Roller chain size chart: ANSI, ISO 606, motorcycle and bicycle chain pitch and roller diameter

A sprocket is drawn for one chain, and two chains that look alike can need different teeth: an ANSI 40 and an ISO 08B both have a half-inch pitch, and their rollers differ by more than half a millimetre. This chart gives the two numbers a sprocket needs, the pitch and the roller diameter, for the chains in common use, and how to tell them apart on the bench.

The figures are the ones the standards print: ASME B29.1 for the ANSI chains, in inches, and ISO 606 for the B series, in millimetres. Each row was checked in two independent sources, and every chain fills in the sprocket generator with one click.

Reading the chain number

ANSI numbers carry the pitch in eighths of an inch in every digit but the last: 40 is 4/8, half an inch; 60 is 6/8; 100 is 10/8, an inch and a quarter. The last digit is 0 for standard roller chain, 1 for the light 41 and 5 for the rollerless chains, 25 and 35, which run on a bushing instead of a roller.

ISO B-series numbers carry the pitch in sixteenths of an inch: 08B is 8/16, half an inch; 12B is 3/4 inch. The exception is 05B, which has a metric 8 mm pitch. Motorcycle numbers start with the pitch in eighths, like ANSI: 415, 420 and 428 are half-inch chains; 520, 525 and 530 are 5/8 inch.

ANSI roller chain sizes (ASME B29.1)

On the 25 and the 35 there is no roller: the tooth seats on the bushing, and its diameter is the one the sprocket needs.

ISO 606 B-series chain sizes

The B series is the European, originally British, roller chain. It shares the pitch with the ANSI chain of the same size and differs in the roller and the width, so the two do not interchange: an 08B chain rides high on a 40 sprocket.

Motorcycle and bicycle chains

Motorcycle chains share the pitch and the roller with an industrial chain -- 520, 525 and 530 have the roller of the 50, and 428 the roller of the 08B -- and differ in width and plate strength. Bicycle chain has a half-inch pitch and a 7.75 mm roller (ISO 9633); 1/8 inch inner width is the single-speed and BMX chain, 3/32 inch the derailleur chain.

How to identify a chain on the bench

1. Read the side plate: most chains are stamped with the number -- 40, 08B, 428 -- next to the maker's mark.
2. Measure the pitch over ten pitches, from the centre of one pin to the centre of the pin ten pitches on, and divide by ten. One pitch alone hides the difference between a worn chain and the next size.
3. Measure the roller with callipers. On a half-inch chain, 7.9 mm is an ANSI 40; 7.75 to 7.8 mm is a 41, a 415 or 420, or a bicycle chain; 8.5 mm is an 08B or a 428.
4. Measure the inner width, between the inner plates, to choose the plate thickness of the sprocket: the tooth has to pass between the plates with clearance.

From the chain to the sprocket

Pitch diameter

d = p / sin(180° / z) p is the chain pitch and z the number of teeth. A 40 chain on 20 teeth: 81.18 mm, 3.196 in.

With the pitch and the roller, the sprocket generator draws the ISO 606 tooth for any number of teeth and gives the DXF for plasma or laser. The sprocket dimensions guide has the formulas and a chart of pitch diameters.

Common mistakes

• Taking an 08B for a 40, or the other way round: same pitch, different roller and width.
• Measuring one pitch instead of ten, and missing a worn chain or a different size.
• Forgetting the bushing on the 25 and the 35: there is no roller, and the diameter to use is the bushing's.
• Fitting a new sprocket to a stretched chain: past about 3 % elongation the chain rides up the teeth, and no sprocket fixes that.

Frequently asked questions

What size is a 40 chain?

Half-inch pitch, 12.70 mm, with a 0.312 in (7.92 mm) roller, by ASME B29.1. The number is the pitch in eighths of an inch, 4/8, followed by 0 for a standard roller chain.

Is an 08B chain the same as a 40?

No. Both have a 12.70 mm pitch, but the 08B roller is 8.51 mm against 7.92 mm on the 40, and the widths differ. They do not run on each other's sprockets.

What does 428 mean on a motorcycle chain?

The first digit is the pitch in eighths of an inch: 4 is half an inch. The 428 has an 8.51 mm roller, the same as the 08B; the 420 is the lighter half-inch chain, with a 7.77 mm roller.

How do I measure chain pitch?

Lay the chain straight and tight, measure from the centre of one pin to the centre of the pin ten pitches along, and divide by ten. Half an inch is 12.70 mm; 5/8 inch is 15.875 mm.