11 25 671 Replacing piston rings on all pistons (N55)

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Necessary preliminary work:

Remove all PISTONS .

Measuring side clearance of piston rings in piston ring groove.

TECHNICAL DATA .

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

Fig 1: Measuring Piston Ring Side Clearance

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

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

NOTE: The M-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.

Insert removed piston ring into cylinder bore hole.

With aid of piston (piston crown), position piston ring in its measuring position.

Determine END CLEARANCE with a feeler gauge.

Fig 3: Checking End Clearance Using Feeler Gauge

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NOTE: Schematic diagram of piston rings.

Installation note:

Plain rectangular compression ring "top"
Piston ring "top"
Steel band with M-flex ring

Fig 4: Identifying Piston Rings
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Installation note:

Piston rings with "TOP" identification (1) must point to piston crown.

Fig 5: Identifying Piston Rings With Top Identification

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NOTE: The M-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 6: 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 7: Identifying 120° Offset Of Piston Ring Contact Points

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Assemble engine.