My dad recently took a minor tumble on his motorcycle. He’s fine, but the bike was banged up a bit, including a bent brake rotor. Consensus among his buddies in the Magna Owners of Texas was that the rotor would have to be replaced, but of course they’re pricey, and since the rotor was “shot” anyway, Dad figured he might as well try to straighten it and see what happened.
Here’s what he did, in his own words:
Since I had mounted the tire/wheel on the axle in my vice to polish the wheel, it was a simple matter to rig up the “feeler” shown in the first picture to check out the rotor flatness. Just a piece of copper wire about AWG 7 to 9 or thereabouts — I had in my electrical junk box. With a light behind the setup, one can use the reflection of the end of the wire from the rotor surface to obtain a very sensitive indication of warp when one spins the tire/wheel. Brought it back to planar using a soft face (brass) hammer. Go slow, it takes some time. “Sneak up on it” by whacking gently, measure, whack a little harder, measure, etc. until it yields just a bit.
Then, concerned that the rotor needed to be flatter than he could detect with the naked eye, he rigged up a second jig to test it:
Looked up the spec in the shop manual. It specifies “runout” at .01 inches or 0.25mm. Found a 1/4-20 screw in the junk box, and a piece of steel from some long discarded bracket off something I don’t even remember. Drilled and tapped for the screw and drilled a clearance hole for the mount bolt. At 20 threads per inch, that would be .050″ per turn. So .01″ would be 1/5 of a turn. Put on a standard six-flat nut for reference. Turn less than one flat would be .050/6 = .0083 inches, a little margin to the spec. So, to use it, you spin [the wheel] and gradually drop the screw until it just touches at the highest point. Turn to the lowest point, and tighten down. Took less than one flat, so I believe I am in spec.
He cautions that he has not ridden the bike any serious distance since he made this repair, and cannot vouch for its safety, but he is plenty confident to use it himself. There’s more detail in his original MOOT forum post here.