OnPoint

Anybody know why the LT1 uses polymer coated main bearings? Is this something new - I don't recall the LS engines having such a coating on the bearings?

Just wondering what the benefits/hazards of such a coating are.

Unreal

Late LS motors have it too. EU regulations and RoHS don't allow the leaded bearings so they switched around 2011-2012 to polymer coated bearings.

Benefits, allows them to sell cars in EU. Hazards, they don't work as well.

PowerLabs

Polymeric coatings have much better lubricity in the absence of oil than metallic ones. Piston skirts have had polymer coatings for over a decade now.

Shaka

Bearing coatings haven't used lead for 20 years in autos. Eco-Babbitt alloy coatings are 90%Sn/7%Zn/3%Cu.
F1 and Indy cars have used poly coatings for nearly 30 years on all metal to metal components. Poly coatings are limited in terms of temperature resistance and corrosion. Pluses are electrically non-conductive and have low thermal conductivity compared to metals. They also have low coefficients of friction. Modern lubricants allow extended life.

http://www.pvd-coatings.co.uk/applications/automotive/

robertf97

I concur, we were switching from DU to DP4 in the late 90's in rack and pinion steering gears and the DP4 had less friction. I like having the polymer coated bearings

Unreal

Good to know. I was way wrong. Learn something new every day. I love this place.

robertf97

Here is a write up on the advantages of the polymer coated bearing. http://www.jhb.com.tw/html/ggbpdf/1561.pdf

OnPoint

Sounds like, on balance, they may be better than non-coated bearings.

OnPoint

Hopefully these are the cause of the LT1s that have gone down/thrown rods.

Old Yellow

I'm way more concerned about the thrust bearing failures that we're seeing.

The new 180 degree thrust bearing on number 3 main is a big concern for me.

Does anyone have insight as to why they made the switch from a 360 degree bearing that has been the industry standard for 100 years: with a history of very few failures for 100,000 mile engines .

This new bearing looks like a conventional main thrust bearing, but only has the thrust flanges on the half that is in the block. the #3 main bearing that resides in the cap does not have a flange to resist the thrust. This reduces the thrust bearing surface by 50%.

Any for & aft movement of the crankshaft will place stress on the connecting rods that can result in broken rods that will poke a hole in the block, similar to the few blown engines that have been reported here.

This reduced thrust surface absorption area can also account for the engine replacements due to improper installation of the torque tube causing damage to the thrust bearing.

I'm looking for a reasonable explanation for the switch to this 180 degree bearing style. All I can come up with is reduced friction to free up horsepower, but it's coming with a reliability price tag.

Ed

OnPoint

Ed, you raise a good question. It would be good know the thinking behind that decision.

Just today there is a new thread in the general section about engine damage on a Stingray. In that thread, even at this early point in the thread, there are 3 posters stating their engines went down with bearing failure.

Not sure what is going on here, whether it's isolated or a more general risk across the fleet, but we're seeing some troubling failures.


https://www.corvetteforum.com/forums...ne-damage.html

dvandentop

in for later

JeffInDFW

Sorry I'm getting in on this late, but I just stumbled across this thread. I do have something important to add here.

Unreal is correct.

2012 and 2013 Z06 on this very forum are seeing a LOT of main bearing failures. Here is why:

"Until the 2012 model year, bearings of traditional, "tri-metal" (steel backing, bronze second layer and a top layer of lead) construction were used in the LS7's #1, 2, 4 and 5 main bearing positions and in the connecting rods. European Union legislation enacted in 2011 prohibits the import of products containing lead, so the main and con rod bearing designs were changed. Lead was replaced with a synthetic polymer making a "bi-metal-with-polymer" design. The center (#3) main bearing remains aluminum on a steel backing. According to Mark Damico, because of the firing order used on all Gen 3, 4 and 5 V8s, the second and fourth main bearings are subjected to the highest loads. After a long period in service, if an LS7's bearings are going to wear, it'll be the #2 and #4 mains which show it, first. The two end bearings may see a lesser level of wear due to (#1) the accessory drive or (#5) the flywheel. The center main experiences the least vertical load and while it is an increased-eccentricity design, it's never required a lead overlay nor polymer coating."

Old Yellow

This is some good information. Do you know the original source?

The number 3 main is the only main bearing that absorbs horizontal (thrust) loads placed on the crankshaft. This is the main bearing that has shown to be prone to failure on it's thrust face of LT1s.

I think I know what increased-eccentricity means, but how does it apply to the #3 main bearing. Does it mean that the diameter is greater on a vertical, or a horizontal plane?

Thanks again for the info, but I'm not familiar with who Mark Damico is.

Ed

JeffInDFW

Hi Ed. That clip is from an article that was written by Hib Halverson (who is a member on this very forum). He wrote an incredible article on the LS7 a few years back, and it looks like he expanded it when that engine became available for the Z/28.

Here is the article as it pertains to the Z/28 (and MAY have updated info):
http://www.camarohomepage.com/ls7/

Here is the original article:
http://www.corvetteactioncenter.com/.../#.UZM6ZqKTh8E

If you do a search on this very forum, you will find many threads in the forums of low mileage engine failures from main bearings on 2012 and 2013 cars. It has impacted quite a few of the 427 convertibles. The concensus has been that GM claims to have fixed the LS7 heads after 2011 (which we now suspect is NOT the case), but replaced that problem with failing bearings due to switching away from lead.

The only reason I am up on these discussions is because I have wanted to buy a Z06 for 3-4 years now. I have followed the dropped exhaust valve saga closely, and then watched as this new problem unfolded. I am a lifelong GM fan (I'm 47), and turn my own wrenches. I've got to tell you, GM has absolutely, totally screwed over the LS7 owners and has really shaken my faith in my favorite automaker. Come on, I've hotrodded small blocks for decades, dropped valves are -very- rare. Yet with the LS7 it happens to bone stock cars with insanely low miles in alarming numbers. It seems like over half of the people who pay to have their valve guides checked come back well out of clearance. Yet GM did not say anything until 2011, and then only said "a small number of cars were affected". They put out a bulletin on the subject to dealers that ridiculed owners who were concerned, claiming it was "internet hysteria". Go spend some time in the Z06 forum and judge for yourself.

My point? After seeing all of that unfold, I no longer have confidence in GM to "do the right thing". They used to. But they have left those guys swinging in the wind for the last 6 years with no relief in sight.

HC Mechanic

The mains surface material won't make any difference to the rods integrity - it will only a change if there is an oil clearance differential/problem between the rod/main.

ticat928

Diameter is greater on the horizontal plane, i.e. the bearing shell is the thickest at the 90-degree mark and thinest at 0 and 180-degree mark.

The idea being that under high RPM the bearing cap is under tension creating an oval cross section containing the bearing shells, this leads to the bearing shells being pinched in at cap parting line.
The eccentricity avoids pinching of the main journal and loss of the hydrodynamic fluid layer supporting the crank.

The eccentricity at rod journals is up to four times greater due to greater stretch at big end of rod thus more of a pinching problem

ticat928

http://www.corvetteactioncenter.com/...-engine14.html

Above is how GM selected exhaust valves

http://www.superchevy.com/how-to/140...#ixzz3G441hLj9

A study of the LS7 valve train dynamics

https://www.corvetteforum.com/forums...tte-forum.html

Above: Better cams and valve train components being tested.

I still have not found a link of my past reading that 2-pc valves cannot be 100% inspected, many get shipped with hidden flaws due to gun drilling and friction welding. I will post it when I find it.

I wonder if GM knew this?

ticat928

The LS7 has been using the polymer coated bearings since 2006, no problems even with much higher bearing loads due to longer stroke and higher RPM