Guides / Design
What Goes Into Machining a Bike Stem
September 13, 2026
A stem looks like a simple part. It is two clamps and a tube between them. The difficulty is that both clamps are fits against a standard, the faceplate has to close evenly and the whole thing carries a rider’s weight through a steering input.
The interfaces you do not get to choose
| Feature | Common sizes | Why it is fixed |
|---|---|---|
| Steerer bore | 28.6 mm for 1 1/8 in | Set by the fork steerer standard |
| Bar clamp bore | 31.8 mm, 35 mm, 25.4 mm | Set by the handlebar |
| Steerer clamp bolts | M5, two, sometimes one M6 | Torque and fit convention |
| Faceplate bolts | M5, four, sometimes two | Convention, and every rider owns a 4 mm key |
| Stack height | 38 to 42 mm typical | Headset and spacer stack |
Get these wrong and the part is scrap regardless of how well it is machined. Put them on the drawing as toleranced dimensions, not as a note.
The fits
Steerer bore. A clamp bore, so it is slightly larger than the steerer and closed by the pinch bolts. Plus or minus 0.05 mm works. Too tight and the stem will not slide down the steerer. Too loose and the clamp closes further than designed, which loads the slot.
Bar bore. Same idea, but it matters more because carbon bars have a torque limit and an uneven clamp crushes them. A round, evenly sized bore with a good surface is what protects the bar. Ra 1.6 or better on the clamp faces, and no sharp edge at the bore entry.
Faceplate joint faces. These need to be flat and parallel. If they are not, the faceplate pinches at one edge, the clamp pressure goes into two lines rather than a band and the bar sees a point load.
The features that decide whether it survives
Slot ends. The pinch slot on the steerer clamp ends in a hole or a radius. A sharp slot end is a crack starter. Drill the end of the slot and radius the entry.
Bolt hole edge distance. Keep enough material between the bolt hole and the edge of the clamp ear. Too little and the ear bends under torque.
The transition from extension to clamp. This is where bending load peaks. Blend it generously. It is also where anodize cracking starts, which is why anodize type matters on a structural part.
Weight pockets on the extension. Fine, and in the low stress zone if you put them on the sides. Pockets that thin the top and bottom skins are removing the material that is actually carrying the bending load. More on this in weight versus strength.
Material and finish
6061-T6 is the default and it is the right default. The stem’s size is set by the bar and the steerer, so the extra strength of 7075 has nowhere to go, and 6061 anodizes evenly, which matters on a part sold in a color range. 7075 turns up on stems sold on weight claims, where the designer has thinned sections to the point that alloy strength does matter.
Bead blast plus Type II anodize is the standard finish. Keep hard anodize off a stem unless you have tested it that way.
Setups, and why the price moves so much
A stem body has features on the top, the bottom, both sides and both bore axes. On a 3-axis machine that is four or five setups, each with its own fixture and program. Put it on a machine with a 4th axis and the two bore axes can be cut in one setup, which removes both cost and the position error between them.
If you are quoting a stem and the prices come back far apart, the spread is usually machine capability, not margin. Say on the RFQ that you expect the part to run on a 4th axis and the quotes get closer together.
A stem drawing checklist
- Steerer bore size, tolerance and surface finish
- Bar bore size, tolerance and surface finish, and whether it is finish machined with the faceplate fitted
- Faceplate joint face flatness and parallelism
- Reach and rise, with the reach measured center to center
- Stack height
- Bolt sizes, thread depth and recommended torque, including whether the torque is marked on the part
- Slot width and end radius
- Material and temper
- Finish, thickness, masking and whether dimensions are before or after it
- Which faces are cosmetic
That is ten lines. It is the difference between one quote round and four.
Frequently asked questions
What bore sizes does a stem need?
Standard steerer is 28.6 mm for a 1 1/8 inch fork. Bar clamps are 31.8 mm on most road and mountain bars and 35 mm on newer mountain bars, with 25.4 mm still around on older and some gravel bars. Anything else is a custom interface and needs to be called out clearly.
Should the faceplate be machined as a pair with the body?
The bore is usually finish machined with the faceplate bolted on, so the two halves are matched. That gives a round clamp with the right preload and it means faceplates are not freely interchangeable between units unless the design allows for it. Decide which you want and say so.
How many setups does a stem take?
On a 3-axis mill, typically four or five for the body plus two for the faceplate. With a 4th axis it comes down, often to two or three. It is the single biggest cost variable on the part.
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