ARP Ring Compressor (901-1000)100. 0 mm Ring Compressor Specialty Kit by ARP 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