What the three digits actually mean
A motorcycle chain is stamped with a three-digit number — 420, 428, 520, 525, 530 — and the number is not a model code. It is two measurements written next to each other.
The first digit is the pitch, in eighths of an inch. Pitch is the distance from one pin centre to the next, and it is the measurement that decides whether a chain will sit on a sprocket at all. A 4-series chain has a 4/8 inch pitch, which is 1/2 inch or 12.70 mm. A 5-series is 5/8 inch, 15.875 mm. A 6-series is 3/4 inch, 19.05 mm.
The last two digits describe the internal width — the gap between the inner side plates, which is what the sprocket teeth have to fit through. ChainMate states the rule directly: those digits are “multiples of 1/8 inch for the internal width between plates. For example, 20 means 2 x 1/8 inch = 1/4 (6.35mm)”.
Read that way, the whole system decodes itself. A 530 is a 5/8 inch pitch chain with a 3/8 inch internal width. A 520 is the same pitch with a 1/4 inch width. They take identical sprockets, because sprocket tooth spacing is set by pitch and nothing else.
The chart
| Size | Pitch | Pitch (mm) | Internal width | Width (mm) | Usually found on |
|---|---|---|---|---|---|
| 415 | 1/2 in | 12.70 | 3/16 in | 4.76 | Mopeds, small commuters |
| 420 | 1/2 in | 12.70 | 1/4 in | 6.35 | Under about 125 cc |
| 428 | 1/2 in | 12.70 | 5/16 in | 7.94 | Roughly 125–250 cc |
| 520 | 5/8 in | 15.875 | 1/4 in | 6.35 | Sportbikes, most conversions |
| 525 | 5/8 in | 15.875 | 5/16 in | 7.94 | Middleweights and up |
| 530 | 5/8 in | 15.875 | 3/8 in | 9.53 | Large sportbikes, cruisers, tourers |
| 630 | 3/4 in | 19.05 | 3/8 in | 9.53 | Older large-capacity bikes |
Pitch figures are exact conversions of the fractions. The internal widths are what published specification tables agree on across suppliers; individual manufacturers vary in the outer dimensions rather than these. The last column is a rough guide to where each size tends to turn up rather than a rule — capacity is not what decides chain size, torque and intended use are, and plenty of bikes sit outside the usual band for their engine size.
ChainMate publishes overall widths for the 5-series that give a sense of how much difference those fractions make once the plates and pins are counted: 520 comes out at 20 to 21.5 mm across, 525 at 22.6 to 23.7 mm, and 530 at 24.9 to 25.3 mm.
428 breaks the rule everyone quotes
Take the numbering rule at face value and apply it to a 428. The last two digits are 28, so the internal width should be 2.8 × 1/8 inch:
2.8 ÷ 8 = 0.350 in = 8.89 mm
Every published spec table gives the 428 an internal width of 5/16 inch, which is 0.3125 in or 7.94 mm. That is not a rounding difference. It is a full millimetre out, and it makes the 428 exactly as wide inside as a 525.
So the rule that gets repeated on every page that explains chain numbering does not describe one of the most common small-bike chains in the world. The 4-series and 5-series widths run 15, 20, 25, 30 — and then 428 sits in the middle of the 4-series with a number that does not fit the pattern it belongs to.
The practical consequence is small but real: you cannot work out a 428’s width from its name, and if you are cross-referencing sizes by arithmetic rather than by table, that is the one that will catch you. Everything else in the chart follows the rule.
Pitch decides fitment. Width decides strength.
This is the distinction that makes the chart useful rather than trivia.
Pitch has to match. A 520 chain on 428 sprockets, or a 428 chain on 520 sprockets, will not mesh. The teeth are spaced for one pitch and one pitch only. Get this wrong and nothing will run.
Width does not have to match the sprocket, because sprocket teeth are cut for the pitch. A 520 chain runs perfectly well on sprockets sold for a 530, since both are 5/8 inch pitch. What changes is how much metal is carrying the load, and therefore how much torque the chain will take before it stretches or breaks. A wider chain is stronger and heavier; a narrower one is lighter and has less margin.
That is the entire logic behind a 520 conversion, which is one of the most common modifications on a sportbike. The bike leaves the factory with a 525 or 530 for durability across a long service life. Swapping the whole set to 520 — chain and both sprockets — drops rotating and unsprung weight for a machine that is not being asked to tow anything. It works because the pitch is unchanged. It is a trade of service life for weight, not a free upgrade.
How to identify the chain already on your bike
The size is usually stamped on the outer side plates, repeated every few links. Clean a section with a rag and look along it in good light; on a worn or greasy chain it can be almost invisible.
If nothing is legible, measure the pitch. Do not measure a single pitch — measure across several and divide, because a millimetre of error on one pitch is enough to put you in the wrong series:
- Find a straight run of chain, on the bottom of the swingarm rather than around a sprocket.
- Measure from the centre of one pin to the centre of a pin ten pitches away.
- Divide by ten. Around 12.7 mm means a 4-series; around 15.9 mm means a 5-series; around 19 mm means a 6-series.
Then measure the internal width by slipping a feeler or a caliper jaw between the inner plates. Combined with the pitch, that puts you on exactly one row of the chart.
Measuring across ten pitches has a second use: on a worn chain the reading will come out longer than the nominal figure, because wear at the pins is what chain stretch actually is. That is the basis of the standard wear check, which sits alongside the slack setting in motorcycle chain adjustment. If your bike turns out not to have a chain at all, chain vs belt vs shaft drive covers what you have instead.
What the size does not tell you
Two chains of the same size can be very different products, and none of that difference is in the number.
- Sealing. A plain chain has bare pins and bushings. An O-ring, X-ring or Z-ring chain traps grease behind rubber seals between the plates. Sealed chains last far longer on the road and cost more drag; unsealed chains are lighter and are still normal on racing and off-road machines that are stripped and lubricated constantly. The seals also change how you are supposed to clean it, which is covered in how to clean and lube a motorcycle chain.
- Tensile strength. Manufacturers publish tensile ratings that vary substantially within a single size, depending on the steel and the treatment. A premium 520 can be stronger than a budget 530.
- Link count. The size says nothing about length. Two 525 chains for two different bikes will have different numbers of links, and the count you need after a sprocket change comes from the pitch, the centre distance and the tooth counts — that calculation is built into our motorcycle gearing calculator.
Buying a chain and sprockets together
Sprockets wear into the shape of the chain running on them, and a chain wears into the shape of the sprockets. Fitting a new chain to worn sprockets transfers the old wear pattern into the new chain within a few hundred miles, so the standard advice is to replace chain, front sprocket and rear sprocket as a set.
Which also makes it the natural moment to change the gearing, since you are buying both sprockets anyway. Before ordering, run the tooth counts you are considering through the calculator: it will give you the final drive ratio, the exact rear-tooth equivalent of one front tooth for your specific bike, the resulting speedometer error, and the chain length in links. The reasoning behind each of those numbers is set out on the gearing calculator page, and the speedometer half of it — including the bikes where a sprocket change makes no difference at all — is in does changing sprockets affect your speedometer.
If you just want to see what a given combination comes out at without doing any arithmetic, the ready-reckoned figures are in the motorcycle gear ratio chart.
Frequently asked questions
What is the difference between 520, 525 and 530 chain?
Only the width. All three have a 5/8 inch pitch, so they use the same sprockets and are interchangeable in terms of fitment. The internal widths are 1/4 inch, 5/16 inch and 3/8 inch respectively, so the wider ones carry more load and weigh more.
Can I put a 520 chain on 530 sprockets?
Yes. Pitch is what has to match, and 520 and 530 share the same 5/8 inch pitch. This is exactly what a 520 conversion does, although converting properly means changing the sprockets too so the chain is not running on teeth machined wider than it needs.
How do I know what chain size my motorcycle needs?
Check the stamp on the side plates of the chain already fitted. If it is unreadable, measure the pin-to-pin distance across ten pitches and divide by ten: about 12.7 mm is a 4-series, about 15.9 mm is a 5-series, about 19 mm is a 6-series. Then measure the gap between the inner plates to identify the width.
What does the 428 in a chain size mean?
A 1/2 inch pitch with a 5/16 inch internal width. It is the one common size where the usual rule does not work: applying the standard “last two digits are eighths of an inch” logic to 28 would give 0.350 inch, but the 428’s actual internal width is 0.3125 inch, the same as a 525.
Is a wider motorcycle chain always better?
No. A wider chain is stronger but heavier, and the extra weight is rotating and unsprung, which is the weight that affects handling most. Manufacturers fit wider chains for service life across the whole ownership of a bike, which is why lighter conversions are common on machines used hard for short periods.