Is it OK to re-use main bearings?
 

Hi guys,
I decided to split the case to check my main bearings after I was shocked by 1 destroyed conrod bearing, I had a relief when I saw the bearings in what appears to be good conditions and more importantly good crankshaft journals as well, please have a look at what the bearings appeared to be and let me know if its fine to re-use them? .. I'm saying that because I have ordered a set of main bearings from Rosepassion.com and they send me a box that has no brand name on it, even the bearing shells themselves have no marking at all except the part number so I thought maybe its better to keep the original Porsche bearings.

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http://forums.pelicanparts.com/uploa...1708238137.jpg
http://forums.pelicanparts.com/uploa...1708238137.jpg
http://forums.pelicanparts.com/uploa...1708238137.jpg
http://forums.pelicanparts.com/uploa...1708238137.jpg
http://forums.pelicanparts.com/uploa...1708238137.jpg

Yes.

I have now come to prefer excellent used bearings, after checking them with Plastigage. Certain brands of recent bearings have had quality problems.

It is hard to assess bearing condition with just pictures, however, your "7D" bearing looks like it has some mark above the "D" and some of the others look like the surface is not smooth. That might just be oil or grease, Make sure they are perfectly smooth and the plastigage ends up between .01mm and .07mm. I think .05 is ideal.

If you have any doubts, make sure a COMPETENT machine shop looks at it and measures the clearance.

Yes, if your budget dictates.
They (good used bearings) are excellent candidates for dry film coatings.

Hello Henry, its not about budget, its about will the available now bearings be better than a good used Porsche ones, I heard a lot of not so good stories about the newer Glyco bearings being not so consistent in terms of clearances. Unfortunately where I live it is impossible to find someone who can dry-film bearings.

Inconsistent Glyco main bearing was during a transitional phase when Federal Mogal took over Glyco.
We aren't seeing any of those old issue at this time. The internet never forgets and generally never gets updated.

Also, I thought the issues were mostly with rod bearings anyway?

This seems an opportune time to ask. A friend pulled apart an engine that was rebuilt and run maybe a thousand miles or so.. The mains have these marks on them.
What does this indicate?

http://forums.pelicanparts.com/uploa...1708290889.jpg

That looks like a set of the transitional main bearings. They looked like that when they were installed and the builder just missed it. When the white metal was applied, it disguised the surface inconsistencies.

Thanks Henry.
The engine was disassembled for other reasons, but would the mains above, have eventually caused some sort of detrimental running condition? I suspect there are hundreds of engines driving around with these transitional bearings in them and one would never know, unless they pulled it apart.

Obviously, it's not good.

What was the approx. time frame of the transitional period? I'm about to assemble an engine with "regular" Glyco bearings (seem to have a date code of 5/2019 and from Poland in case that matters), and now I'm wondering if they are OK.

As ugly as those bearings look, they probably wouldn't have caused a problem.
Simply polishing them gently, with a 600 grit wet/dry should make them usable.
A light polish and dry film coating and they are better than new.
Remember we're dealing with a 6 cylinder engine with 8 main bearing. A design that is really over the top.
I remember some Mercedes 6 cylinder engine with 3 main bearings.

I would use the old trick of polishing them with a kitchen paper towel. It's really just cosmetic.

You can dry film them yourself,

There can be a greater benefit to light sanding than simple cosmetics.
We have been sanding air-cooled bearing since the early 70s as a way to increase clearances on racing engines. It seems there can be a benefit to greater clearances combined with higher oil pressures. SOP on Chevy's I think?

The Nissan GTP engines I worked with went the other direction. They ran crazy tight clearances. The problem was, you had to heat the engine before you could start it up.

Doing it properly is not as easy as it looks.
It's more that clean and spray. It take an understanding of blasting, spray and drying techniques.
Of course it can be done but paying an expert may prevent the need to start over.

A long-time member here suffered that fate not that long ago. It is a nauseating situation.

It is still surfacing from some stock floating around, apparently.

You can get Chevy bearings in standard sizes and 0.001" over or under bearings and would use one of each half shell to get the right clearances. But Chevy bearings are a dime a dozen and available everywhere. Porsche, not so much so. Bearing clearances are determined (mostly) by the intended oil viscosity. Thicker oil requires added bearing clearances. In Cup, we were down to 0W-9 engine oil. Bearing clearances were just under a thousandth. Oil pressure was actually lower than Porsche engines.

I too have sanded the backside of the bearings to get the correct tolerances when no other option was available. You gotta do what you gotta do.

I'd send the bearings out for coating too. Leave it to the professionals that do it every day. No way that I'm experimenting on an engine that I care about.

Actually, the 996 GT3 bearings came in three different sizes. The crank journals are often marked with red, blue or yellow so that each journal can get the correct size. As far as I know, only the red bearing (largest clearance) are available.
As I've said before, the mains on a 7000 rpm air-cooled engine are not generally critical. proper clearance + is generally a good rule.

Henry, others, FYI: Per Smokey Yunick's book, on his NASCAR Chevy engines, he wanted .0025-.0027" on the mains. He wanted to pump extra oil through the bearings to help keep them cool at 8000 RPM.

I'll note that the Tech Specs for the 3.0 SC and 3.3 Turbo engines allows .010mm to .072mm (.0004" to .003") on the 1-7 mains; and .03mm - .088mm (.0012"- .0035"), which seems like a pretty wide tolerance spread. I don't have one for the 3.2 engine handy, but I'll bet it's the same. "Proper clearance +" is a good rule.

On pull type clutches, you see wear on the thrust surface of the rear main bearing. If trying to reuse mains check axial crankshaft clearance and compare to specs. I check during disassembly to see where I’m at. Rare, but I’ve found crank thrust flange wear from clutch riding.