Roller chain sprockets and idlers have a series of teeth around a central bore, or hole, that engage with roller chains to transmit power between rotational shafts. Fixed-bore roller chain sprockets have a finished bore that is machined to a set diameter and are ready to install. Plain-bore roller chain sprockets have a starting bore that can be machined to the correct size of the shaft. Bushed-bore roller chain sprockets need a bushing to mount to the drive shaft and adapt the bore to the shaft size. Idler sprockets maintain tension, change the direction of the chain, or position chains away from obstacles.