These ring compressors are an affordable, simple way to install pistons. Made for engine blocks with studs, they're CNC'd to accommodate bore sizes 4.000" through 4.310". Anodized aluminum for durability with easy-to-read size etched on each ring. By squeezing slightly on the piston ring, the compressor eases the piston into the cylinder bore. So get your vehicle out of the garage and on the road faster! Sold 1/pkg.