It's the oversize. If your cylinder bore is worn or damaged, you bore it out a little larger until the damage/wear is gone. Because the bore is larger now, you need oversized pistons and rings. The +.xxmm is how many millimeters standard bore the engine is bored out. There are standard overbore sizes, i.e. the ones listed.

The overboring repairs the damage/wear and also increases the displacement a small amount and compression ratio a small amount.


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