Custom CNC Machined Bike Stems
A stem is two clamps and a tube between them, which makes it look simple. Both clamps are fits against a standard, the faceplate has to close evenly onto a bar that may be carbon, and the whole part carries a rider through a steering input. It is the most commonly machined bike component and the one where a sloppy drawing shows up fastest.
What this covers
- Road, gravel and MTB stems
- Faceplates sold separately
- Direct mount and integrated cockpit stems
- Headset top caps and spacers
- Stem shims and adapters
Interfaces you do not get to choose
- Steerer bore28.6 mm for a 1 1/8 in steerer
- Bar clamp bore31.8 mm, 35 mm, 25.4 mm on older bars
- Steerer clamp boltsTwo M5, sometimes one M6
- Faceplate boltsFour M5, two M5 on minimal designs
- Stack height38 to 42 mm typical
Full tables in the standards reference.
Material choice
What each option actually buys you on this part, rather than in general.
| Material | Why it gets used here |
|---|---|
| Aluminum 6061-T6 | The default. Size is set by the bar and steerer, so alloy strength has nowhere to go, and 6061 anodizes evenly across a color range. |
| Aluminum 7075-T6 | For stems sold on weight, where sections are thinned far enough that alloy strength does start to matter. Harder to color match. |
| Titanium Grade 5 | Boutique short runs. Heavy for the strength gained on a part this size, so it is a positioning choice more than an engineering one. |
Where the tolerance belongs
These are typical working ranges, not promises. Tolerance these dimensions and leave the rest at a general tolerance, because every step tighter than you need buys machine time you will pay for.
| Feature | Typical | Why it matters |
|---|---|---|
| Steerer bore | ±0.05 mm | Clamp bore, opened by the pinch bolts. Too tight and it will not slide onto the steerer. |
| Bar clamp bore | ±0.02 to ±0.05 mm | Carbon bars have a torque limit. An oval or oversize bore crushes at two lines instead of clamping in a band. |
| Faceplate joint faces | Flat and parallel | If they pinch at one edge the bar sees a point load. |
| Reach and rise | ±0.2 mm | Fit dimension for the rider, not a mechanical fit. |
| Cosmetic surfaces | ±0.1 to ±0.2 mm | Nobody measures them. |
Finish
Bead blast plus Type II anodize is standard. Keep Type III off a stem unless you have fatigue tested it that way, because hard anodize costs fatigue life on the exact part you least want it on.
Realistic quantities
Prototypes from 1 piece. Most shops are comfortable from 50, and price settles from about 200. Anodizing batch minimums usually set the real floor, not the machining.
What goes wrong on this part
Failure modes specific to this component, collected so you can design them out rather than discover them.
- Sharp slot endsThe pinch slot must end in a drilled hole or a radius. A square slot end is a crack starter on a part that carries a rider.
- Setup countFour or five setups on a 3-axis machine, two or three with a 4th axis. This is the single biggest price variable, and it is why quotes for the same stem differ by a factor of two.
- Pockets in the wrong placeBending load runs through the top and bottom skins of the extension. Pocket the sides, not the skins.
- Faceplate interchangeabilityBore is usually finish machined with the faceplate bolted on, so faceplates are matched to a body. Decide whether you want them interchangeable and say so.
What to send
A drawing with these on it gets quoted in a day. One without them gets quoted with padding in it, or gets four rounds of questions first.
- STEP file plus a PDF drawing
- Steerer and bar bore sizes with tolerances
- Reach, rise and stack height
- Whether the bore is finish machined with the faceplate fitted
- Bolt sizes, thread depth and marked torque values
- Finish, color and any laser mark
Printable version: the RFQ checklist.
Go deeper
- Design
Machining a Bike Stem: Spec, Fits and Cost Drivers
Steerer and bar bore fits, faceplate flatness, pinch bolt geometry and the setups a stem really needs. A spec checklist before you send the drawing out to quote.
- Design
Design for Machining: Bike Parts That Quote Cheap
Wall thickness, pocket depth, internal radii, tool access and setup count. The drawing decisions that decide half the price of a machined bike component.
- Finishing
Type II vs Type III Anodizing for Machined Bike Parts
Decorative or hard anodize? What each does to color, wear, your bore sizes and the fatigue life of a structural part, plus what to put on the drawing.
Other parts
Chainrings and spiders
A flat part with the tightest tolerance on the site. Tooth width decides whether a 1x chain stays on.
Cranks and pedal hardware
Fatigue critical, envelope fixed by the frame and the rider. The part where anodize choice changes test results.
Hub shells and freehubs
Turning plus milling in one part. Concentricity between the bearing seats decides whether it drags.
Brackets, mounts and hardware
The long tail. Low part cost, fixed interfaces, and finishing batch minimums that dominate the quote.
E-bike drive hardware
Machined like a bike part, loaded like a light vehicle part, regulated like an electrical product.