Which Rod Bearings to Use for the BMW S65 and S85: WPC, BE, or ACL?
Over the years there has been a lot of discussion on what might be causing premature bearing failure on the S85 engine in the E60 M5 and the S65 engine in the E90/92 M3. Very early on people did some measuring on bearings and found that the rod bearing clearance on stock motors was a bit tighter than BMW has used in previous M series engines, and tighter than the commonly accepted bearing clearances. There are several theories for why BMW might have intentionally done this on these engines but I won’t get into those here. Our belief is that bearing clearance may play a role in premature failure, but it’s also possible that other factors play a role. Historically many BMW S series engines have suffered premature rod bearing failure despite having industry acceptable bearing clearances. Other factors such as insufficient bearing surface area, insufficient oil pressure and excessive detonation could all play a role to accelerate bearing wear.
Unfortunately it is impossible for the aftermarket world of engine builders to determine the exact cause of why this is happening, what we do know is that regular replacement of rod bearings will prevent engine failure on these engines.
We saw some variance in crankshaft journal diameters which seemed to indicate that stock clearances were in the .0016 to .0021, depending on who you asked and what crankshaft was measured. Most engine builders would agree this is too tight and we would prefer to see closer to .0025″ for this journal which is almost exactly 2.0465″ on average. Unfortunately we also noticed that these crankshafts seemed to have about .0005″ tolerance on these journals. This amount of variance wasn’t a good match for tight clearances.
The question remains, what bearing would be best to use during these bearing replacements? Early on a lot of options didn’t exist. Since BMW bearings were the only option, we tried to open them up a bit using WPC treatment. While WPC claims their treatment doesn’t modify clearances in a significant way, we had seen in the past that on bearings we would often get increases in clearance of .0002″ to .0003″, not a significant amount but it would help.
Down the line BE bearings started selling Clevite bearings. BE was able to achieve a nominal clearance of .0024″ by using a STD bearing on the rod side and a .001″ oversize bearing on the cap side, this is common practice when building race engines to achieve ideal bearing clearances. Early on the BMW bearings with WPC treatment were still a bit cheaper than BE bearings, so many people continued to purchase WPC bearings. Unfortunately cost on Genuine BMW bearings went up considerably and BE bearings became the lowest cost solution for doing rod bearing replacements and achieving industry accepted rod bearing clearance.
In 2018 ACL began to make rod bearings for the S65 and S85 in two varieties, a STD size and an HX (.001) oversize. The cost of these bearings was considerably less than both the BE and Genuine BMW alternatives so we decided to purchase both sets and measure them. We found the STD ACL bearings to be just slightly tighter on average than Genuine BMW bearings, on average they produced about .0018″ clearance, we were not comfortable selling these to our customers or installing them in the cars we service. Similarly the HX bearing is a bit to loose producing around .0028″ clearance. We decided to implement the same solution BE did with Clevite bearings on the ACL products. We were able to achieve a similar .0024″ rod bearing clearance using this method and this has become our “go to” bearing at the the shop. The ACL bearings share many of the beneficial features that the Clevite bearings carry, with the added benefit of reduced cost. At the time of this writing the ACL bearings are less than half the cost of BE bearings and about 1/3 the cost of the Genuine BMW WPC treated bearings.
Great write up and comments. My question is if cost of the bearings is not a factor, which ones would you recommend?
What is the downside of using full HX ACL bearings ? not the custom set but the one with increased bearing clearance up to .0028″. Are you implying that this clearance will start to have diminishing returns in terms of oil retention and bearing wear ?
With bearing clearances it’s not a “more is better” situation. There is an ideal number and for this engine it’s right around .0024″. Looser than that is not ideal but yes it will run and work just fine with .0030″ clearance bearings.
Curious if there have been any updates since your original post. I keep wondering, if there isn’t data on aftermarket RB wear, who SHOULD be doing that research? You know better than I, but you would think that higher clearance aftermarket RBs SHOULD have a longer lifetime than OEM RBs (all conditions being equal). I mean, that’s why they (high clearance RBs) were created in the first place, right? To not fail the way OEM RBs do? My BE high clearance RBs have 80k on them, and all traditional precautions have been taken to extend their life—no track, only 4k+ once at operating temp, oil changes at <5k, Liqui Moly and Cera Tech. But who knows how much of a difference it all makes? I suppose Blackstone has the largest sample size of data that can be sorted by engine, RBs, engine miles, oil type, oil change frequency, and additives (though all data are owner-reported). That would be an interesting future episode on their pod.
We don’t really agree with the premise of your entire argument that rod bearing clearance is the only problem with these engines. The total surface area of the bearing is quite small and while the bearing clearance is a little tight on these bearings they aren’t far off from industry standards. Size of the bearing is relatively small though for this power level. Unless you increase the bearing size these will always be wear items.
What is the typical lifespan of the BE or are they too new to know?
What is the typical lifespan of the VANOS lines if replaced?
I just had BE installed at 70K and also the VANOS lines while they were in there to save on labor doing it after the fact at a later date. So both replaced with NEW at 70K
Vehicle now has 75K and I just did a dealer oil change to ensure proper flushing and OEM grade components.
Just curious if there is new data out there since all of these multiple choices.
Thank you in Advance.
Kevin
Hi Kevin, great and well thought out question.
There short answer is, there is no data. I don’t think anyone has tracked long term wear in the same situations, in the same cars, between each bearing option. As a general rule here, we think that replacements should last the same as the original bearings, about 70k miles or so. We recommend to our clients that rod bearings be changed every 70-75k miles as a maintenance item. I would have to give the same reply for the VANOS lines, though we don’t treat the lines as mandatory, just the bearings.
I have a 2006 e60 m5 and i have always worried about bearing failure. I am currently sitting at 118k miles and will do my bearing within the next two weeks. I have noticed botch vanos units being noisy and idle fluctuates. any help would be appreciated as I am a college student and don’t have infinite funds. thanks
Emille, thank you for reaching out. Contact us on the appointments page when you’re ready to get scheduled and we’ll get you taken care of. We can take a listen to your VANOS while your in the shop as well.