bearing and not overheating the oil.

For high rpms and resulting high tension loads, the situation is different. The load on the rod big end bottom part is higher, sure, but the main issue is the deformation of the bore big end. At high rpms, the bearing grabs the journal from the sides. Smaller the clearance, greater the risk of this. The best answer is greater eccentricity of the bearing shells. As long as you stick with bearings made for Porsche 928, by my knowledge you've got no eccentricity options, so that's a dead end. So the remaining option is to increase the bearing clearance such that even with deformation at TDC the bearing doesn't grab. With the bigger leak, you now need higher viscosity oil and even with that you can wear out the bearing quickly as the larger clearance results in a higher peak pressure in the bearing. It's still better than having the bearing grab the journal at the parting lines. Once you go thru that door, going to a harder bearing (such as those Clevite race bearings for Toyota Supra) is the logical next step.

This is why forced induction is so nice on the 928 engine. You don't need the extra rpms, so you can keep the clearances about stock. Stock or near stock clearances combined with great oil cooling and high viscosity oil give fantastic ability to hold up against compression loads. And the compression loads won't cause the rod bearing to grab the journal at the parting line. So you don't need higher eccentricity or higher clearances to prevent that. It just runs, provided that you can get the oil to the bearings.
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Old 02-15-2018, 01:54 AM
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When I pulled my old bearings, they were all stamped "STD". Does that mean they were all yellows? I believe my engine was rebuilt before I owned it, so the old bearings might have been non-Porsche branded.
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Old 02-15-2018, 02:11 AM
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They're all stamped 'STD' on the backside, if they're factory bearing halves the paint mark is on the side. Here's a sampling of 20 new rod bearings from a batch bought in 2014- all 20 are 'yellow'.

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Old 02-15-2018, 04:23 AM
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STD means standard size, 1st undersize is marked 0,25 (0,25mm smaller journals), 2nd undersize is marked 0,50. The system with color codes for different tolerance groups of bearings can be found for many motorcycle and automotive engines. A pity Porsche do not have a separate part number for each type of color coded bearing (blue, yellow and red). The only way to know you are right is to carefully measure the crankshaft journals and bearings before installation.
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Old 02-15-2018, 02:01 PM
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Originally Posted by Snark Shark View Post
When I pulled my old bearings, they were all stamped "STD". Does that mean they were all yellows? I believe my engine was rebuilt before I owned it, so the old bearings might have been non-Porsche branded.

No. "Standard" is a generic term, in this case. There are still the three subgroups of blue, yellow, and red, which are not stamped in, but painted on.
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Old 02-15-2018, 02:04 PM
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Originally Posted by Strosek Ultra View Post
STD means standard size, 1st undersize is marked 0,25 (0,25mm smaller journals), 2nd undersize is marked 0,50. The system with color codes for different tolerance groups of bearings can be found for many motorcycle and automotive engines. A pity Porsche do not have a separate part number for each type of color coded bearing (blue, yellow and red). The only way to know you are right is to carefully measure the crankshaft journals and bearings before installation.
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Porsche originally did have the subgroups, for the 928 engines....and for all the other engines they built.

Motorsports has the different sizes for the race engines available.
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Old 02-15-2018, 04:08 PM
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Originally Posted by ptuomov View Post
Per a conversation with the bearing manufacturer about a decade ago, the Glyco and "Porsche branded" bearings were, at least at the time, bi-metal bearings. The bonding layer coloration is an interesting detail about these bi-metal aluminum-steel bearings.

I agree with Greg that the rod bearing clearance being correct is critical.

Factory tolerance range mid point is likely to work well for most cases that even remotely resemble the factory stock setup. Bottom end of the range probably not so much.

If you use higher viscosity oil, you may want to have a clearance a little higher than that to keep the oil flow up to cool the bearing.

If the engine sees very high compression gas loads, then increasing the bearing clearance is not beneficial. Larger clearance will concentrate the load on a smaller area of the bearing, and that will wear out the bearing quicker. For high compression loads, you want the smallest clearance that you can get away with while still flowing enough oil to cool the bearing and not overheating the oil.

For high rpms and resulting high tension loads, the situation is different. The load on the rod big end bottom part is higher, sure, but the main issue is the deformation of the bore big end. At high rpms, the bearing grabs the journal from the sides. Smaller the clearance, greater the risk of this. The best answer is greater eccentricity of the bearing shells. As long as you stick with bearings made for Porsche 928, by my knowledge you've got no eccentricity options, so that's a dead end. So the remaining option is to increase the bearing clearance such that even with deformation at TDC the bearing doesn't grab. With the bigger leak, you now need higher viscosity oil and even with that you can wear out the bearing quickly as the larger clearance results in a higher peak pressure in the bearing. It's still better than having the bearing grab the journal at the parting lines. Once you go thru that door, going to a harder bearing (such as those Clevite race bearings for Toyota Supra) is the logical next step.

This is why forced induction is so nice on the 928 engine. You don't need the extra rpms, so you can keep the clearances about stock. Stock or near stock clearances combined with great oil cooling and high viscosity oil give fantastic ability to hold up against compression loads. And the compression loads won't cause the rod bearing to grab the journal at the parting line. So you don't need higher eccentricity or higher clearances to prevent that. It just runs, provided that you can get the oil to the bearings.
Bearing clearance is very dependent upon what oil is being used. These engines were designed to use 20/50 weight engine oil, unless they were in cold climates.

The factory specification for bearing clearance is .0013" to .0036". I agree that the middle of this range would be a great target....for the designed oil viscosity.

Getting anywhere near the lower limit would require some very gentle break-in.

It is common to see the upper limit in older engines, without any obvious issues.....one would not know that there was .0035" of bearing clearance, unless the engine was apart and physically being measured. Of course, that statement is true only if the engine was being operated with the proper weight oil.....someone dumps in some 5-30 weight oil and the oil pressure idiot light will most likely come on at idle on a warm day.
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