Axle muffs are insulated cylinders of acetal to facilitate the joining of two wheels on their stub axles.

The muffs are bored 1.45mm such that they are a firm fit around the 1.5mm diameter stub axle.

It is recommended that axles are pressed onto the muff using a tool to keep them square. The Association quartering jig is designed for this purpose or you might use a small vice.

An alternatively method is to ream the muff out to 1.5mm diameter and then glue the stub axles in to maintain back to back and quartering.

Muffs are available with various outside diameters to suity specific purposes. This is usually driven by the bore of various gears which might be mounted on the muff. Generally imperial gears require a muff of 1/8" OD whereas metric gears require a 3mm muff. Some muffs are designed to be ungeared. These are generally cheaper but are not manufactured to the same tolerances as geared muffs.