Figure 1 - Two different sizes of bicycle chain
There are two basic types of bicycle chain: single speed chain and multi-speed chain. Single speed chain is mainly used on kids' bikes, BMX bikes, coaster brake cruisers, and heavy cargo bikes. Multi-speed chain is used on standard speed bikes and mountain bikes that require the use of a front and rear derailleur to change gears. Both types of bicycle chain have a pitch of 1/2 inch (ANSI standard #40). This measurement indicates the length of the links. Although every type of bicycle chain and freewheel have a 1/2 in pitch, the width of chain varies quite a bit, from 3/32" to 1/8".
Single speed bicycle chain is wider, having a width of 1/8 inch. This type of chain will not fit a multi-speed freewheel nor will it fit properly through a derailleur cage. Multi-speed chain comes in various widths, with 3/32" being the most common size.
Multi-speed chain is designed with a lot more side-to-side flex to allow it to function properly with a derailleur system. Flexibility is very important in a multi-speed system as the alignment of front and rear chain rings could be off by as much as 3 inches, depending on which gears are being used. Figure 1 shows the two common sizes of bicycle chain; 1/8" on the top and 3/32" on the bottom. At this angle, both chain types look very similar since you can only see the pitch, not the width.
Figure 2 - Single speed (top) and multi-speed (bottom)
Figure 2 gives you a much clearer view of the difference between a 1/8" single speed chain (top) and a 3/32" multi-speed chain (bottom). The multi-speed chain is obviously narrower to fit the narrower chain rings on a multi-speed freewheel, and it also includes a beveled edge on the inner link to allow for better meshing with the teeth when switching gears.
Figure 3 - A bicycle chain link tool
When bicycle building becomes your hobby, one of those "must have" tools will be a chain link tool as shown in Figure 3. For under $20, this small tool will give you a lifetime of service, able to break and rejoin any size of bicycle chain in a few seconds. The other method involves using a punch, a hammer, and a finishing nail, but I assure you, the chain link tool is so much easier and makes a worthwhile investment. To open a link, place the chain into the holder as shown in Figure 3, and then turn the vice handle clockwise to press out the link pin.
More of this and other free DIY plans at AtomicZombie.com .