Repairing a Bicycle Crank

Alex du Pre tackles an unusual workshop task

Apart from providing the means to fulfil the satisfying hobby of model engineering, owning a well-equipped home workshop often allows you to tackle the other little jobs that life throws up from time to time. Sometimes, the workshop can even help with other hobbies. As well as a model engineering, I also enjoy cycling and one such task recently came my way when I managed to strip the thread on my pedal crank. The root cause of this was temporarily fitting a cheap replacement pedal with a poorly-formed thread, thus also acting as a reminder of the old adage ‘buy quality, buy once’. Neither my local bike shop nor the internet was able to come up with a like-for-like replacement crank, so I thought I would attempt a repair in the workshop. As this was an interesting machining challenge with some learning along the way, I thought I would share it with you.

The Challenge

The crank in question was the left-hand crank of a Trek road bike. The crank is an irregularly-shaped alloy forging. Critical dimensions are the throw, which is the distance between the pedal axle and the crank axle (175mm in this case) and the offset, which sets the lateral distance between the two pedals. The pedal axle must be parallel to the crank axle.

The standard pedal thread is 9/16in x 20 TPI Cycle Pitch. The left hand crank has a left hand thread to prevent the pedal from unscrewing in use. The right hand crank has a right hand thread.

The challenge was to find a way of holding the work and then to bore out and replace the ruined thread whilst retaining the necessary dimensions and parallelism and giving a result that could withstand the pedalling forces.

The Repair Job

Before starting, I bought a suitable tap, at no small expense, but forgot that I needed a left-hand thread. So, I went back for the correct tap, mulling another adage, ‘check twice, buy once’.