When an engine block cylinder is worn, scored, or damaged, a machinist will bore it out to a slightly larger diameter to restore a perfectly round and straight wall. Oversize pistons and rings are manufactured to match these new dimensions. They are typically sold in standard increments like 10, 20, or 30 thousandths of an inch over the stock bore.
Heard on the job
The cylinder walls were too badly scored to hone, so we're going 30 thousandths oversize on the new pistons.
Insider tip
Always wait until the machine shop finishes boring the block before ordering your oversize pistons, in case they have to cut deeper than expected to clear the damage.
Also heard as
- overbore