M11 Series – Service Manual – 001-043   Piston

All M Series engines are equipped with articulated pistons.

The articulated piston is a two-piece piston consisting of a forged-steel crown and an aluminum skirt.

A closed chamber oil gallery is located on the underside of the crown of the piston to provide more oil contact with the piston and give better piston cooling. Piston cooling nozzle targeting is critical because of the closed chamber oil gallery.

The piston and connecting rod

must be removed as an assembly.

To remove the piston and connecting rod, refer to Procedure 001-054.

Refer to Procedure 001-054 to disassemble the piston and connecting rod assembly.

NOTE: Refer to Procedure 001-047 for piston ring inspection.

CAUTION  Be sure the cleaning solvent is approved for aluminum. Damage to the pistons can result if the wrong solution is used.

Allow the pistons to soak for a minimum of 30 minutes in a tank containing
an approved cleaning solvent for aluminum.

Use a hot, soapy solution and a nonmetallic brush to remove carbon deposits.

WARNING  When using a steam cleaner, wear protective clothing, and safety glasses or a face shield. Hot steam can cause serious personal injury.

Use steam to clean the pistons.

Dry with compressed air.

Inspect the piston bowl (1) and crown piston pin bore (2) for cracks or damage.

Do

not use pistons with cracks. If the crown is cracked,
it

must be replaced.

Inspect the piston gallery cover plate in the piston crown for cracks or looseness.

If the cover plate has a radial crack (1), the crown can be reused.

If the cover plate has a circumferential crack (2), the piston crown

must be replaced.

If the gallery is loose, the piston crown

must be replaced.

Inspect the piston skirt pin bore (1) and sides (2) for cracks or damage.

Do

not use pistons with cracks. If the skirt is cracked,
it

must be replaced.

The ring groove can be inspected with a new ring and a feeler gauge.

Hold a new ring in the groove even with the outside diameter of the piston. Install a 0.15-mm [0.006-in]
feeler gauge. If the feeler gauge enters the groove without resistance, there is too much wear. Replace the piston.

Measure the piston pin bore inside diameter.

Measure the piston pin outside diameter and length.

Refer to Procedure 001-054 for piston and connecting rod assembly instructions.

The piston and connecting rod

must be installed as an assembly on M series engines.

Refer to Procedure 001-054 for installation instructions.