ARP tapered piston ring compressors are CNC-machined from 6061-T6 billet aluminum tube material and feature a true radius for each individual bore diameter. They're also relieved to accommodate wire O-rings on the bottom. Type 3 anodizing is used for long life, and each bore size is prominently engraved in 3/4 in. high numbers for easy identification. The true-radius design of ARP tapered ring compressors is far superior to conventional tapered devices, and they're widely acclaimed by professional engine builders. Dozens of sizes are available for both standard and metric applications.