Guides / Design
Which Dimensions on a Bike Part Actually Need a Tolerance
September 11, 2026
A tolerance is a request for machine time. Every step tighter adds a finishing pass, a slower feed, a measurement or all three. On a bike part, a handful of dimensions genuinely need to be tight and the rest are along for the ride.
The dimensions that always matter
Bores that take a bearing. Cartridge bearings in hubs, pulleys and pivots want an interference fit measured in hundredths. A 6902 bearing seat at 28 mm typically wants something in the region of plus 0.000 to plus 0.010 mm for a press fit. Get it loose and the bearing spins in the shell. Get it tight and the bearing is crushed and rough out of the box.
Bores that clamp a standard. A 28.6 mm steerer bore or a 31.8 mm bar clamp is opened by a pinch bolt, so it is more forgiving, but it is still a fit. Plus or minus 0.05 mm is usually enough and plus or minus 0.02 mm is being careful.
Chainline and offset. The distance from the mounting face to the chainring teeth sets the chainline. A tenth of a millimeter is invisible, half a millimeter is a shifting complaint.
Bolt circle position. Not the diameter alone, the position of each hole relative to the center and to each other. This is where a positional tolerance says exactly what you mean.
Thread positions and depths. Especially on brake mounts, where the caliper has limited adjustment.
Perpendicularity and parallelism on clamp and mounting faces. A face that is 0.2 mm out of square puts a bending load into a bolt that was never meant to see one.
The dimensions that almost never matter
The outside profile of a stem body. The depth of a cosmetic pocket. The exact width of a rib. The radius of a styling scallop. The overall height, as long as it clears. These can sit comfortably at plus or minus 0.1 or 0.2 mm and nobody will ever know.
If the dimension does not touch another part, does not set a fit and is not measured by the customer, it does not need a tight number.
What a tight tolerance actually costs
| Tolerance | What the shop does | Relative effect |
|---|---|---|
| ±0.2 mm | Normal cutting, no special measures | Baseline |
| ±0.1 mm | Normal cutting, occasional check | Baseline |
| ±0.05 mm | Finishing pass, regular checks | Small increase |
| ±0.02 mm | Dedicated finishing pass, temperature awareness, frequent gauging | Noticeable |
| ±0.01 mm and tighter | Slow finish, gauge for every part, higher scrap, possibly a different machine | Significant |
Putting ±0.02 mm on forty dimensions instead of four does not make the part four times better. It makes it several times more expensive and adds an inspection routine that slows every batch.
Say when the tolerance applies
The single most common source of argument on a machined and anodized bike part is whether the dimension is before or after finishing.
Anodizing changes dimensions. A bore loses roughly the coating thickness on diameter, 5 to 25 microns for Type II and 25 to 50 for Type III. That is more than a bearing fit tolerance.
Your drawing needs one of these notes, explicitly:
- “All dimensions after anodizing” and a coating thickness range, or
- “Dimensions before anodizing, bore A masked” or
- “Dimensions before anodizing, machine to allow for 20 micron coating on diameter”
Whichever you pick, both you and the shop are then measuring the same thing.
Datums, briefly
Pick the feature that locates the part in use and make it your primary datum. On a hub shell that is the axle bore. On a stem it is the steerer bore. On a chainring it is the mounting face and the center bore.
Then dimension the important features from that datum rather than chaining one dimension off another. Chained dimensions stack their tolerances, so five chained ±0.05 mm dimensions give you a ±0.25 mm position at the end of the chain, which was never what you meant.
A drawing that gets a good quote
- Title block general tolerance of ±0.1 mm
- Three to six dimensions individually toleranced tighter, each with a reason you could explain out loud
- A stated datum scheme
- Surface finish callouts where they matter, Ra 1.6 on a bearing seat and nothing anywhere else
- Material and temper spelled out, 6061-T6 not “aluminum”
- The finish, its thickness and whether dimensions are before or after it
- A note on which faces are cosmetic
That drawing takes an hour longer to make and saves a week of back and forth. Send it with the RFQ and the quotes come back tight.
Frequently asked questions
What general tolerance should I put on the title block?
Plus or minus 0.1 mm is a sensible default for a machined aluminum bike part, or plus or minus 0.2 mm on larger non-critical features. Then call out the handful of dimensions that need tighter, individually.
Do I need GD and T on a bike part drawing?
Not always, but position and perpendicularity callouts earn their keep on bolt circles and bearing seats. A chainring with a positional tolerance on the bolt circle is unambiguous. The same ring dimensioned as five separate hole positions is not.
Should the tolerance be before or after anodizing?
State it. A bore dimensioned after anodizing tells the shop to machine oversize by the coating growth. A bore dimensioned before anodizing tells the shop nothing about what the customer actually receives. Most disputes start here.
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