How to read N55 engines for Rod Bearing codes

BMW’s high-performance nature means that every one of their engines is built with strict tolerances. In the more mechanical days, engine builders carefully measured their clearances to get them right and knew the correct tolerances via a book. Nowadays, BMW publishes engine clearances and technical data via ISTA/AIR, a paid access portal. All engine builders must do is log online and match codes stamped into the block to see which bearings the engine will need.

BMW N55 Main Bearings

Main bearings are assigned on the engine block and the crankshaft itself. Upper main bearing assignments for positions 1-7 can be found on the engine block (code letters A, B, or C), while lower bearing assignments are etched into the crankshaft (code numbers 1, 2, or 3). These numbers represent a grinding stage for bearings, and a combination of these bearings creates the in-spec tolerance required per journal on the crankshaft.

Upper Main Bearings

Code letters on the engine block designate the bearing type and its position. This information can be found written vertically on the right side of the engine block directly to the right of the coolant feed pipe.  Below is an N55 engine block with the location of this information circled in red for reference and a close-up of what you'll read.

• Code letter A = bearing shell, yellow with lubricating groove.
• Code letter B = bearing shell, green with lubricating groove.
• Code letter C = bearing shell, red with lubricating groove.

Using the assignment from the second image above as an example:

*BBABABB

The asterisk denotes the front of the engine block (timing chain end). Left to right is the order of positions 1-7. In this case, this engine block requires the following colors/part numbers by position:

1. B = Green = 11217627038
2. B = Green = 11217627038
3. A = Yellow = 11217627037
4. B = Green = 11218605857
5. A = Yellow = 11217627037
6. B = Green = 11217627038
7. B = Green = 11217627030

Part Numbers For Positions 1, 2, 3, 5, 6:

Part Numbers For Position 4 (thrust bearing):

Part Numbers For Position 7:

Lower main bearings:

The lower main bearings are coded to the crankshaft itself using numbers. Similar to the system used on the engine block, these numbers correlate to a specific color, which in turn correlates to a specific grinding stage of the bearing, allowing for a precise tolerance between the bearings and the crankshaft main journals. 

The numbers on the crankshaft counterweight represent a color (grinding stage), coded by position to specific part numbers. The letters represent the following:

• Coefficient 1 = bearing shell (2) yellow without lubricating groove.
• Coefficient 2 = bearing shell (2) green without lubricating groove.
• Coefficient 3 = bearing shell (2) red without lubricating groove.

Part numbers for positions 1, 2, 3, 4, 5, 6:

Part numbers for position 7:

Here is an example of the numbers on the crankshaft denoting main bearing assignments 1-7:

An example of a main bearing assignment code on a crankshaft:

"2222233"

1. 2 = Green = 11217627034
2. 2 = Green = 11217627034
3. 2 = Green = 11217627034
4. 2 = Green = 11217627034
5. 2 = Green = 11217627034
6. 3 = Red = 11217627035
7. 3 = Red = 11217627027

Caution:

Per BMW’s technical documentation, yellow and red main bearings must not be mixed. If a yellow bearing is assigned on the block and a red bearing is assigned on the crankshaft for the same journal, you must use a green and upper bearing. 

The reason for this is pretty straightforward. Yellow and red-coded bearings are on the tighter end of the spectrum for clearance. If we take the maximum tolerance for each bearing shell (55.989-55.970, respectively), we could end up with a hypothetical clearance of 0.019mm, which is too tight and out of spec for the recommended main bearing clearance of 0.020-0.046mm.

A Complete BMW N55 Main Bearing Assignment Example:

BMW N55 Connecting Rod Bearings

Connecting rod bearing assignments are located on the crankshaft. There are only two options for rod-bearing clearances on the crankshaft, with a lower and upper bearing shell needing to be installed in matched pairs. The two color options for the small end (connecting rod bearing cap) are red or blue, which is coded on the crankshaft as “r” or “b.” On the big end (connecting rod), the bearing shell color corresponds to the color used on the small end. Red on the small end matches with yellow on the big end, and blue on the small end fits with violet on the big end.

Here is an example of how connecting rod bearing assignments are ordered 

An example of a code on a crankshaft and how part numbers are assigned:

"brrrrr"

1. r = red = 11247628034 and 11247576714
2. b = blue = 11247628035 and 11247576715
3. b = blue = 11247628035 and 11247576715
4. b = blue = 11247628035 and 11247576715
5. b = blue = 11247628035 and 11247576715
6. b = blue = 11247628035 and 11247576715

BMW N55 Clearance Specifications:

Information in this article pertains to Genuine BMW factory bearings only. Aftermarket rod bearing and main bearing sets include upper and lower in an entire package. They’re ground somewhere in the middle of the spectrum so that a standard set can apply to almost all engine block/connecting rod and crankshaft combinations. The factory bearing tolerances are tight, and if you’re looking to build an engine according to BMW’s engineers, Genuine bearings are your only option.

 If you use an aftermarket bearing set, FCP Euro recommends checking clearances during engine assembly. Plastigauge PG1 (green) is the recommended method by BMW for a quick check, as the tolerances are machined from the factory. However, more accurate options are available to you if performing a complete rebuild where the block or crankshaft has had machine work.