Seatpost diameter and shim compatibility: separate the three interfaces
Identify shaft outside diameter, frame bore and collar before considering an exact adapter. A diameter difference is not a shim design, tolerance or assembly approval.
Published 9 October 2026 · OpenBikeFit
Seatpost field guide · not to scale
Name both sides of the interface.
Fictional concentric annulus: inner 27.2 mm / outer 30.9 mm.
Diameter difference 3.7 mm; radial wall 1.85 mm.
No frame outside or collar size is supplied. Not a shim design, tolerance or part approval.
On this page
- 01Name the post outside surface
- 02Distinguish frame bore from collar size
- 03Read an adapter as an ordered pair
- 04Diameter difference is not radial wall
- 05Matching nominal numbers leaves conditions open
- 06Non-round and wedge systems are separate
- 07Engagement remains a separate check
- 08Rail and head identity are still unresolved
- 09Make a short unresolved-interface hand-off
Identify before changing
Keep the unresolved interfaces on paper
The free unfilled worksheet creates no bike or installed status. Exact manuals and physical checks remain separate.
Open toolA replacement search often begins with a number such as 27.2, 30.9 or 31.6 mm. Before using it, name the object: outside diameter of a round post shaft, inside bore of a frame, outside diameter at a frame collar, or one side of an adapter pair. These dimensions refer to different surfaces. An advertisement with the right familiar number can still describe the wrong interface.
This guide helps prepare an identification record, not a list of parts that will fit. Do not force a post, tighten an unfamiliar clamp, cut a shim or improvise packing material to test an idea. Preserve the installed system and consult exact current frame, post, collar and adapter documentation. If any identity remains uncertain, the result of this path is an unresolved question, not a substitute size.
Name the post outside surface
Park Tool distinguishes a round post diameter from frame outside diameter and notes that a post marking can help identify its size. Record the exact marking beside manufacturer, model and generation. A specification is nominal identity evidence; a number typed into a shopping filter is not a physical inspection. Keep an observed reading separately labelled rather than declaring that reading to be the manufacturer specification.
A round example does not cover an aero, oval, D-shaped or otherwise keyed shaft. Those shapes cannot be reduced to a single circular diameter to establish interchangeability. If accessing a marking would require removal or mechanical work, leave that observation unknown and request documentation or mechanic identification. This page supplies no removal procedure and no authority to disturb an installed system.
Distinguish frame bore from collar size
The receiving frame bore and the outside surface gripped by a separate collar are different interfaces. A frame wall lies between them. A collar-size label therefore cannot be used directly as a post-size label. Integrated wedges and other proprietary clamps may not have the same circular collar arrangement at all. Write which interface each quoted dimension addresses.
An original fictional section illustrates the distinction: a receiving nominal bore of 30.9 mm is not a declaration that the frame outside diameter is 30.9 mm. This construction provides no frame wall thickness and no collar dimension. Those fields remain unknown. Inventing them from a photograph would collapse two unanswered questions into one confident but unsupported number.
Read an adapter as an ordered pair
A proposed shim has an inner side intended for the post and an outer side intended for the receiving system. Keep the order explicit: fictional inner 27.2 mm, outer 30.9 mm. A label containing only one of those values is incomplete evidence for the pair. It also omits length, material, clamping arrangement and any stated engagement conditions.
Cane Creek lists paired adapter sizes and excludes internal wedge-style clamps for the adapters on that page. That is a specific manufacturer restriction, not a claim that every other system is allowed. Do not transfer the listed pair or exclusion to a different adapter maker, frame or generation. An identified product and the applicable permissions from all involved manufacturers are still required.
Diameter difference is not radial wall
For an original ideal concentric annulus with outer diameter 30.9 mm and inner diameter 27.2 mm, the diameter difference is 3.7 mm. The radial wall is half that difference: (30.9 − 27.2) / 2 = 1.85 mm. This is pure circular geometry. It does not specify an adapter wall, slit, manufacturing tolerance, contact length or material.
The fictional picture contains no real part and no clamping load. Do not order raw material or assess a used shim from the 1.85 mm result. Actual products may have features and conditions absent from a continuous annulus. The calculation only prevents confusing a diametral difference with a radial thickness; it cannot turn two catalogue numbers into an approved assembly.
Matching nominal numbers leaves conditions open
Even when labels agree, a dimensional record still needs exact component identity and applicable conditions. Material, surface state, shape, clamp design and the permitted adapter arrangement are not encoded in a diameter alone. Do not make a near-size choice because a ruler cannot resolve the difference. There is no browser tolerance that converts approximately matching values into demonstrated fit.
Keep readings with method and resolution where they are safely available. A rounded measurement should not silently become a different nominal size. If an observation disagrees with identified documentation, investigate the endpoints, part identity and method with a mechanic or manufacturer. Do not sand, squeeze or use tightening as a test of which number is correct.
Non-round and wedge systems are separate
Park Tool describes proprietary non-round and seatmast arrangements separately from common round posts. Use the exact frame system rather than applying the concentric drawing to a shaped profile. Rotation keying, wedge contacts and a mast topper can introduce independent identity questions. An apparently matching visible outline is not a complete interface specification.
For these systems, record the frame model and generation, post or topper identification and exact clamp parts. Leave circular diameter arithmetic out of the approval question unless the applicable manufacturer explicitly defines that interface. A circular shim example does not prove that a wedge can clamp it, that a keyed post can rotate, or that a topper is interchangeable.
Engagement remains a separate check
A matched diameter pair does not establish sufficient inserted length. The post, frame and applicable adapter can have independent engagement requirements and insertion constraints. Use the next guide to distinguish lower engagement bounds from upper insertion bounds; do not simply add every printed length. Each limit needs the same declared datum before an interval can be compared.
Adapter length is not automatically a permitted exposed length or a safe minimum for the frame. Retain its exact conditions rather than borrowing a generic multiple of diameter. This article deliberately provides no universal engagement rule. Without applicable limits, label the compatibility record incomplete even if the nominal round sizes happen to agree.
Rail and head identity are still unresolved
The shaft may be identified while the head-to-saddle interface remains unknown. The Ritchey specification lists shaft diameters and a particular supplied rail clamp separately. That separation is the lesson, not a recommendation for those parts. Continue to the existing rail/clamp guide for exact cross-section, clamp-parts and permitted-zone evidence.
Likewise, shaft size cannot answer post offset or the installed BB-relative saddle point. A useful comparison keeps these records side by side without treating one as a shortcut for another. You can finish this guide with a fully identified round shaft and still lack the evidence needed to select or install a replacement. That is an honest intermediate result.
Make a short unresolved-interface hand-off
On the blank worksheet, keep a line for each item: current post identity and nominal outside diameter; frame identity and receiving interface; collar or wedge identity; proposed adapter inner/outer pair and source; engagement conditions; rail/clamp identity. Mark unavailable observations as unknown. Do not enter zero to stand for a missing size or infer a private customer requirement from a public example.
Ask the manufacturer or mechanic the remaining exact question, such as whether an identified adapter is permitted with an identified frame clamp and post material. This guide neither supplies that answer nor creates an installed record. Opening a tool or worksheet remains free. The result is a clearer evidence request, not fit approval, a purchasing verdict or a promise about riding outcomes.
Practical questions
Frequently asked questions
Is seatpost diameter the same as seat collar size?
No. Post outside diameter, receiving bore and frame outside/collar interface are different surfaces. Proprietary systems need their own identity.
Does a 27.2-to-30.9 pair approve a shim?
No. It names two nominal interfaces only. Exact adapter, frame, post, clamp and engagement conditions remain necessary.
Is the wall for a 3.7 mm diameter difference 3.7 mm?
An ideal concentric annulus has a 1.85 mm radial wall. That fictional arithmetic is not a real shim specification or tolerance.
Can a round shim be assumed to work with a wedge clamp?
No. The cited Cane Creek adapters specifically exclude internal wedge-style clamps. Other systems require their own exact applicable permission.