ARP's Tapered Ring Compressors are CNC-machined from 6061-T6 billet tube material with a true radius for every bore diameter. Type 3 anodizing is used for long life, and the bore size is prominently engraved in 3/4" high numbers for easy identification. Standard stocking sizes from 3.552" to 4.750" (SAE) and 75mm to 100mm (metric). The true radius design is far superior to conventional “tapered” devices, and widely acclaimed by professional engine builders.