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.
My name is Richard. I was a Contouraholic.
NOW: '02 Mazda B3000 Dual Sport, Black
BEFORE: '99 Contour SE Sport
Duratec ATX Spruce Green
PIAA 510's, Foglight MOD, K&N Drop-in
|