Repairing Worn Bearings – A Real Grind
We are definitely in an advanced bicycle repair technology here, a bit past the cone-wrench and grease procedure. As far as I know, I am the only one who rebels against the simple act of replacing aworn-out bottom bracket spindle or wheel bearing cone. They are not terribly expensive, and somewhat readily available. Although the spindles are getting hard to find, due to the proliferation ofcartridge bottom bracket bearing assemblies. What prompted me to seek an alternative to replacement was a typical (for me, anyway) scenario where I am in the middle of a bottom bracket or hub rebuild when I spy the telltale pitted bearing cone or spindle. I guess I could stop the project, find the proper replacement part, get it, and continue the job. I’m impatient, I hate to leave a disassembled component languishing in my shop whilst I scrounge up the possibly elusive part. Not having to wait for a mail-ordered part (LBSs long since have discontinued using separate spindles in favor of cartridges) is a big bonus.
So what to do? Borrowing a page from the old-time auto rebuilders, my thoughts drifted to a re-grind. Old cars had plain crank bearings with journals that could be re-ground and fitted with oversize inserts. Our subject cones and spindles can be mated only to a specific size hardened steel ball bearing, but the pits resulting from wear are just a few thousands of an inch deep and not too problematic.
Ideally, a machine shop would chuck up the part in a lathe and turndown the bearing surface to fresh metal. Ordinary cutting tools would not do, due to the hardness encountered, but a grinding stone would (literally) cut it. Not having a lathe, I chucked up the worn component in my drill press and had at it with a grindingstone, which in turn I chucked into my rotary tool, or die grinder.See photo for the set-up. Follow up the grinding with a polishing achieved with a piece of emery cloth wrapped around a screwdrivershaft. Use grinding stones with a tip the same radius as the ball bearing – 1/8 inch radius for 1/4 inch ball races such as spindles and rear wheel bearing cones.
Spindles with provisions for nutted attachment of crank arms may be chucked up in a drill press directly by the threads. Just use light pressure on the chuck key, don’t damage the threads. Wheel cones may be threaded onto an axle and locked in place with an axle nut or lock nut. Then chuck up the whole axle into the drill press. Spindles with bolts for attachment of crank arms will require a stud be made up for mounting in the drill press.Wearing eye protection, start both drill press and grinder and apply light pressure as shown. Make sure the spindle or axle spins true,and don’t allow the grinding stone to bounce. Keep this up until the pits in the spindle or axle disappear. Then follow up with the emery as shown. A finish equal to that on a new part should be almost attainable with this method. Close enough anyway.
After my initial attempts at this shade-tree wizardry, I was delighted to see the surface pits fade and a bright shiny finish emerge. After cleaning, lubrication, assembly and adjustment these reground bearings run apparently as smooth as new. What may actually be happening is that I have re-contoured the cone so that the balls (use new ones if necessary) ride in a different position, and not on whatever may remain of the pitted surface. Either way, I found no evidence of any rumble or shake in the bearing after lubrication and adjustment.
No telling how long this lash-up will last, but for now, this looks like a quick, cheap, and handy way to freshen up those worn bike bearings. Given the relatively low speed and light loading on bicycle bearings, this crude procedure will probably work pretty well. If you have any doubt that bike bearings are not equal to high-speed and high-load automotive and industrial bearings, check out a brand new cone or spindle surface with a high power magnifier. What you see are grooves or marks left by the manufacturing process which may be machining or simple die casting. These are not precision bearings, nor do they need to be.