Why should connecting rod bearing replacement be part of the maintenance schedule for many BMW engines?

BMW’s high-performance engines such as S65, S85, N55, S55 and in recent years also the extremely popular B58, are highly appreciated by enthusiasts and tuners. However, these and several other BMW engines share one surprising common factor that many owners wonder about: connecting rod bearings are a wear item. The long-term reliability of the engine depends greatly on how proactively they are inspected and replaced.

Upgraded and stronger ACL Race bearings are available directly from Futurez, as well as from our authorized resellers across Finland. Available in standard size, with extra clearance (HX), and undersized options.

 

Even though for example the B58 has gained a reputation of being “bulletproof”, the truth is that even these engines can suffer from bearing issues, especially with heavy tuning, high torque levels and track use.

One of the most well-known examples is the S65B40, powering the E90/E92/E93 M3 models. While the vehicle itself isn’t the most common daily driver out there, nearly everyone in the car scene knows about its rod bearing reputation. It’s an M-engine after all – so why on earth does this happen?

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S65 – The gloriously high-revving V8.

This engine is a perfect example, because here rod bearing wear isn’t a matter of “if” – it’s a matter of “when” and “at how many kilometers”.

One of the key factors is the relatively tight bearing clearances used by BMW, especially in the S65 and S85 engines. The bearing manufacturer and specification changed during production, but on average the tolerance is noticeably tighter than in many comparable engines.

Considering that bearing clearance and oil viscosity go hand in hand, the OEM specification of 10W-60 is surprisingly thick. 10W is not very flow-friendly when cold, and it can lead to situations where cold starts create metal-to-metal contact – or when an eager driver applies load and RPM too early before the engine has fully warmed up.

Even small scratches and marks will gradually grow worse over time, as worn bearing surfaces reduce the oil film stability and load-bearing capacity.

Additionally, it’s worth remembering that when a high-performance V8 screams past 8,000 rpm – most likely for a significant part of its driven mileage, considering the type of car – its lifespan cannot realistically be compared to, for example, a 1.9 TDI used for relaxed commuting.

Key contributing factors include:

• Tight factory bearing clearances

• Thick (10W-60) oil as factory recommendation (cold operation)

• Very high 8,400+ rpm redline

• Demanding usage profile based on car character

 

How often should S65 bearings be replaced?

Many consider 100,000 km a reasonable general rule. But if you take pride in proper maintenance – and enjoy driving the car as intended – there is absolutely no harm in being proactive and doing it earlier.

Bearings are easy to replace – crankshafts (or engine blocks…) are not.

 

N55 / S55 – the list of suspects continues.

Several other BMW engines suffer from the same symptoms. Due to their large numbers on the road and their strong tuning potential, the N55 and S55 engines appear prominently in these statistics as well.

And no wonder. High torque from low RPM, and in the hands of tuners often significantly increased power and cylinder pressures, typically with ethanol fuel and high boost levels.

In these engines, oiling and stable oil pressure can sometimes leave something to be desired, especially in the N55.

Rod bearings can also wear during completely normal street driving, but under tuning conditions the wear rate increases dramatically.

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B58 – You too, my Brutus…?

The BMW B58 has quickly earned its reputation as an exceptionally durable engine, but the aggressive tuning trends of recent years (even 500–700 hp setups on a stock engine!) make it clear that at some point, as mileage builds up, metal starts to give.

When torque rises from the stock 450–500 Nm to 650–800 Nm levels, the bearings are placed under significantly higher surface pressure.

 

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Why is correct bearing clearance critically important?

When building a street or performance engine, one of the most important factors is how much clearance the bearing has to work with.

Important: ALWAYS — and we emphasize ALWAYS — when replacing or installing bearings, the installer must know exactly what bearing clearance the engine will be running.

Your mechanic will know the ideal clearance for your specific build — but as a general guideline we can say that “five tenths” (0.05 mm) is a commonly targeted value for connecting rod bearings.

Clearance can be measured easily with a Plastigauge-type strip, which is placed between the bearing during assembly and then checked for the actual achieved clearance.

• too tight clearance → bearing overheats, oil film breaks down

• too loose → oil pressure suffers, unnecessarily thick oil is required

 

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More room to play with HX size

Often the crankshaft has polished itself slightly over time, which naturally creates a bit more clearance. But if the crankshaft happens to be perfectly within spec and the desired clearance is not achieved, ACL’s HX size can help.

HX size provides an additional .025 of clearance compared to the standard bearing when needed.

 

High-quality engine oil goes a long way

The metal surfaces inside the bearing should never make contact — there must always be a strong, unbroken oil film between them. Bearing durability is only as good as the oil that separates the metal surfaces.

This is not an area to cut corners. Always use high-quality oil with no compromises made in formulation. If you run ethanol fuel, remember that your oil must also withstand ethanol’s “cutting” or thinning effect.

Our most popular oils for BMW engines are the Signature and Dominator series oils.

 

Do the bearing service on time — it saves serious money.

The BMW engines mentioned are incredibly capable powerplants, but they do require proper maintenance.
Rod bearings are something that must be taken seriously, because failure in this area can destroy the entire engine.

Shorten the replacement interval even further if:

• the car has been tuned

• you attend track days or otherwise drive the engine hard

• the service history is incomplete or uncertain

• there are knocking/metallic noises or fluctuating oil pressure

• (Or generally: it’s an S65, N55, S55 or a heavily-driven B58)