Join Date: Aug 2011
Location: Puyallup, WA
Bikes: Tommasini Super Prestige, Kamra Triathlee, Nishiki Tri-A equipe', Sakai 2000
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The chain in use on modern bicycles has a 1/2" pitch, which is ANSI standard #40, where the 4 indicates the pitch of the chain in eighths of an inch, and metric #8, where the 8 indicates the pitch in sixteenths of an inch.
Chains comes in either 3/32", 1/8", 5/32" or 3/16" roller widths: 5/32" is used on cargo bikes and trikes, 1/8" with low-cost coaster brake (back pedal) bikes, hub gear and fixed gear bicycles, and on track bikes. (Fixed sprockets and freewheels are also available in 3/32" widths; fixed gear and single-speed bikes can be set up to use either 1/8" or 3/32" chains.) Derailleur equipped bikes such as racing, touring and mountain bikes use 3/32" chains exclusively.
The Wikibook, "Bicycle Maintenance and Repair", explains that the difference between derailleur chains commonly labelled 8-speed, 9-speed, and 10-speed is in its external width (all are 3/32" chains, that is, in their minimum internal clearances).