Piston: Service and Repair

a Valve recesses
b Trough diameter

NOTE: As standard, the pistons are subdivided into six tolerance groups (group numbers). The following information is stamped into the piston crown:

1. Piston diameter e.g.. 96.505
2. Piston code number e.g.. 72
3. Group number e.g.. 1+
4. Direction of driving arrow





The group number is also stamped into the crankcase mating surface.

The piston group number (e.g. 2) must correspond with the group number of the cylinder bores (series manufacture).

This will ensure that the piston clearance is maintained.

In the event of repairs, hone cylinder bores according to the dimensions of the existing, pistons plus the piston clearance. Specifications

The piston pins of the engines 117.967/968 have different inside diameters for weight compensation purposes together with the piston. In the case of repairs, the piston pins must remain with their respective piston.

Removal





1 Remove connecting rod with piston in upward direction. For this purpose, cover connecting rod with cover sleeves to prevent damage to the cylinder wall.
2 Remove piston pin circlip and push out piston pin.
3 Recondition connecting rod and square. Service and Repair


Installation





4 On used pistons, check piston ring gap and axial clearance.
5 Lubricate piston pins and connecting rod bushings.





1 Connecting rod contact side
3 Locating grooves

6 Mount pistons in such a way that the arrow points in the direction of driving and the locating grooves (3) in the connecting rod are facing to the engine outside.





7 Push piston pin in manually.

CAUTION: Do not heat piston.





8 Insert piston pin circlip into the groove.
9 Lubricate cleaned cylinder bores, connecting rod bearing journals, connecting rod bearing shells and pistons.
10 Distribute piston ring gaps uniformly around the piston circumference.





11 Position piston ring clamping strap and introduce pistons into cylinder bore with arrow in the direction of driving.
To do so, fit cover sleeves to connecting rod to avoid damage to the cylinder wall.





12 Mount connecting rod bearing caps along connecting rod with matching identification number and tighten connecting rod nuts with 40 - 50 Nm initial torque and 90 - 100 ° angle of rotation torque.
13 Rotate crankshaft and check clearance between piston pin eye and connecting rod.