Rod and main bearings: clearances, classification, plastigage, torque stages

Bearing work is where a rebuild is won or lost. The tolerances are hundredths of millimeters, the fasteners are one-time-use, and BMW did not make it easy: shells are graded by color on the N54 and by paired letters on the B58, and the crankshafts of the N55 and S55 cannot be reground at all. Here is the whole bottom-end data set.

The clearance windows, factory values

The N55 carries the same numbers (side clearance 0.060–0.250 mm; its crank friction check is below). These windows are the pass/fail line for every plastigage reading you take. A clearance outside the window is either the wrong shell grade, a worn journal, or a damaged bore — never a "close enough" situation.

Journal grades: why bearings come in colors and letters

BMW builds select-fit engines: the block, crank, and rods are measured at the factory and stamped or printed with grade marks, and the shells are picked to match. There is no universal "N54 bearing" — there is your engine's bearing.

N54 mains. Crank journal grades are marked S1, S2, S3 in descending size:

N54 rods. Rod journals come in two stages, marked b and r on the crank web: b = 49.9730–49.9834 mm, r = 49.9835–49.9930 mm.

N54 rod shell colors. The rod bearing shells are color-classified per side: r-class rods take yellow in the rod and red in the cap; b-class rods take violet in the rod and blue in the cap. The factory's rules: only one color per surface, colors differ between rod and cap, and the blue/red combination was phased out — match your engine's actual marks, not a forum chart.

S55 mains. Same architecture, different grades — journal 1 runs its own (smaller) grade set, journals 2–6 another, journal 7 the 64 mm guide set. And the headline: every S55 grinding stage is marked "no machining possible." The journals are surface-treated; wear past the grade window is a new crankshaft.

B58 mains. The B58TÜ1 uses a letter-pair system: a capital letter and a lowercase letter on the crankcase cross-referenced in factory tables to shell grade 1, 2, or 3. Main shell (with lubricating groove) goes in the crankcase side; the plain shell goes in the bearing cap; the guide bearing (position 4) is its own table.

Plastigage, done properly

Plastigage is a crush gauge: a strip of calibrated plastic flattens between journal and bearing, and the flattened width reads out the clearance. It is accurate enough for this work when you respect its rules:

1. 01Clean the journal and shell dry — one film of oil inflates the reading into fiction.
2. 02Install the shell, lay the strip along the journal axis (parallel to the crank), on the bearing you are checking.
3. 03Assemble and torque to spec — the full sequence, angles included, bolts new. Plastigage without the real torque value is decoration.
4. 04Do not rotate anything. Assemble, torque, disassemble. Rotation smears the strip and lies to you.
5. 05Read the widest flattened point against the envelope's scale, at the journal's bottom (12 o'clock of the shell).

Torque stages: every fastener, factory values

The N55 rod bolt is identical: M9, replace, 20 Nm + 70° + 70°. The B58 carries the same law — renew the connecting rod bolts, 20 Nm jointing torque, then 70° and 70°.

The details that actually kill bottom ends

• **Groove orientation.** The N54 main shell with the lubricating groove fits in the crankcase upper section; the plain shell fits in the bedplate. Backwards, and the oil hole index is wrong.
• **Oil spray nozzles.** The factory procedure sets the piston-cooling nozzles with a dedicated fixture (tool 11 8 510) whenever main bearings come out. A bent nozzle sprays the cylinder wall instead of the piston crown.
• **Crank friction check.** The N55/S55 data carries a rotation torque for the assembled crank: 2 to 2.5 Nm. After the mains are torqued, rotating the crank should feel like that — a dial torque wrench on the crank bolt gives you the number. Tight = wrong shell or a fouled thrust face.
• **End play.** Pry the crank fore-aft with the guide bearing torqued and measure with a dial indicator at the nose: 0.060–0.250 mm, all four platforms. Out of window = thrust bearing or crank thrust faces.
• **Bearing position 1.** On the N54, position 1 is at the timing-chain drive end — count from the front, not the flywheel.

Grinding: what can be cut and what cannot

That last line surprises people with an S55 in the shop. The factory data is unambiguous: the journals are treated, and the treatment does not survive a grind.

Where this connects

Ring and piston numbers live in the piston guide; the crankhub and chain torque sequence that shares this disassembly lives in the crankhub guides; and the priming procedure that protects all of this work is in the oil and break-in guide.