ANSI Chain Sizes: Numbering Rule, Pitch in mm, and Common Sizing Traps

A caliper across two pin centres reads 12.70 mm, the side plate is stamped 40, and the OEM parts list says 08B. All three describe a half inch pitch chain, and only one of them will seat properly on the sprocket in front of you.

ANSI chain sizes are the easiest sizing system in power transmission to read, and the easiest to get half right. The number on the plate encodes pitch, but pitch alone tells you very little about whether a replacement will run: roller diameter, inner width and plate thickness decide whether the chain sits down in the tooth seat, carries the load and clears the guide rails. What follows is how the numbering actually works, where it stops being reliable, and what to put on a purchase enquiry so the wrong box never arrives.

How ANSI Chain Numbers Encode Pitch

Divide the ANSI number by 8 and you have the pitch in inches; multiply by 25.4 to get millimetres. Chain #40 is 4/8 in, or 12.70 mm. Chain #80 is 8/8 in, or 25.40 mm. Chain #160 is 16/8 in, or 50.80 mm. This single rule covers the whole A series from #25 upward and is the fastest way to sanity check a parts list.

Pitch in millimetres equals the chain number divided by 8, then multiplied by 25.4. The rule gives you the pitch. It does not give you the roller.

#41 is the trap. It shares the 1/2 in pitch of #40, so a length cut for one will hang on the other, but its roller is 7.77 mm instead of 7.92 mm and its inner width is 6.35 mm instead of 7.85 mm. Run it on a #40 sprocket and the load lands on a narrow band of the tooth flank, wearing both parts faster than either would wear alone. #25, the smallest common size, is a bushed chain in the ANSI definition, and its 3.18 mm inner width leaves very little room for misalignment; the detailed #25 chain specification covers where that size genuinely belongs.

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ANSI and BS Chains: Same Pitch, Different Roller

The A series (ANSI) and the B series (BS and DIN) use the same inch based pitch steps, but the roller dimensions differ, so a #40 and an 08B will hang on the same shaft and run badly on the same sprocket. Pitch gets you in the door; the roller decides whether the chain sits in the tooth seat.

The gap widens at small sizes. #35 uses a 5.08 mm roller, while 06B at the identical 9.525 mm pitch uses 6.35 mm, a 25 percent difference in the one feature that transfers load into the tooth. Sprockets are cut to the roller, not to the pitch, so a B series chain on an A series sprocket rides high on the tips and wears quickly. Where a plant mixes imported and domestic machinery, log the standard as well as the number against every chain on the line.

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Measuring a Chain When the Stamping Is Gone

Measure across ten pitches, divide by ten to get the pitch, then confirm the roller diameter: pitch narrows the chain to a family, and the roller pins it to one number.

Pitch grows as pins and bushings wear, so the same measurement doubles as a wear check. Drive chains are usually retired at around 1 percent elongation, and conveyor chains in clean service can run to roughly 1.5 percent, but a chain past its limit has already reshaped the sprocket to match its stretched pitch.

Matching the Sprocket to the Same Numbers

A sprocket is defined by pitch and roller diameter together, so a sprocket that accepts one 12.70 mm chain will not necessarily accept another. Tooth count then sets the ratio, and flank wear decides how long the new chain lasts.

• Pitch and roller seat: the tooth form is cut to the ANSI or BS roller, so the standard has to match the chain, not just the pitch.
• Tooth count: the ratio is driven teeth divided by driver teeth, and pinions below 17 teeth increase chordal action and wear.
• Wear pattern: hooked or sharpened teeth mean the sprocket has already adopted the pitch of a stretched chain.
• Alignment: a straight edge across both sprocket faces should show minimal offset on short centre distances.

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What to Put on a Chain Enquiry

Send the chain number with its standard, the measured pitch and roller diameter, the strand count, the length in pitches and the material grade. A number on its own leaves too much to assumption, especially where the same number exists in stainless and carbon steel with very different load ratings.

• Standard and number: ANSI 40, BS 08B, or the measured dimensions if the stamping is unreadable
• Length in pitches rather than metres, because a 120 pitch loop is unambiguous and a cut length is not
• Strand count and attachments: simplex, duplex or triplex, plus attachment spacing and hole size
• Material and environment: carbon steel, SS304, SS316 or a precipitation hardening grade such as SS630, with washdown, caustic or high temperature conditions described
• Sprocket tooth counts and centre distance, if chain and sprockets are being replaced as a set

If the stamping is gone and the measurement sits between two sizes, send the span, the roller diameter and a photo of the sprocket to our engineering team and we will match it against the standard before anything is cut.

ANSI Chain Sizes: Common Questions

Is a #40 chain the same as an 08B chain?

No. Both share a 12.70 mm pitch, but #40 uses a 7.92 mm roller and a 7.85 mm inner width, while 08B uses an 8.51 mm roller and a 7.75 mm inner width. The chain will physically hang on the other sprocket, but the roller will not seat correctly in the tooth.

How do I convert an ANSI number to millimetres?

Divide the number by 8 to get the pitch in inches and multiply by 25.4. #50 is 5/8 in or 15.875 mm, #60 is 3/4 in or 19.05 mm, and #100 is 1 1/4 in or 31.75 mm.

Can I replace a #40 chain with #41?

Only if the sprocket and the load allow it. #41 shares the 1/2 in pitch but has thinner plates, a 7.77 mm roller and a 6.35 mm inner width, which means lower tensile strength and a narrower bearing surface on the tooth.

What does the suffix mean in 40-1 or 40-2?

The suffix gives the strand count. 40-1 is simplex, 40-2 is duplex and 40-3 is triplex. Pitch and roller diameter stay the same, so the sprocket must be cut for the matching strand count.