Necessary preliminary tasks

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Removing all PISTONS

Measuring axial clearance of piston rings in piston ring groove.

Technical Data . See PISTONS WITH RINGS AND PINS N52K/B30 ; PISTONS WITH RINGS AND PINS N52K/B25 ; PISTONS WITH RINGS AND PINS N52K/B30 or PISTONS WITH RINGS AND PINS N52K/B25 .

NOTE: It is not possible to measure the axial clearance of the U-flex rings.

Fig 1: Measuring Axial Clearance Of Piston Rings

Courtesy of BMW OF NORTH AMERICA, INC.

Remove plain compression ring and stepped ring upwards with piston ring pliers.

The U-flex ring comprises two steel band rings and a support spring.

NOTE: The U-flex ring cannot be removed with piston ring pliers.
Put aside all piston rings in correct sequence and installation position.
It might not be possible to find the identification on used piston rings.

Installation:

New pistons may only be installed together with new piston rings.

Fig 2: Removing Compression Ring And Stepped Ring

Courtesy of BMW OF NORTH AMERICA, INC.

Determine end clearance . See PISTONS WITH RINGS AND PINS N52K/B30 ; PISTONS WITH RINGS AND PINS N52K/B25 ; PISTONS WITH RINGS AND PINS N52K/B30 or PISTONS WITH RINGS AND PINS N52K/B25 with a feeler gauge.

Fig 3: Measuring End Clearance Using Feeler Gauge

Courtesy of BMW OF NORTH AMERICA, INC.

NOTE: Schematic diagram of piston rings.

Installation:

Piston rings with "TOP" identification must point to piston crown.

Plain compression ring
Stepped ring "TOP"
U-flex ring

Fig 4: Identifying Piston Rings

Courtesy of BMW OF NORTH AMERICA, INC.

NOTE: The U-flex ring comprises two steel band rings (1) and a support spring (2).

Installation:

Insert support spring (2) into piston ring groove and then fit steel band rings (1) so that contact points are offset by approx. 120°.

Fig 5: Identifying Steel Band Rings And Support Spring

Courtesy of BMW OF NORTH AMERICA, INC.

The contact points (1) of the piston rings must be arranged offset by approx. 120°. However, the contact points (1) must not be arranged over the piston pin boss.

NOTE: Picture shows N52.

Fig 6: Identifying Contact Points Of Piston Rings

Courtesy of BMW OF NORTH AMERICA, INC.

Assemble engine.