Larry

Your description of the piston recess area does not seem to correspond to the descriptions/illustration in the OM616/617 engine manual......

The illustrations show two versions of pistons used on these engines. The early version has a round tapered recess area that tapers to a small dented area that provides clearence for the prechamber neck. According to the book these pistons should use the flat tipped prechambers.

The later pistons have the star shapped recess area. These pistons should be used with the round tipped prechambers. The book is quite explicit that the prechambers must be matched to the correct piston type.

Both piston versions also have relief areas cut to provide valve clearance.

Where did you get your pistons? Did the pistons you removed have the star recess in the pistons?

Tim