Rod Bearing Controversy

[911pcars]

Originally Posted by Europeanmotors

I picked up a set of rod bearings (one blew and tore up my crank and one of my cams on my 3.2) from the host like 5 years ago, 20,000 miles in and no complaints yet. I wonder who the host sourced these rod bearings from.

Spun rod bearings happen more frequently than we'd like due to a number of reasons. The question is how/why is this attributed to the manufacturer vs builder vs owner/driver?.

Not sure why a coating is needed on bearing surfaces. I can see some benefit for cold starts, but I don't see any thin coating like this lasting and/or reducing significant wear after a few hundred miles (if that). And let's hope the coating is benign once wiped/worn off the bearing surface.

Sherwood

PS: I should add:
- There are several dry film lubricants. Not all are equivalent.
- Some dry film coatings may also provide journal protection if normal oil pressure is lost. It's a temporary solution to avoid major damage until the engine can be shut down

[tocobill]

I would like to think that the coatings are for the initial break in. During that time the coating would end up working the material into the bearing to retain some of these properties. Thats why I got my bearings coated plus other engine builder recommendations. I pulled this tid-bit off the net via HotRod Mag ....

"In the engine's bottom end, dry-film lubricants are usually applied to piston skirts, engine bearings, and the main and rod journals. On the top side, dry-film lubricants are sometimes applied to valve stems, rocker arms, pushrods, lifters, valvesprings, and cam lobes. Under heavy loads, moly-based dry-film lubricants can actually hold oil on their surfaces, enhancing the integrity of the oil film between the metal parts. These same lubricants can delay metal-to-metal contact if oil pressure is lost. Although some claim that by reducing friction, lubricity coatings should offer some power gains, normally an engine's journals and bearing surfaces are separated by the oil film and never come into contact, so it's hard to see how coating the bearings would add any power under standard operating conditions. Therefore, the main advantage is really as an insurance policy, enhancing longevity and providing emergency protection if something goes wrong."

[j911brick]

Interesting read. I usually build several motors every year and I have noticed the quality of Glyco bearings have gone down hill, but I have not yet had motor fail due to a bearing issue. A couple years ago I noticed that bearing tolerances on some bearings were too tight. Now it seems like they are too loose. But the reason I ended up on this thread is because of some bearings for a 996 motor I'm rebuilding. These bearings (Glyco #71-3913 996.103.121.81) not only measured at twice the clearance I was expecting, they are also simply 1/8 inch too wide. No question they would have self destructed instantly since they were practically hanging out the side of the rod. They are also .0007 thinner than the (old) stock bearing which is a total of .0014 extra clearance. But under further investigation they seem to be an exact replacement for a 3.0 rod bearing, give/take a couple 10 thousandths.

[KTL]

The .81 bearings are for the 996 GT3 RSR if I recall correctly from memory. I have a set on my shelf that are all black and I think they are the .81 part I'll take a look when I get home.

[j911brick]

This is how the replacement bearings actually fit.


Original 996 bearing on left; replacement bearing n center. 3.0 bearing on right.

[KTL]

What 996 motor are you building? The GT3 996/997 bearings are dimensionally interchangeable with the older air-cooleds because the GT3 (and twin turbo) engine case/bottom end is based on the air cooled case. The run of the mill 996 engines are completely different in pretty much every way with no interchange of anything.

[Evan Fullerton]

While you are correct that the run of the mill 996 engines are completely different (m96/m97 engines), it is reasonably common practice to use narrowed GT3 rod bearings in m96/m97 engine rebuilds. Wrightwood racing can supply them.

[j911brick]

Originally Posted by KTL

What 996 motor are you building? The GT3 996/997 bearings are dimensionally interchangeable with the older air-cooleds because the GT3 (and twin turbo) engine case/bottom end is based on the air cooled case. The run of the mill 996 engines are completely different in pretty much every way with no interchange of anything.

Its a regular 966 engine. The Glyco label on the box says these are the correct bearings. The part number on the box matches the part number on the bearings.

[j911brick]

Originally Posted by Evan Fullerton

While you are correct that the run of the mill 996 engines are completely different (m96/m97 engines), it is reasonably common practice to use narrowed GT3 rod bearings in m96/m97 engine rebuilds. Wrightwood racing can supply them.

The running clearance would still be too great.

[Evan Fullerton]

Is it out side of factory specs? Sure, is that what some race motor builders are after, seems to be.

[j911brick]

Originally Posted by Evan Fullerton

Is it out side of factory specs? Sure, is that what some race motor builders are after, seems to be.

There is no published factory specs for M96 motors. Based on my experience the factory runs about .001 to ,0015". I set up air cooled motors at .002 to .0022" These bearings would run about .003". I like .0018 to .002 for the M96 motors.

[Evan Fullerton]

I'm not necessarily advocating the practice, simply stating that is being done.

[VFR750]

Do GT3 rod bearing come loose? Or in individual boxes?

GT-3 bearings are just excellent. These come in 3 sizes so make sure you order the right ones after measuring everything. Your machinist should be capable of resizing the rods to fit.

[KTL]

They come loose in individual plastic bags (heavy mil thickness for protection). I don't know how to obtain the other two sizes mentioned to adjust clearances as desired. I only know of the one part number I mentioned.

[cornernfool]

I just recieved my rod bearings for my 3.0 rebuild. They are Glyco with a part number of 996-103-121-81. Is the -81 bearing different from the -94? Thanks.

[KTL]

-81 is the "race" version. -94 is the std GT3 bearing shell

Porsche 996 (1999-2005) - Main Engine Components - Page 3

[cornernfool]

Kevin, whats the difference? Is the -81 ok to run in a street car? Thanks.

[KTL]

Haven't had the opportunity yet to handle the -81 shells. I suspect they allow for a teeny bit more of clearance. Can you post a picture? There were some comments I recalled from Mike Bruns awhile back that always stuck with me, as he builds high dollar engines and can't compromise.

3.0L stock rods & bolts

[cornernfool]

Kevin, here are some pics but they are lousy pics. Tried with both of my cameras with the same outcome. Let me know exactly what you are after and I will try again.

[KTL]

Mike,

I take back what I said about the -81 shells. I do have a set of them on the shelf. I got them from a guy selling them "cheap" for $100. They were in the same box as you show and same makeup of bearing- black bearing surface. Unfortunately I haven't had a chance to measure them. I was a bit disappointed in them when I received them (understatement) because they were just tossed in the box and that caused them to be all nicked up on the bearing surfaces.

I'll throw a couple of -94 shells in a rod (bolts stretched to spec) and do the same with the -81 shells to see what difference in size I measure with the bore gauge.