11 25 671 Replacing piston rings on all pistons (N12)

WARNING: This page is about a different variant/trim than selected.

Necessary preliminary tasks:

Measuring axial clearance of piston rings in piston ring groove.

NOTE: It is not possible to measure the axial clearance of the oil scraper rings.

Fig 1: Checking Piston Ring Clearance

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

Oil scraper ring comprises two steel band rings and a support spring.

NOTE: Oil scraper ring cannot be removed with piston ring pliers.

Put aside piston rings in correct sequence and installation position.

It might not be possible to find the identification on used piston rings.

Fig 2: Removing Compression Ring And Stepped Ring

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Installation:

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

Determine GAP with a feeler gauge.

Fig 3: Measuring Gap Using Feeler Gauge

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Installation:

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

Fig 4: Identifying Piston Rings With TOP Identification

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Installation:

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

Fig 5: Identifying Compression Ring, Taper Face Ring And Oil Scraper Ring

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IMPORTANT: Contact point on oil scraper ring (3) can overlap - risk of damage! - during installation in cylinder.

NOTE: The oil control ring comprises a steel band ring (1) and a support spring (2).

Installation:

Insert support spring (2) in piston ring groove and then fit steel band ring (1) so that end faces point to each other.

Fig 6: Identifying Steel Band Ring And Support Spring

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Offset the contact points (1) of the piston rings by approx. 120° to each other but do not position above the piston pin boss.

Coat special tool 11 9 670 with sufficient engine oil. Risk of damage! to piston rings!

Fig 7: Identifying Piston Rings Contact Points

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Assemble engine.