The Golden Wrench

A blog about bicycle repair and maintenance by the mechanics at Freewheel Bike.

Monday, December 13, 2010

Repairing a Stripped Pedal Thread

Occasionally we will have a customer come to us with a right pedal jammed into the left crank arm, or vice versa. In most cases, the pedal has a steel spindle and the crank is aluminum. Aluminum, being much softer than steel will accept the forced thread, but not for long. Once a load is put on the pedal the threads begin to strip out and the crank is toast! Or is it?

Of course, on lower end bikes, a left side crank arm can be purchased for a fairly reasonable price. However, right side cranks or higher end cranks can be much more expensive. In most cases a pedal thread can be repaired.

For science, I went against all of my mechanical training and experience and forced this right (Red for Right) pedal into the left crank arm. It was quite liberating.

Now that our left crank arm is good and wrecked, its time to get fixin'.

Remove the crank bolt.

Extract crank arm.

Up close of the damaged threads. if you look straight down the center, you can see where the newly forced right-hand threads cross the preexisting left-hand threads.

Left-hand in silver, Right-hand in gold. they can also be identified by the direction the threads slope. Left-hand threads slope up to the left and right-hand threads slope up to the right. Same as pedal threads.

Double check the tap to make sure you're using the correct one. The last thing you want to do it tap a right-hand thread in the left crank. This particular thread repair kit uses a special tap with a tapered reamer. It enlarges the hole and then taps the new threads all with one bit. Also, Make sure to use plenty of cutting oil to keep the tap cool and lubricated.

There will be plenty of metal shavings. The tap needs to be sent through all the way.

Pristine new threads!

Grease the pedal threads and install thread insert on pedal spindle.

Apply high strength thread lock compound to the external threads of the sleeve insert. This will keep it from being removed from the crank when the pedal is taken on and off.

Thread insert into the crank. Wipe off excess thread lock compound before threading in all the way.

There will be a bit of overhang on the back side of the crank.

The quickest way to remove this is with a bench grinder.

After the thread lock compound cures overnight, the pedal can be removed and reinstalled as if the new threads were one with the crank.

Time for reassembly.

Grease those bolts! This will allow proper torque to be achieved and protect the threads from corrosion. Park Tool's PolyLube 1000 works great as an assembly grease.

Tighten crank bolt to spec. In this case, Shimano recommends 35 - 50 Nm.

Reinstall pedal.

Here is the finished product. It's hard to tell that there was every anything wrong with this crank in the first place.

Our Shop bike is reassembled and ready for lunch trips to Chipotle once again!