"In my V10 Maintenance Series, I compile various entries touching on maintenance across the E60 chassis. Evidently, the majority of these touch on the S85B50 and its SMG3 gearbox.

In this entry, I document my rod bearing service on my original S85B50 engine (001). I briefly lay the fundamentals to bearings and discuss the rational for my bearing & bolts selection, along with the signature picture."
- Matt

I'm skipping this debate.

After a decade on BMW M groups and forums, I've found the rod bearing dilemma to be excessively religious in nature. Many deities exist: BMW M, Blackstone Labs, BearingWikis and YOLOs.  

Ultimately, there are two questions you should ask yourself when considering rod bearing service: 

1. Do you believe the Genuine BMW rod bearings were specified with too little clearances? 
2. Do you believe the aftermarket has proven its ability to deliver an improved solution?  

If you want to learn the in and outs of S65 and S85 rod bearing science, it starts here with Robert Collins' oil flow analysis.

I trust in Science. 

Science answers "very likely" whether your Genuine BMW rod bearings will catastrophically fail within the lifetime of your engine. The bearings surface will degrade and wear down to metals that will make contact with your crankshaft. 

My crankshaft's journals were looking good as we removed the original bearings. Some are unfortunately scratched by previously worn rod bearings. Some can be polished, others are beyond saving. 

Flow matters.

Increased bearing clearance reduces the time to achieve optimal oil pressure & oil flow to lubricate the bearings. This in itself is a huge benefit to reduce bearing wear and increase bearing longevity.

The S85 has a massive main oil pump in the oil main, driven by the VANOS high pressure pump off the crankshaft. There are also scavenger pumps on each side of the oil pan to help move oil to and from the head. 

Material matters. 

The original BMW bearings used in the S85 from 2006 to 2010 did have something good going for them: they use a tri-metal lead and copper composition. They were designated as 088/089 bearings. 

This combination is softer on the crankshaft's journals, and reduce the propensity for scratches. 

In 2010, Europe legislated the removal of copper in automotive bearings for environmental purposes. BMW M had to switch to a Trimetal aluminum composition and were designated as 702/703 bearings. 

Unfortunately, aluminum is harder than copper and lead. The tendencies for bearings to scratch the crankshaft's journals are increased. Evaluating wear on the bearings is also more difficult due to the absence of visible wear. 

The bearings on my 2012 M3 V8 are aluminum. This is how they were at +/- 120,000km. 

Other components matter. 

For the benefits on briefness, I am skipping the conversation about rod bearing bolts and whether the main bearings are problematic as well. There are plenty of arguments about bolt eccentricity and torque procedures during installation. 

I chose ACL [...]

Various aftermarket companies have delivered on increased clearance rod bearing and matching bolt solutions: I opted for the ACL custom clearance bearings.

The ACL custom clearance bearings are bearings mix and matched amongst the ACL lineup of S65 and S85 rod bearings. The goal is to achieve the target increased clearances over the original configuration. 

The bearings were clearly marked as lower and upper shells.

The ACL bearings are also made using the original 088/089 tri-metal lead and copper composition. 

ACL is made in Australia and provides an aftermarket solution, I assume they are not subject to European mandates for OEM manufacturers. 

Each shell as a small tab designed to fit with the OEM connecting rods. Make sure to inspect each shell for any potential transport damage or manufacturing defect.

Manufacturing bearings is a dirty business. Do not expect these to look mint and homogenous. 

I had planned on using the ARP-2000 bolts: they are a step up in strength over OEM, can be re-used and require a standard torquing procedures with 45 lbs ft. I had my bolts, and my lube ready. 

The ARP-2000 bolts are custom machined to length for the S65 and S85 connecting rods.

[...] and Genuine BMW connecting rod bolts. 

My shop likes to use things it knows - AH! Sounds familiar? Phil @ Innov is part of shops that prefer putting in Genuine BMW hardware over aftermarket, even warrantying the engine. 

We made a compromise: he'd put in the ACL bearings but he'd use Original BMW bolts. They require a 3 step torquing procedure using 20Nm of torque and 130 degrees. 

I was just starting to get my cars serviced with Phil and I sought to build a relationship over presenting Wiki pages. You pick your battles.

The main event. 

With the theory and unboxing out of the way. It's down to business. The mechs at InnovAuto have done dozens of these, including various of their own cars. The speed at which all that and then some was removed from the car was blistering quick. 

The front wheels, subframe, skidplate, undertray and exhaust all came off in a flash. 

With everything out of the way, the pan was off and voila: the VANOS pump was left hanging and the crankshaft was in full display. 

Place your bets. 

My S85B50 had around 70,000miles / 120,000km when this service was performed. It had been used in Winter conditions in Alberta, using the specified 10W60 oil. It had been well maintained under BMW's original and extended warranty. 

Shops around the globe all use a variant of a table to lay out the OEM shells per cylinder # and position. It does for impressive pictures and helps you keep track of progress, along with probable diagnostics across the engine. There are 10 cylinders, and 20 shells in total. It adds up!

Starting with cylinder 1, it was already looking weird. The top shell was worn heavily to the copper layer, while the lower shell was still on its first layer. 

With the first few cylinders done, we were already seeing a pattern emerge. 

The top shells on cylinders 1-3-5-7-9 were also excessively worn in absolute and relative terms to the other shells. 

Cylinder 2-4-7-8-10 were in better shape, though wear was still happening. 

With taps on our back, we told ourselves we had saved a V10. I don't think this would have seen another Winter - but it's merely projection as I ended up blowing the V10 for entirely different reasons :)

While you're-in-there.

For a rare occasion, we didn't perform other maintenance while the pan was down. The VANOS high pressure oil line had been recently maintained and we didn't bother upgrading to the new oil squirters.

Time was a limited factor as this car was my Winter daily driver back then. 

TL;DR: Change your rod bearings.

Do your research, and buy the brand that meets your own thresholds and requirements. 

If I was given a choice to fit ARP bolts, I would have without hesitation. 

You can source the S85 ACL custom rod bearings & bolt kit year long here.