11 25 671 Replacing piston rings on all pistons (N52)

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Necessary preliminary work:

Remove all PISTONS .

Measuring axial clearance of piston rings in piston ring groove:

refer to TECHNICAL DATA .

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

Fig 1: Measuring Side Clearance

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

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

Fig 2: Removing Plain Rectangular Compression Ring And Stepped Ring

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Installation note:

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

Determine END CLEARANCE with a feeler gauge.

Fig 3: Measuring End Clearance

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NOTE: Schematic diagram of piston rings.

Installation note:

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

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

Fig 4: Identifying Piston Rings
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NOTE: The U-flex ring comprises two steel band rings (1) and a bearing spring (2).

Installation note:

Insert bearing 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 Bearing Spring Of M-Flex Ring

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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 gudgeon pin boss.

NOTE: Picture shows N52.

Fig 6: Identifying 120° Offset Of Piston Ring Contact Points

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Assemble engine.