Cotterless (square taper) cranks have long been superseded by splined and ISIS (tapered spline) designs. But there are still plenty of cotterless cranked unis out there. Cotterless cranks are somewhat notorious, particularly on the left side, where the slightest crank play (looseness) results in the retaining bolt unscrewing and the crank quickly becoming unridably loose. On the right side, play on the fit tends to tighten the retaining bolt, typically self-correcting the problem.
I recently had a recurring problem with a left crank coming loose on my very old steel frame 36er. I had torqued the bolts carefully when I fitted the cranks. The 2nd time, I used thread lock (e.g. Loctite) on the bolt threads. The third time I applied my engineering attention to the issue…
On close inspection, I discovered that the taper angle on the shaft did not perfectly match the taper on the crank, with the crank’s taper being slightly too narrow and the taper angle slightly too shallow. So I very very carefully filed the aluminium crank taper to match the shaft taper. After getting the angles on each of the four faces roughly correct, I applied permanent marker pen to the shaft’s taper faces and fitted the crank with mild tightening of the bolt. This left ink marks on the crank’s taper faces where contact was made, allowing me to finely file these “high spots” away in order to improve the fit. I did a few iterations of this until the fit was excellent and I obtained ink marks across all four faces of the cranks taper. I then cleaned the tapered faces of the shaft and crank, fitted the crank and correctly torqued the retaining bolt. The crank remains firmly fitted and has not worked loose again. Problem solved it seems (and I’m confident it will remain so).
I must emphasis that this was a very very careful filing job and neither easy, nor quick. If you get the angle wrong, or take too much material away anywhere, the crank is ruined. It is also important to appreciate that as you remove material from the crank’s tapered faces, the fitted crank sits further toward the other crank, so keep an eye on the clearance with the bearing, bearing retainer and frame. Hopefully this post will help someone else with the same problem.