Modifying a track hub sprocket to fit a threaded Torpedo hub driver

Original threaded cogs for classic Torpedo hubs like this one is getting harder to find:

Old threaded torpedo sprocket
But luckily contemporary sprockets made for track/"fixie" bikes have the same BSA/ISO 1.37" x 24TPI AKA FG 34.8mm thread size. However, modern sprockets have a slightly wider threaded section, causing the left-hand threaded lock ring of the Torpedo driver to engage in fewer threads and not secure the sprocket sufficiently. This page shows how to make a sprocket narrower so it will fit.

A threaded Torpedo Freilauf driver designed for 6 mm deep sprocket threads:

Threaded Torpedo Freilauf driver
An example of a track sprocket being produced today, the Miche system of sprocket carriers and sprockets width 7.7 mm deep threads:

Contemporary track sprockets
Contemporary track sprockets with carrier
The separate threaded adaptor/carrier makes it easier and cheaper to change just the wear part of the sprocket.

Sprocket carrier profile
A separate lock ring (not shown in these photos) secures the cog against the carrier. Do not use the standard Miche lock ring made for Miche track hubs on a Torpedo hub, as it is a special Italian size (1.32" x 24TPI, around 33,5mm) and will have a loose fit. The Torpedo driver lock ring thread is 1.29" x 24TPI, around 32.8mm which is still used on non-Italian track hubs.

Sprocket carrier threads
Sprocket carrier interface towards cog
The carrier has a width of 7.55 mm (7.7 mm with sprocket mounted). The original Torpedo threaded cogs are 6 mm wide at the threaded section, so to make it compatible, the carrier should be 1.7 mm narrower (for a carrier target width of 5.85 mm, giving a total width of 6 mm with the sprocket mounted):

Sprocket carrier width
The carrier is made of hardened steel, so it practically cannot be hand filed or sanded. Instead, an angle grinder can be used. Take very light passes over the surface and check periodically that the thickness is even all around, also to make sure the temperature of the steel does not get too high which could affect the hardening.

Reducing width of sprocket carrier
The final finish should be done with a diamond file or emery paper on a flat surface. Check with a caliper that the target thickness is within 0,05 mm all around the edge.

Sprocket carrier hand filing
Bevel the edge of the threads to make it start easier and to make sure it does not bottom out on the driver's thread before the filed surface reaches the driver/dustcap "shoulder".

Grinding a bevel on sprocket carrier threads
Test-mounted on a threaded driver:

Sprocket and carrier mounted on threaded driver
Good fit confirmed.

Sprocket and carrier mounted on threaded driver profile
An added benefit is that the sprocket should be able to be changed much easier because the carrier (which will be threaded on very tightly) can stay on the driver and only the left-hand thread lock ring needs to come off.

Page last updated 2023-02-24 20:49. Some rights reserved (CC by 3.0)