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Bicycle stem size standards

Updated: 17/08/2020.

Table of contents:
0. Introduction
1. Types of stems
….1.1. Quill stem
….1.2. Threadless stem
2. Important dimensions and how to measure
….2.1. Handlebar (clamp) attachment diameter
….2.2. Steering column attachment diameter
3. Stem dimension standards
….3.1. Handlebar clamp dimension standards
….3.2. Steering column attachment dimension standards
……..3.2.1. Threaded stems
……..3.2.2. Threadless stems

0. Introduction

This post explains stem size standards in terms of steering column attachment and handlebar attachment at the opposite end of the stem. Other dimensions, such as stem length and angle will be explained in a separate post.

When purchasing, or replacing a stem, it is important to know what type and what size is needed. Both the side of steering column attachment, as well as on the side where the bars are attached. If bars are not already owned (at hand), author’s recommendation (personal preference based on knowledge and experience) is to choose a stem with a modern OS (“oversized) bar attachment diameter (table 1 lists the standard dimensions). Dimensions and type of steering column attachment depend on the fork, that is depended on the frame, so that’s something a bit harder to “change” – though, if choosing a frame, this author’s recommendation is to look for one with a 1 1/8″ steering column diameter and a threadless fork – for several reasons. The remainder of this post will explain stem types, important dimensions, how to measure them and, finally, list the dimension standards.

1. Types of stems

There are two types of stems (in terms of standard attachment dimension): quill (for threaded forks) and threadless (a.k.a. ahead) stems (for threadless forks).

1.1. Quill stem

Quill stem is attached to the steering column by inserting it into the steering column and tightening a bolt on top of the stem, that pulls the quill upwards, making the lower stem diameter wider. Without much going into details and various designs, the basic principle is shown in picture 1.

1.2. Threadless stem

Threadless stems have a slot at the steering column attachment end. It is fastened by sliding it onto the steering column, and then tightening the pinch bolt(s) that hold it fastened.

Threadless ("ahead") stem Picture 2
Threadless (“ahead”) stem
Picture 2

2. Important dimensions and how to measure

Stems are made with various lengths and angles. Picture 2a shows a quill stem with a shorter neck, angled downwards, next to a threadless stem with a longer neck. Lenghts and angles differ and are important when fitting a bicycle to suit a rider. However, in terms of (attachment) standards, important dimensions are the handlebar attachment diameter and steering column attachment diameter.

2.1. Handlebar (clamp) attachment diameter

This dimension is measured practically identically for all the stems. The name says what it is. Measurement method is shown in picture 3.

Picture 3b doesn’t show clearly enough, but the calipers are placed at the inner side of the black part of the stem, just near the slit – there would be no use measuring at the slit itself, of course.

One could also measure the (outer) bar diameter (at the stem attachment point) – if a new stem is sought to match the bars.

2.2. Steering column attachment diameter

For threadless stems, this is measured the same as the handlebar attachment diameter. Picture 4 says it all.

Of course, if a new stem is sought for a fork at hand, then the fork is measured. Picture 5 shows how.

3. Stem dimension standards

After dimensions were defined and their measurement explained, now an overview of the standard dimensions will be given. First table will show bar attachment diameter standard. Second table will show threadless stems for attachment diameters. While the third one will show that for the quill stems.

3.1. Handlebar clamp dimension standards

TABLE 1
Handlebar attachment diameter – measurement units: mm (inches) Standard name
22.2 (7/8″)for steel bars of old MTB and BMX bikes
23.8 (15/16″)Obsolete British standard common on old British 3-speeds
25Obsolete French standard
25.4 (1″)Standard ISO size, common for bars from the end of the 20th century
26Italian road bike (and bar) standard. Obsolete
27Titan – obsolete standard
31.7 – 31.8 – depending on how manufacturers round down the inch dimension which is (1 1/4″)Modern “oversized” standard
35 (1 3/8″)Easton standard

3.2. Steering column attachment dimension standards

3.2.1. Threaded stems (quill stems)

TABLE 2
Stem “neck” outer diameter – measurement units: mm (inches) Standard name
21.15 (13/16″)Old American and BMX standard
22Old French standard
22.2 (7/8″)Standard for most threaded forks with an outer diameter of 25.4 mm (1″)
25.4 (1″)Standard for most threaded forks with an outer diameter of 31.8 mm (1 1/8″) – not very common

3.2.2. Threadless stems

TABLE 3
Steering column outer diameter (and inner diameter for the attachment on the stem)Standard name
measurement units: mm (inches)
25.4 (1″)One inch standard
28.6 (1 1/8″)1-1/8 inch standard
31.8 (1 1/4″)1-1/4 inch standard
38.1 (1 1/2″)1-1/2 inch standard

Related post – Bicycle handlebar dimension standards:

Bicycle handlebar dimension standards
Bicycle handlebar dimension standards
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5 thoughts on “Bicycle stem size standards”

  1. i am looking to replace a bent bracket on a Diamond back BMX mini bike. It looks like it has a threadedless stem which is 7/8 OD by 6 inches long question I have is how to ensure that the top part which is a welded bottom plate to stem with top clamp for handle bar and for bolts will be correct when ordering since sites don’t show that dimension

    Reply
  2. I am looking for a handle bar or a substitute handlebar clamp. I have a restored single speed bike from the 1970’s. The handlebars were way uncomfortable as they wrapped around. the handlebar clamp attaches with a very long screw into the stem. The circumference of the area on the handlebar were the clamp would be is 3 inches. Diameter is just under 0.95 of an inch (24.254895017319mm). Is there a larger handlebar clamp out there? Should I just buy a new stem and if so what size stem? Can I buy a stem that will fit the bike and fit the more standard size mountain bike style handlebars?

    Reply
    • Give me the metric system, or give me death!
      🙂

      This article lists all the standards.
      I’m not sure what kind of stem that is, but guess it’s a quill stem.
      Measure the diameter as shown in picture 4a.
      If it’s 22.2 mm, then it’s relatively common nowadays and finding one that has handlebar clamp for 25.4 mm (1″) should be relatively easy and they are quite cheap.
      Finding one for more modern, “oversized” handlebars (1 1/4″), is not as simple, but there still are 25.4 mm bars available (at least in Serbia).

      24.25 mm is something I haven’t seen (or at least measured). Not sure where I’d look for bars that fit such stem.

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