Chainrings/Parts

Chainrings engage the chain to transfer power to the (usually rear) wheel. They usually have teeth spaced to engage every link of the chain as it passes over; however, in the past, some designs (called skip-tooth or inch-pitch) have had one tooth for every other link of the chain.
By convention, the largest chainring is outboard and the smallest is inboard. Chainrings vary in size from as few as 20 teeth to as many as 60 and potentially more.

Chainrings also come in several nominal widths:

• 3/16" (4.76 mm) for old-time bikes (especially skip-tooth or inch-pitch), heavy duty BMX, Worksman, and exercise bikes
• 1/8" (3.18 mm) for track, BMX, cruiser bikes, one-speed, three-speeds, and the rare older 3 or 4 speed derailleur bike.
• 3/32" (2.38 mm) for road, hybrid, mountain bikes, single-speed and 5-, 6-, 7- and 8 speed freewheels or cassettes.
• 5/64" (1.98 mm) for any bike with 9- or 10-speed cassettes

What's BCD?

Many modern bicycles have removable chainrings, to allow for replacement when worn, or to change the gear ratio provided (although the change is limited).
The holes on the spider arms used for attaching a chainring can have a variety of dimensions, referred to as the bolt circle diameter, commonly abbreviated as BCD. This measurement is sometimes referred to as pitch-circle diameter (PCD). Cranks designed to mount one or two chainrings will almost always use a single bolt circle diameter. Cranks designed to mount three chainrings will almost always use two different bolt circle diameters; the larger to mount the two outer rings and the smaller to mount the inner ring.