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Intermediate shaft bearing weirdness

I've got my 78 930 case apart. This is an engine that had the bearings replaced 630 miles ago. I find the intermediate shaft bearings are strangely worn -- the front one on the left case is down to the brass in a couple of zones and the rear on on the right side of the case is also down to the brass in one area. But the opposing bearing halves (right front and left rear) look fine. When I disassembled the engine for the original rebuild 630 miles ago, I don't recall any strange wear on these bearings, though I did replace them with new bearings. At that time, the intermediate shaft measured out correctly and was reused without any change (though no alignment checks were done that I'm aware of). Any idea what sort of a problem I have and what should be done about it?

Also, note the fretting (dark discoloration) on the main bearing webs where they contact each other near the case through bolts. Is this an issue? Would shuffle pinning alleviate this or is it not an issue?

Left side:


Right side:
Old 02-22-2005, 06:23 PM
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The nature of the chain tension on the shaft causes only those two bearing shells to wear but if all was working well the shaft should not wear through to the copper under the babbit.

If the case was properly torqued I would not expect a 930 case to fret like this.

What is the reason for breaking down the motor again after only 650 miles?

Judging by the thick white sealant everywhere I would suspect the previous rebuild was not done well.
Old 02-22-2005, 07:13 PM
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The reason for disassembling the engine with so few miles was because I'm going up a notch with EFI and I want to make sure everything is done properly. I had the heads off anyway for the change and it was no big deal to split the case and look inside. I'm glad I did...

You're right. The wear would correspond to loads from chain tension. But why would chain loads do this? Could the tension have been higher than normal? Not likely -- can you think of what could cause the tension to be higher than normal? Standard Carrera tensioners were used. Everything in the cam train seemed alright and was new. Admittedly, the clearances of the intermediate shaft and new bearings were not checked, and maybe they should have been. On the next round, they will be...

I've posted another view of the forward left intermediate shaft bearing shell. Note that it's scuffed through the babbit primarily on one side of the oil hole but also a bit on the other side, but not at all directly in line with the oil hole. Any ideas why that may be?

While I'm inside the engine, I'm likely to have the crank cross drilled, make sure the spigot decks are flat and even, and verify alignment of the bores, including the intermediate shaft. And I'm still considering shuffle pinning.

The engine was assembled carefully 630 miles ago. Attention to detail was good, but not to the level of measuring all the clearances in the engine by torquing the case together and verifying with a bore gauge. The engine ran perfectly, made no strange sounds, didn't leak a drop, and produced 526 hp at the crank. This time around, it will be done more fastidiously, with "blueprint" attention to all the details. I wasn't expecting to find this artifact, though, so I want to be sure to address it properly.

The white sealant is the choice of an engine builder who prefers it to Loctite 574 -- it was chosen very conciously by a pro who has plenty of data to back up this choice (Two pros, actually) I'm very aware of the 574 product and why it's used. I trust that other opinions are valid as well, especially when backed up by the kind of experience these guys have. But I don't want to debate the merits of different sealants here.

The fretting is interesting. The case was torqued absolutely correctly -- that is *not* the reason for the fretting. I understand fretting is not that uncommon -- that's why I posted the thread about case and crank mods further down this list. The majority of people who responded seemed to think shuffle pinning isn't necessary on a turbo motor. Could this change some minds? Here's a closer look at the fretting, which, by the way, you can't really feel with your finger, but it certainly shows discoloration.

[img]
Old 02-22-2005, 07:59 PM
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Was the white sealant used on the main bearing webs and is it a pliable sealant?

If it was used there its possible it acted as a lubricant film under load and caused the 'fretting'.

As far as there being a little wear on the other intermediate bearing shells its due to the fact that the cam chains also pull the shaft up in additition to the sides. Uneven wear across the bearings could indicate that the bearing shells were not even in the bore when the case was assembled assuming the bores are not damaged and there was no debris under the shells.

Last edited by 350HP930; 02-22-2005 at 08:43 PM..
Old 02-22-2005, 08:30 PM
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Thanks for your thoughts.

No, there was no sealant used on the main bearing webs -- only on the case perimeter where it is necessary and proper. Any other ideas on the fretting?

I don't know how the intermediate bearing shells could have been uneven in any way within their bores. They're keyed just like any other bearings, and they were properly seated upon disassembly (and it was a sanitary installation too -- with little chance of contamination under the bearing shells). As I said, I didn't check the bore during a dry preassembly to verify the bearing clearance. I will probably do that this time. But what could cause this wear pattern? I certainly don't want to rule out an assembly error, a mistake could have been made. If so, I just don't want to make it again when I reassemble this time.
Old 02-22-2005, 10:56 PM
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Rob,
I have seen this bearing wear many,many times,in very low milage and in very high milage,the culprit seem to fall in some extent into the uneven loads that the chain drive may impose into the shaft,also the loads imposed by the oil pump may have something to do with it, either way I will not be to much concerned about it,that bearing with the copper showing should last lots of miles still. On the other topic, you should use Loctite 574 on ---all---contact surfaces of the case,including the bearing saddles, if you don't you will get I slight shuffling of the
case, (the torque applied to the bolts will change on those areas)causing those shadowy marks on the case saddles.,cheers, Antonio.
Old 02-24-2005, 06:20 PM
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white sealant could be Dow 730, that is what I used, see Bruce Andersons book, and I don't think it is reccomended that it is put on the center webs,

Jim
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Old 02-24-2005, 06:39 PM
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Antonio,

Thank you for your thoughts.

I just spoke with a very experienced Porsche engine builder and machinist and he also said that it's not uncommon to see the brass through the silvery coating of the intermediate shaft bearing shells. I hesitate to call that silvery coating babbit -- I don't know what it is, but what he said was that it was "flash" coated and extremely thin -- much, much thinner than the main or rod bearing shells, where evidence of brass showing would be very bad indeed. So, I guess I won't worry too much. I haven't seen enough of these engines apart to have any perspective on this, but it sure didn't look good (to me) when I first observed it. I will certainly be replacing those bearings, and I'll carefully check everything out to make sure it's up to spec before the engine gets buttoned up. Funny thing, though, when I disassembled this engine for the first time 600 miles ago (with probably 100K miles on the engine) I don't recall seeing this kind of wear on the original bearings. It makes me wonder if different bearing manufacturers make their products differently. I may go to the Porsche dealer for the intermediate bearing shells this time around.

Regarding fretting, he didn't seem concerned about the choice of sealant. He said that when he sees fretting, it's (usually) either because of some internal problem with the engine (like an imbalance) or detonation. I didn't own the engine for the first 100K, so I don't know the history of the engine. It's very possible that the evidence of fretting existed at that time and that I didn't notice it. I'm just looking at things more carefully now.

I will be re-evaluating what sealant to use this time. There are lots of opinions about this. I've used 574 in the past, and I may do it again. Or I may use Threebond 1104 and 1211, as recommended by several people in another thread on Pelican on case sealing options (in 2002). I will look into whether I should apply the sealant to the bearing webs themselves. That's something I've never heard of before.
Old 02-24-2005, 06:44 PM
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Quote:
Originally posted by Rob 930
I will be re-evaluating what sealant to use this time. There are lots of opinions about this. I've used 574 in the past, and I may do it again. Or I may use Threebond 1104 and 1211, as recommended by several people in another thread on Pelican on case sealing options (in 2002). I will look into whether I should apply the sealant to the bearing webs themselves. That's something I've never heard of before.
See this Case Sealing Opinions Wanted thread where Henry Schmidt writes:
Quote:
For many years we used 574 Loctite but now ( for the last 4 years) we use ThreeBond 1104 and 1211 for perimeter sealing on the case ( 1211 on # 8 main bearing) Where the case joins at the through bolts we use 574. For the the cam towers we use 1104 and where there are gaskets we use a very thin coat (very thin mind you) of 574. On the cylinder base gaskets we use Curil T. Four glues equal no leaks. Over kill ? MAYBE No leaks GUARANTEED.
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Old 02-24-2005, 07:08 PM
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Rob,
Just take a look at the factory manuals, and see where they (factory)recomends appling the stuff.(574),cheers,Antonio.
Old 02-25-2005, 09:25 AM
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Thanks, I will look at the factory manual, which I haven't done recently. From the way you presented it, I assume the factory manual suggests that sealant be applied to the bearing webs as well. I wasn't aware of that. And I haven't seen it done before, by anyone. Even on the original diassembly, there was no evidence of sealant on the webs that I can recall, though there was (and still is) evidence of a brown sealant that the factory applied to the case perimeter sealing surfaces. Now that we've opened this whole topic of sealant and how to use it (and my mind is more at ease about the intermediate shaft bearing problem) I'd be interested in pusuing it.

Antonio, are you suggesting that the presence of sealant actually restricts movement of the bearing webs and prevents fretting, or do you think it cushions and protects the two surfaces by separating them with a film of material so they don't fret? I suppose the use of sealant on the bearing webs may help keep oil from leaking in around the through bolt holes, which are oil galleries. But I would think that if that were a concern, the factory would have put o-rings there. Furthermore, I would think that to have sealant too close to those holes, such that excess sealant could ooze into the oil galleries, could be risky, even if it's an anerobic like 574.

Chris, thanks for the thread about sealants. Very interesting. Sounds like Henry Schmidt uses a complex scheme that includes 574 on the bearing webs.

On a side note, I heard that an old trick for sealing the case perimeter (which I've never tried and won't use) is to lay a very thin silk thread all around the perimeter sealing surface, in the middle of the gasket surface. Apparently, silk thread (and it must be silk) crushes but maintains an impermeable barrier that aids the sealing qualities of the sealant. I think I heard of this being used on VW cases 20 years or more ago. I'm not going to do this, but has anyone else ever heard of this?
Old 02-25-2005, 09:57 AM
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Rob,
The correct way for appling the stuff,(and I have been doing this for 20 yrs.!),is exactly as the factory does it, use a tiny roller(foam type),deep it on 574, and apply it to all the mating surfaces,that's it,no more no less!, and forget about it,and sleep well,cheers,Antonio.
Old 02-25-2005, 03:29 PM
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I wouldn't be afraid to keep using the white stuff to seal the case but I would try to use less of it. You have quite abit squishing out into the case. Also it should not be used all the way around the number 8 main bearing like you did. This area is sealed with the O-ring. The bearing can and does move and the sealant is not meant for a moving area.

With the amount of power you are looking for I'd shuffle pin. The shop I use did this for me an I think it'll cut down or eliminate the "fretting". Only draw back is the you'll probably need to have the case align bored to the next bearing size and the over size bearings are expensive.

-Andy
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Old 02-25-2005, 10:00 PM
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Shuffle pinning should not require line boring.
Old 02-26-2005, 04:38 AM
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It depends on who does it. My shop won't do it without line boring afterward. It's tough to get the pins so exact that it doesn't offset the bores by a couple at least a fraction.

-Andy
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Old 02-27-2005, 07:44 PM
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Quote:
Originally posted by ANTONIO
Rob,
...On the other topic, you should use Loctite 574 on ---all---contact surfaces of the case,including the bearing saddles, if you don't you will get I slight shuffling of the
case...

Are you implying in this sentence that the loctite will somehow keep the case halves from shuffling? I'm really skeptical of that statement!
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Old 02-27-2005, 10:48 PM
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Yes.,Antonio.
Old 02-28-2005, 03:33 AM
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I'd be really suprised that locite was strong enough to with stand the shear force.... what lead you to this conclusion?
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Old 02-28-2005, 07:24 AM
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