The Simple Fix for a Perfect Seatpost Fit – XLC Calibration Bushing SP-X20

You’ve found the perfect seatpost. The right diameter, the right length, the right weight. There’s just one problem — your frame’s seat tube is slightly larger than the seatpost. Without the right bushing, you’re looking at play, creaking, and potential damage to both the post and the frame. The XLC Calibration Bushing SP-X20 is the elegant solution: a precision-machined aluminium sleeve that bridges the gap between a 31.6mm seatpost and a 34.9mm seat tube, letting you run the post you want in the frame you have.

At just 55 grams and 120mm long, the SP-X20 is a lightweight, low-profile adapter that installs in seconds and disappears inside your frame. No compromise on stiffness, no creaking, no play — just a perfectly fitted seatpost, every time.

Key Features

Bridges 31.6mm to 34.9mm: Allows a 31.6mm seatpost to be fitted securely in a frame with a 34.9mm seat tube inner diameter. Eliminates play and prevents damage caused by an oversized seat tube bore.
120mm length: The 120mm length provides a generous contact area between the bushing and the seat tube, ensuring a secure, rattle-free fit and even distribution of clamping force along the seatpost.
Precision-machined aluminium: The SP-X20 is machined from aluminium for a precise fit, low weight (55g), and excellent corrosion resistance. The silver finish blends cleanly with most seatposts and frames.
Slotted design: The longitudinal slot allows the bushing to compress slightly when the seat clamp is tightened, ensuring a firm, secure grip on both the seatpost and the seat tube wall.
Universal compatibility: Suitable for use with any round 31.6mm seatpost (rigid or dropper) in any frame with a 34.9mm seat tube bore — road, MTB, gravel, or city bike.
Easy installation: Simply slide the bushing over the seatpost, insert into the seat tube, and tighten the seat clamp to the manufacturer’s specified torque. No tools, adhesives, or special skills required.

Specifications

Brand: XLC
Model: SP-X20
Inner diameter: Ø31.6 mm (fits 31.6mm seatpost)
Outer diameter: Ø34.9 mm (fits 34.9mm seat tube)
Length: 120 mm
Weight: 55 g
Material: Aluminium
Color: Silver
Includes: 1 x calibration bushing (Kalibrierbuchse)
Manufacturer article number: SP-X20
EAN: 4055149475406

Frequently Asked Questions (FAQ)

What is a calibration bushing (seatpost shim) and why do I need one?

A calibration bushing (also called a seatpost shim or reducer) is a thin-walled sleeve that fits between a seatpost and a seat tube when the seat tube’s inner diameter is larger than the seatpost’s outer diameter. Without a bushing, the seatpost will have play in the frame, which causes creaking, instability, and potential damage to both the post and the frame over time. The XLC SP-X20 fills the gap precisely, allowing a 31.6mm seatpost to fit securely in a 34.9mm seat tube.

How do I know if I need this specific bushing?

You need this bushing if your seatpost is 31.6mm in diameter and your frame’s seat tube has an inner diameter of 34.9mm. Check your frame’s specifications (usually found in the owner’s manual or on the manufacturer’s website) and your seatpost’s diameter (usually stamped on the post itself). If the numbers match — 31.6mm post, 34.9mm seat tube — this is the correct bushing. XLC also offers the SP-X20 in other diameter combinations for different frame and seatpost pairings.

Is this bushing compatible with dropper seatposts?

Yes — the XLC SP-X20 is compatible with both rigid and dropper seatposts with a 31.6mm outer diameter, provided the dropper post’s cable/hose routing does not conflict with the bushing. Most internal-routing dropper posts route the cable through the frame before the seatpost is inserted, so the bushing simply slides over the post as normal. For external-routing dropper posts, check that the bushing does not obstruct the cable port at the top of the seat tube.

How do I install the calibration bushing?

Installation is straightforward: slide the bushing over the seatpost (the bushing’s inner diameter should slide smoothly over the post), then insert the post and bushing together into the seat tube. Set your saddle height, then tighten the seat clamp to the torque specified by your frame manufacturer. The slotted design of the bushing allows it to compress slightly under clamping force, gripping both the seatpost and the seat tube wall securely. Apply a thin layer of carbon assembly paste or grease to the bushing’s outer surface if recommended by your frame manufacturer.

What torque should I use when tightening the seat clamp?

Always follow your frame manufacturer’s specified torque for the seat clamp bolt — this is typically printed on the frame near the seat clamp or in the owner’s manual. Common values range from 4–6 Nm for aluminium frames and 3–5 Nm for carbon frames. Do not exceed the specified torque, as over-tightening can damage the bushing, the seatpost, or the frame. If in doubt, consult a bike mechanic.

Will this bushing work with a carbon seatpost?

Yes — the XLC SP-X20 is compatible with carbon seatposts with a 31.6mm outer diameter. When using a carbon seatpost, apply carbon assembly paste (torque paste) to the seatpost inside the bushing and to the outer surface of the bushing inside the seat tube. This prevents slipping at lower torque values and protects the carbon surface from point loading. Always follow the seatpost and frame manufacturer’s torque specifications.

Can I cut the bushing to a shorter length?

The SP-X20 can technically be cut to a shorter length using a pipe cutter or hacksaw, but we recommend doing so only if you are confident in the measurement and have the right tools. The minimum recommended insertion depth for a seatpost is typically 2.5 times the seat tube diameter — for a 31.6mm post, that’s approximately 79mm. Cutting the bushing shorter than this may compromise the security of the seatpost. Always deburr and smooth the cut end before installation.

Are there other diameter combinations available?

Yes — XLC offers the SP-X20 calibration bushing in multiple diameter combinations to cover the most common frame and seatpost pairings. The most common sizes include 27.2mm→30.9mm, 27.2mm→31.6mm, 30.9mm→31.6mm, and 31.6mm→34.9mm (this product). Check the XLC SP-X20 range for the combination that matches your frame and seatpost. We also stock the XLC SP-X20 Ø30.9mm → Ø31.6mm version if you need a different size.

XLC Calibration Bushing SP-X20 – Ø31.6mm to Ø34.9mm, 120mm, Silver