Is a missing cassette tooth a shift gate or damage?

An apparent gap in a cassette is not automatically a broken tooth. Modern and older cassettes use deliberately shortened, shaped, or occasionally omitted teeth to help the chain move between sprockets. But impact damage and fatigue can also remove a tooth, so identify the cassette and inspect the evidence before riding.

The short answer: clean the cassette, find its exact model, and compare the tooth pattern with the maker’s exploded view or a known-good example of the same model and ratio. Smooth, repeated, aligned profiles usually indicate designed shift features. A fresh jagged surface, bent neighbouring teeth, cracks, or new skipping points to damage.

Why cassette teeth are not all identical

A recent r/bikewrench discussion showed an older SRAM road sprocket marked for 13 teeth but with an apparent missing tooth. Several initial answers called it broken before contributors identified the OpenGlide cassette and its intentional pattern. That disagreement is the useful lesson: counting teeth without identifying the part can produce the wrong diagnosis.

Indexed shifting asks the chain to leave one sprocket and engage another at controlled locations. Manufacturers create those opportunities with shaped teeth, ramps, gates, pins, and timed alignment between adjacent sprockets. Shimano describes Hyperglide cassettes as using specifically positioned shift ramps and tooth profiles. SRAM’s archived technical manual identifies the OG-1070 as an OpenGlide 10-speed road cassette.

The visual result can include:

• teeth shorter than their neighbours;
• asymmetric tips;
• scooped or chamfered sides;
• aligned features across several sprockets;
• on some historical designs, a conspicuous gap.

Do not use the appearance of a new cassette family to judge an older one.

Identify the cassette before counting anything

Look for model markings on the lockring, largest sprocket, carrier, or individual cogs. Record:

• manufacturer;
• complete model number;
• number of rear speeds;
• tooth range printed on the lockring or sprockets;
• whether sprockets are loose, pinned, riveted, or carried on a spider;
• freehub interface;
• bicycle or component year if known.

Search the manufacturer’s manual archive, exploded view, or spare-parts catalogue. Match the exact ratio because tooth patterns can differ across options within one model family.

If the cassette cannot be identified, ask a mechanic to inspect it in person. A cropped photograph cannot reveal looseness, cracks around rivets, or how the chain engages under gentle load.

Signs the shape is intentional

Designed shift features tend to show a pattern:

• the edges are smoothly formed and carry the same finish or aged surface as nearby teeth;
• similar features appear at planned intervals;
• neighbouring sprockets have ramps or shortened teeth aligned with the feature;
• the maker’s diagram or a verified identical cassette shows the same geometry;
• the chain settles normally and shifting behaviour has not changed.

A sprocket labelled “13T” can still have the pitch diameter and chain engagement expected for that ratio even when a historical design includes an intentional gap. The stamped number identifies the sprocket’s nominal tooth count or ratio position; it is not, by itself, proof that every visible peak must look full height.

Signs a tooth is actually broken

Damage is more likely when you find:

• bright raw metal or a jagged granular fracture;
• a tooth bent sideways;
• impact marks on adjacent teeth, derailleur, chainstay, or spokes;
• a crack running into the sprocket body or around a rivet;
• a loose sprocket or carrier;
• one isolated defect absent from documentation and matching sprockets;
• new skipping, banging, binding, or derailment in one gear;
• a damaged chain link at the corresponding event.

Rust and dirt can hide a fresh edge. Clean with the method approved for the drivetrain, using enough light to inspect both sides. Do not file or reshape the tooth: that destroys evidence and can make chain engagement worse.

Test without creating a dangerous load

If there are no cracks, loose parts, sharp fractures, or damaged chain links, rotate the cranks slowly in a work stand or with the rear wheel safely lifted. Watch the chain enter and leave the suspect sprocket while shifting one click at a time under very light load.

Stop if the chain:

• rises onto tooth tips;
• catches at the same point each revolution;
• jumps forward;
• tries to enter the spokes or frame;
• pulls a visibly damaged link tight.

Do not perform a hard standing test to “see if it holds.” If actual riding produces skipping, use the chain-skips-under-load diagnosis and inspect chain wear, cassette wear, derailleur alignment, indexing, and freehub security as a system.

One odd tooth is not a cassette-wear gauge

Ordinary cassette wear does not always look like one cleanly missing tooth. Chain and sprocket wear develop together, and a cassette can shift acceptably in the stand yet skip with a new chain under load. Conversely, dramatic-looking factory ramps can work normally for years.

Use history and function:

• Was there an impact or chain jam?
• Did the symptom begin suddenly?
• Is the chain beyond its wear limit?
• Does skipping occur only with a recently replaced chain?
• Is the lockring secure to the maker’s specification?
• Does the exact sprocket pattern match official documentation?

The chain-or-cassette replacement guide explains how to separate wear pairing from visible damage.

Replace the right unit

Some cassettes allow individual sprocket replacement; others use matched groups or carriers. Even when a loose cog is sold separately, its position, thickness, timing, and spacers must match. Do not substitute a similar tooth count from another family based only on diameter.

Use the manufacturer part number and exploded view. If the cassette is obsolete, compare the cost and availability of the correct sprocket with a complete compatible cassette and chain-wear inspection.

Your next step

Clean the cassette and photograph its model markings plus the suspect area from both sides. Find the official exploded view for that exact model and ratio. If the pattern is undocumented, fractured, loose, or accompanied by skipping, stop riding and have the cassette and chain inspected.

Sources and further reading

• SRAM archived technical manual: OG-1070 OpenGlide road cassette
• Shimano: Hyperglide cassette shift ramps and tooth profiles
• r/bikewrench reader context: an intentional missing-tooth pattern mistaken for damage — used as a current identification signal; comments were not treated as product documentation.