BMW Rod and Main Engine Bearings
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BMW Rod and Main Engine Bearings

The BMW engine crankshaft and connecting rods ride on thin metal rings called bearing shells. Over time these shells wear out and require replacement as any other normal wear item. In a perfect world the bearings wear out instead of the crankshaft. And when the bearings are fully worn you can simply replace them with original spec bearings. If you preemptively replace bearings you should not need to measure for the right size (in theory). This page is a work-in-progress and should not be taken as gospel or the final word in engine bearing selection, measuring, or ordering.

BMW has never publicly released a replacement interval or expected lifespan for any bearing shells. A used oil analysis may give you early warning signs of bearing wear. Otherwise, the only way to tell is by removing a bearing and inspecting it. This is not as involved or technically challenging as it sounds, especially if you already have the oil pan removed. Rod bearings can be replaced independent of main bearings but not vice versa.

Bearings are produced based on two factors: inside diameter and outside diameter of the two bearing shells. The ID is going to be a fixed number to maintain a specific tolerance gap around the crankshaft for oil to occupy. The OD will vary by cylinder to account for slight differences in the machining of the block, crankshaft, or connecting rod. This measurement is color coded. When looking at bearing part numbers in ETK/RealOEM there will usually be multiple bearing part numbers for each size, labeled by color. This color needs to be matched to paint marks on the crankshaft and/or engine block and it won't be unusual to have a mix of one or two different colors.

If a crankshaft is damaged from a worn-out or dislodged (spun) bearing it will likely need machining to be usable again. Machining the crankshaft reduces the OD of the shaft, requiring bearing shells with a smaller ID. BMW makes undersized bearings for this purpose. Machining is typically done via "stages" so that bearings can be manufactured to pre-determined specs and expensive custom-sized bearings are not needed.

For example, an original spec may be 56.00mm. First machined bearings will be 55.75mm (.25mm removed). Second machined bearings will be 55.50mm (.50mm removed).

Measuring for Bearings

M and S Engines up to 2006
There is always a standard bearing spec for each engine. However the best approach is to confirm the size to ensure machining hasn't already been done. The engine builder will confirm the size by hand using ultra-precise micrometer measuring tools capable of measuring to the thousandth of an inch. Then the size is matched with the color code on the block/crank/rod.

N and S Engines from 2006
Starting with the N-series engines in 2006, BMW laser etches the rod bearing and main bearing sizes onto components using a code so mechanics can know what was originally equipped. The combination of these alpha-numeric codes corresponds to a matrix to indicate which bearing to use.

Upper main bearing shell sizes are etched with a alphabetic code on the side of the engine block.
Lower main bearing shells are etched with a numeric code onto the first counterweight of the crankshaft.
Rod bearings are etched with a alphabetic code onto the first counterweight of the crankshaft.

Upper Main Bearing Codes:
Code string is located vertically on a flat surface on the passenger side of the engine block. An asterisk "*" denotes the transmission end of the code string (see note below). There are 7 main bearings with the middle (#4) being the thrust bearing. The upper bearings are designed with an oil lubricating groove. The code is alphabetical - A-F.
A = yellow code
B = green code
C = red code
D = blue code
E = brown code
F = purple code

NOTE: BMW N55 tech documents state: "Bearing seat assignment from 1 to 7, the first bearing position is indicated by an asterisk (1) on the transmission side." We think "transmission" is a misprint and it should be "timing side" like the lower and rod bearings are labeled. But we don't have confirmation on this. If it is transmission side then the order is reversed compared to the lower bearings!
Update: on the S55 document they are logical: "the seventh bearing position is located on the clutch side and is indicated by an asterisk". So we can only assume that the original N55 document was a misprint or just plain confusing.

Lower Main Bearing Codes:
Code string is located on the first/front counterweight of the crankshaft. There are 7 main bearings and the code is numerical - 1-6. The lower bearings have a smooth surface with no oil groove.
1 = yellow
2 = green
3 = red
4 = blue
5 = brown
6 = purple

Main bearing code example: BBCBCBB on the block and 1222211. In this case your cylinders are laid out with these combinations:
Cyl 1 = green/yellow
Cyl 2 = green/green
Cyl 3 = red/green
Thrust = green/green
Cyl 4 = red/green
Cyl 5 = green/yellow
Cyl 6 = green/yellow
Note: yellow and red are never used together. BMW calls for a green/green combination in this case.

Rod Bearing Codes:
Code string is located on the first/front counterweight of the crankshaft. The code is alphabetical with one letter being for a upper and lower bearing shell combination. Rod bearings have 4 colors that are split into two color combinations:
b = blue combination consists of a violet/purple upper shell and a blue lower shell 
r = red combination consists of a yellow upper shell and red lower shell
Note: colors are never mixed beyond these two combinations.

Rod bearing code example: bbrbrr. In this case your cylinders are laid out with this combination:
Cyl 1 = violet/blue
Cyl 2 = violet/blue
Cyl 3 = yellow/red
Cyl 4 = violet/blue
Cyl 5 = yellow/red
Cyl 6 = yellow/red


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