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Historical Background of S65 Bearing Issue

It was very easy to ignore the very first blown motors because most seemed to originate from highly modified cars. Very early there was one blown NA motor from a guy in Thailand who was "custom tuned" by a local tuner. His motor blew and it cost a lot of money to replace it. Tuning by a local, unknown tuner was another reason to ignore this blown motor.

But about the same time, the first few blown motors showed up on bone stock M3's as well. These cars were totally stock with very low mileage. As time went on, more and more blown motors showed up, again NA motors with low miles. It seemed if you made it past 25,000 miles, then your motor would likely survive the warranty period.

Various theories were discussed to explain this phenomena. The theories ranged from inferior oil pump design to bearing clearance issues. But nobody had any data to support one argument over the other.

Discovering S65 Bearing Issue

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The bearings shown above came from a supercharged stroker motor. The engine was 4.6L and ran a very modest 6.0 PSI boost. After 24,000 miles, the engine was disassembled in July 2010 to rebuild as a low comperssion 4.7L motor. The bearings were photographed and stashed away. I always thought the bearings looked pretty "ratty" but at the time, nobody thought anything else about it. Today these photos might be ground-zero, exhibit-one in the discussion of rod bearing clearance.

By September 2011, the engine was ready to be reassembled when the race shop made the discovery. Van Dyne Engineering in Huntington Beach called to explain that the engine couldn't be reassembled because the bearing clearance was too small. They measured as little as 0.0011 inch rod bearing clearance, which Van Dyne said would lead to oil starvation and potentially catastrophic engine failure. To confirm these measurements we gathered one more factory crankshaft, and samples of two sets of connecting rods from different engines. Van Dyne confirmed the measurements all matched, and the factory BMW clearance was dangerously too small.

Van Dyne explained that the industry standard clearance should be 0.001" clearance per inch of journal diameter. The BMW factory measurements were approximately half those of standard values. Van Dyne proposed a solution to send the crankshaft out for machining to have the journals resized to allow proper clearance. By November 2011, the machining had been completed, and the engine was ready for reassembly. During the engine assembly process, the clearances were all checked and verified to ensure the clearance problem was fixed.

The industry standard clearance Van Dyne was talking about can be found in numerous automotive engineering books, best practices, trade journals, and web articles.  Google search the topic, and tens of thousands of hits will come up, all leading to 0.001" clearance per inch of journal diameter.  Clevte, BMW's bearing manufacturer thought this subject was so important, they wrote a white paper about it, then eventually incorporated this white paper into its own chapter in their automotive bearing catalog.

Mahle-Clevite White Paper on Bearing Clearance Issues

This quote comes directly from a Mahle-Clevite white paper on bearing clearance, then I'll summarize at the end.

Using this formula will provide a safe starting point for most applications. For High Performance engines it is recommended that .0005” be added to the maximum value determined by the above calculation. The recommendation for our 2.000” shaft would be .0025” of clearance. 
...
High Performance engines on the other hand, typically employ greater bearing clearances for a number of reasons. Their higher operating speeds result in considerably higher oil temperatures and an accompanying loss in oil viscosity due to fluid film friction that increases with shaft speed. Increased clearance provides less sensitivity to shaft, block, and connecting rod deflections and the resulting misalignments that result from the higher levels of loading in these engines. Use of synthetic oils with their better flow properties
can help to reduce fluid film friction. 
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Use of these coated bearings may result in slightly less clearance than the uncoated CLEVITE 77® high performance parts for the same application. This will typically be in the range of .0005.” This is because the coating, although expected to remain in place during service, is considered to be somewhat of a sacrificial layer. Some amount of the coating will be removed during break-in and operation, resulting in a slight increase in clearance.

ere's what this means to you. If you run high horsepower and high RPM, then you need extra bearing clearance, not less of it. The coated bearings are great, but you have to size your journals for them;Here's what this means to you. If you run high horsepower and high RPM, then you need extra bearing clearance, not less of it. The coated bearings are great, but you have to size your journals for them; or you must custom make the bearings designed for the coating. If you have coated parting lines you must remove the coating in this area before using these bearings. 

Mahle-Clevite recommends adding an extra 0.0005 for good measure for high horsepower, high RPM, and coated bearing applications. To see some numbers in real life, please see the following examples.

Factory clearance: 0.00125
Factory clearance with Calico coated bearings: 0.00085 - 0.00105 (16-32% smaller).
Factory clearance with TriArmor coated bearigns: 0.00075 - 0.00095 (24-40% smaller).

Recommended clearance for mains (60mm journal):
0.00100/inch + 0.0005 = 0.00286
0.00075/inch + 0.0005 = 0.00182

Recommended clearance for rods (52mm journal):
0.00100/inch + 0.0005 = 0.00254
0.00075/inch + 0.0005 = 0.00203

So there you have it: ideal mains should be 0.0018 - 0.0029 and ideal rods should be 0.00203 - 0.00254. BMW gave you 0.00125 clearance from the factory and some may be thinking of reducing it another 16-40% with coated bearings without sizing the journals because they believe it gives them extra protection. The extra protection is true if you kept all things the same including the bearing clearance; but reducing the bearing clearance by 16-40% to get the extra protection of the coating is not recommended.

Why do the motors Blow with rod bearing failure?

BMW designed the S65 and S85 with approximately half the industry standard oil clearance.  This is a trendy move among some auto manufacturers to increase gas mileage and increase horsepower.  But when they do it, like in the Toyota Prius, the auto manufacturer also specifies a very thin oil to match the very small oil clearance.  BMW dropped the ball here and is using very small oil clearance and very thick 10W60 oil.  Many consider this its own recipe for disaster.  But here's what is believed to be happening in the S65 and S85 engines.

Engines with clearance too small, couple with oil too thick, leads to premature rod-bearing related engine failures. With clearance too small and oil too thick, the lubrication around the bearing is marginal.  Some metal-to-metal contact is made between the bearings and rod journal.  The oil and rod bearings overheat.  Some bearings seize on the crankshaft, while others will "spin" and cause a crankshaft failure.  Here's the recipe how it can happen.

Tolerance Stacking

Every manufactured part has a manufacturing tolerance range that goes along with it.  The parts are supposed to match the specification, but manufacturing variances will lead to these tolerance differences.  When one part is on its outside tolerance, it's probably no big deal.  When two parts are at the opposite of their tolerance limits, it can lead to catastophic failure as I will show.

Oil clearance is affected by three different parts, each having their own tolerance.  Each of these specifications and tolerances were either specified by BMW, or measured in the field using multiple example components to build a database of component tolerance.

  1. Connecting rod big end bore:  2.20474 [-0.00000, +0.00050] inch.  That means the connecting rod big end can vary between 2.2047 - 2.2052.  Nominal size would be 2.2049 inch.
  2. Crankshaft connecting rod journal:  2.20465 [-0.00010, +0.00030] inch.  That means the connecting rod journal diameter can vary between 2.0464 - 2.0468 inch.
  3. Connecting rod bearing:  0.07850 [-.00015, +0.00015] inch.  That means the connecting rod bearing thickness can vary between 0.07835 - 0.07865 inch.

The bearing clearance is calculated as the difference between the (connecting rod big end bore, two times the bearing thickness, and the connecting rod journal diameter.  This is typically measured with a bore gauge when the bearings are assembled in the connecting rod, and size of the connecting rod journal is subtracted from the value found on the bore gauge.

When we put all nominal parts together, here's how they stack up:

That's already dangerously too small and almost half the Clevite recommended clearance.  So let's look at what happens when we put all of the parts together at their maximum stack up (for minimum oil clearance).  Keep in mind each of these parts is within it's allowed tolerance limit and would pass inspection.  But look what happens when you put them all together.

An S65/S85 engine with only 0.00060 inch clearance is less than one-quarter the Clevite recommended clearance.  Finally let's look at what happens when we put all of the parts together at their minimum stack up (for maximum oil clearance)

At miniumum stack up, the S65/S85 engine will last a very long time, even using the very thick 10W60 oil specified by BMW.

It is believed at maximum stack up, coupled with thick 10W60 oil leads to premature engine failures.  These are the engines believed to blow with 25000 miles or less.  A few engines have blow with 6000 miles or less; and one engine blew with only 1500 miles.  The solution to this problem is increased bearing clearance.

When this problem first came to light, many people begin replacing their rod bearings.  The following pictures are evidence of oil starvation, and bearing clearnce too small.

Some Photo Evidence of Oil Starvation

Time to add some pictures to the discussion to see what a bearing looks like due to lack of clearance. The first few pictures come straight from Mahle-Clevite. Thanks to BMRLVR for posting the B&W version of the Clevite bearing failure guide. I found the color version of the same thing. It looks like the color version has better pictures, but the B&W version might have some better illustrations and explanations. So here's both versions for reference.

B&W: http://www.wilmink.nl/Clevite/Clevite_lagerschade_tech_info.pdf
Color: http://catalog.mahleclevite.com/bearing

Using the color bearing failure guide, see example #12 "Oil Starvation / Marginal Oil Film Thickness." The two pictures below come from this example. According to Mahle-Clevite, the number-one cause of failure of this type is too little oil clearance.

S65, 4000 Miles, Bone Stock, 2009.S65, 4000 Miles, Bone Stock, 2009. More Photos
Factory Bearings: 088/089
Category: 03-Mild
Description: When this engine opened to build stroker, main bearing failure was imminent.
       

S65, 24000 Miles, Naturally Aspirated, 2009 More Photos
Factory Bearings: 088/089
Category: 06-Med-Heavy
Description: No other details given.


S65, 24000 Miles, Supercharged, Built Internals, 2008. More Photos
Factory Bearings: 088/089
Category: 04-Moderate
Description: Supercharged Stroker motor with built internals. 22000 miles NA, 2000 mile supercharged.
NOTE: This engine has aftermarket crank, rods, and pistons. They are different designs from stock, have different mass and moments of inertia, possibly different materials, different manufacturing process, different tolerances, and different fasteners. Each of these key difference contribute to the overall stiffness in the connecting rod bore holding the bearing and may affect the observed bearing wear in the following photos. 
       

S65, 30000 Miles, Bone Stock, 2008. More Photos
Factory Bearings: 088/089
Category: 04/05-Moderate
Description: Bone stock engine disassembled to make stroker motor.
       

S65, 31000 Miles, Naturally Aspirated, 2008. More Photos
Factory Bearings: 088/089
Category: 04-Moderate
Description: 27,000 Miles Naturally Aspirated, 4000 Miles Supercharged
        

S65, 33000 Miles (30k) Naturally Aspirated, 3000 Miles Supercharged, 2008. More Photos
Factory Bearings: 088/089
Category: 06-Medium/Heavy
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S65, 40000 Miles, Bone Stock, 2008. More Photos
Factory Bearings: 088/089
Category: 09-Catastrophic
Description: This engine suffered complete engine failure. No holes in the block, but all internals caked in metal shavings.
       

S65, 47000 Miles, Naturally Aspirated, 1.8k Miles Supercharged, 2008
More Photos
Factory Bearings: 088/089
Category: 05-Moderate
Notes: The #3 bearing was not wiped to copper when it was removed from the engine. The shop replacing the bearings sanded it to copper for testing purposes.
       

S65, 60000 Miles (55k) Naturally Aspirated, 5000 Miles Supercharged, 2008. More Photos
Factory Bearings: 088/089
Category: 08-Near Catastrophic
Description: Auto Talent estimates this engine had less than one week to live with the bearings found in this condition
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S65, 72000 Miles, Supercharged, 2008 More Photos
Factory Bearings: 088/089
Category: 06-Med-Heavy
Description: Owner opted for rod bearing replacement at 72000 Miles during Supercharger Upgrade.
       

S65, 90000 Miles, Supercharged, 2008 More Photos
Factory Bearings: 088/089
Category: 06-Moderate
Description: Owner opted for rod bearing replacement at 90000 Miles during Supercharger Upgrade. 1000 Miles later, oil line burst and motor suffered severe, but non-fatal damage.


S65, 106000 Miles (92k) Naturally Aspirated, 14000 Miles Supercharged, 2008. More Photos
Factory Bearings: 088/089
Category: 08-Near Catastrophic
Description: Part of the EAS Ongoing Rod Bearing Journal Thread

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The connection between these pictures and the reference Mahle-Clevite pictures is obvious. Even though there can always be multiple causes of oil starvation and bearing failure, the number one cause of the damage shown in these pictures is listed by Mahle-Clevite as too little bearing clearance.The connection between these pictures and the reference Mahle-Clevite pictures is obvious.

I hope these photos have been helpful to see with your own eyes what this problem looks like, and why it's happening.