Nuts n Bolts: Cranky About Stripped Threads?

[[image:cranky_mini.jpg::inline:1]]During an idle moment at the Bike Oven, waiting for the inevitable arrival of yet another challenging old bike repair job, I glanced over the stash of junque bike parts. My eye fell upon an aluminum drive side crank arm.

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Published August 13, 2007 by C.I.C.L.E.
Contributed by Harv :: Photos by Leticia 

During an idle moment at the Bike Oven, waiting for the inevitable arrival of yet another challenging old bike repair job, I glanced over the stash of junque bike parts. My eye fell upon an aluminum drive side crank arm. I picked it up and checked it out. Hmmmm… just what I need for my latest project. But, what's this? The threads for the pedal were completely stripped out. Another hmmmm… what can I do to repair it? Preferably at no cost, or very little cost. A crank arm thread repair kit costs from $80-$100 or more. Unless you will be repairing stripped out crank arms regularly, you probably will be interested in the following procedure.

[[image:crank3a.jpg::inline:1]]I allowed my subconscious mind to review this problem overnight. On the morning of the next day, I tinkered around with some crank arms and various pedal spindles. I noticed that a 1/2 inch pedal spindle would go into the stripped-out 9/16 threaded hole easily, but did not protrude enough, past the edge, to mount a nut behind the crank arm. Well, the thick aluminum could be partially hogged out to permit a nut to be used to retain the pedal. Conveniently enough, the 1/2 inch pedal thread turned out to be 20 TPI. Even more conveniently, 1/2 – 20 is a common size of auto wheel lug nut. These things are laying around the floor of every auto tyre shop in town. What's more, they are beveled so that a lug nut used to secure a bike pedal would center itself in the hogged-out hole in the crank arm. Not only that, but some old auto lug nuts are left hand threaded to accommodate left side wheels, like on an old Dodge van that I used to own. This repair could then work for either a left or a right crank arm!

It would be nice if we had a milling machine to do this job. But we don't so out came the hack saw, files, and a Dremel tool with a round rasp cutter. You only need to hog out just enough of the inside of the crank arm pedal hole to get a quarter inch of the lug nut bevel into it. About 1/8 inch of crank at the pedal entrance should be hacked off so that a few more pedal threads will engage in the lug nut. Make sure you have enough clearance for the lug nut to swing around and not hit any part of the bicycle frame after the nut has been tightened home. This changes the Q factor (width between pedals) of the crank slightly, but in the right direction to offset the longer side of the bottom bracket."But my pedal has 9/16 threads, not 1/2 inch", you say. Not a problem Grasshoppah. Most pedal spindles are close enough in length so that you can rebuild almost any 9/16 pedal onto a 1/2 inch spindle. Or the other way around, which I have done more than once. So find a BMX pedal, or any one-piece crank pedal with a 1/2 inch threaded spindle and disassemble it. If you can't find an old bike with a one-piece crank in the trash, you need to move to another neighborhood.

[[image:crank4a.jpg::inline:1]]Now, I don't recommend this if you are a heavyweight and have been known to break chains, pedals and/or snap crankarms. But I weigh 122 pounds and have never broken anything on a bicycle due to stress, so I am OK with this fix. Be sure to snug down the lug nut after riding the bike for a few miles, and check it every once in a while to see if it is going to stay tight. If it continues to loosen up, don't ride the bike. Pop for a new crank arm. Or if it makes you feel better, use some Locktite on the threads. I don't, however.

So with a bit of scrounging for a 1/2 inch junque pedal and a lug nut, and some work with a hack saw and a dremel tool, you can save that stripped-out expensive crank arm and be on the road again. It's crude, it's ghetto, it's clunky-looking. But it works just fine. The hardened steel pedal spindle will be thusly retained by a steel lug nut designed to handle over a hundred pound-feet of torque. This tip is for people who have more time than money. Or else it may be what you need to do when you are in downtown Bangladesh or the jungle of Borneo and you strip out a crankarm thread with no LBS in sight.