Assembling Pistons

Information 

• The connecting rods are cracked and together with the matching numbers cannot be mixed up!
• Connecting rod bolts and bearing shells must always be replaced.
• The marking on the connecting rod bearings is located on the flywheel or drive plate flange of the crankshaft.

  > 134819 REMOVING AND INSTALLING CRANKSHAFT 

Information 

The tightening specifications for the basic engine are stored centrally.

> Refer to 1X001N Technical values in the appropriate service information. 

> Refer to 2X001N Technical values in the appropriate service information. 

or

> 1001IN TIGHTENING TORQUES, TIGHTENING SEQUENCES AND ASSEMBLY OVERVIEWS

Tightening torques and assembly instructions for other assemblies (e.g. chassis, electrics, transmission or body) or for peripheral engine equipment can be found in the corresponding documents.

Fig 1: Piston, Connecting Rod Components Location

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Piston, connecting rod

Fig 2: Connecting-Rod Assembly, Bearing Shells Components Location

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Connecting-rod assembly, bearing shells

Fig 3: Piston Classification, Naturally Aspirated Engine Components Location

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Piston classification, naturally aspirated engine

Fig 4: Piston Classification, Turbo Engine Components Location

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Piston classification, turbo engine

1. First fit the two-piece oil scraper ring. To do this, insert the oil scraper spring into ring groove 3. Carefully force the oil scraper ring (TOP marking facing up towards the piston crown -A-  ) apart with your hand and fit it into the third piston ring groove above the oil scraper spring.

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   Piston ring marking

2. When fitted, the gap in the oil scraper ring must be offset by exactly 180° with respect to the gap in the oil scraper spring. Make sure that the oil scraper spring is in the middle of the oil scraper ring.
   CAUTION: Risk of engine damage if piston rings are overstretched! 
   → Do not overstretch piston rings with the piston ring pliers. Only open up the piston rings until they just fit over the piston.

   Information 

   • The two compression rings have different strengths and can only be installed in the appropriate groove. 
   • If the piston ring has the inscription TOP, this must point to the piston crown. 
3. Insert piston ring Into the piston ring pliers. Make sure that, when Installed, the marking on the piston rings is pointing in the direction of the piston crown -A-  Spread the piston ring until it fits across the piston. > Piston ring pliers on piston ring 

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   Piston ring pliers on piston ring

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   Piston ring designation

4. Fit piston rings. Using the piston ring pliers, insert the middle piston ring into the second ring groove in such a way that the inscription TOP is facing the piston crown. Insert the top piston ring into ring groove 1 in such a way that the inscription TOP is facing the piston crown.
5. Check that the piston rings are correctly seated and positioned. Make sure that the oil scraper spring is inside the oil scraper ring. In the case of the piston ring in ring groove 2, the inscription TOP must point to the piston crown. The top piston ring must be installed in such a way that the inscription TOP is facing the piston crown. The gaps in the individual piston rings must always be offset by 120°.
6. Fit a piston-pin circlip. To do this, insert the piston-pin circlip into the ring groove of the piston pin eye so that the ring gap is offset with respect to the piston crown (6 or 11 o'clock position). Press this end into the ring groove with your thumb

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   Piston circlip

7. Lever the piston circlip into the ring groove using a small screwdriver while continuing to press it down.

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   Inserting the piston circlip

8. If the piston circlip does not slide into the ring groove right away, press the second gap end of the piston circlip into the ring groove. Ensure correct installation seating.

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   Pressing the piston circlip into the ring groove

9. Secure piston with fitted piston-pin circlip facing downwards and connecting rod on > 9597 - retaining device  (> Retaining device).

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   Retaining device

10. Place piston pin and connecting rod in installation position. Ensure that the connecting rod marking is facing the exhaust side and the combustion chamber recess is facing the centre of the engine during assembly. Direction of travel arrow on the piston must be In direction of travel.
11. Slide the oiled piston pin through the piston pin eye and through the connecting rou eye. Tap in carefully with a rubber hammer if necessary.
12. Fit the second piston-pin circlip using the same procedure. Align the ring gap so that it is facing downwards, away from the piston crown (6 o'clock position, 11'o'clock position optional)
13. Check connecting rod and piston for smooth operation
14. Fit connecting-rod bearing shells in connecting-rod big end and connecting-rod bearing cover. The retaining lugs must be positioned in the appropriate grooves and on the same side -B and C-  . The matching numbers -A-  must be the same and must be on the same side (pointing to the exhaust side when installed). Tighten the connecting-rod bolts by hand and position the complete connecting rod ready for assembly.

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    Position of bearing shells

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    Surface with connecting rod number marking (exhaust side)