Join Date: Dec 2008
Location: Under the Downunder
Bikes: MTBs, BMX, Pocket MTB
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This is common with one piece cranks and is usually not a biggie, but like you said when you like the chain tension to be just perfect on the tight side (for doing no-footers or whatever), then it becomes a problem. One of the culprits for this is the "bore" or hole in the middle of the chainring is usually machined with a tolerance slightly bigger than the machined axle shank that's supposed to locate it dead-center.
To "minimize" this (I've had good results, but haven't been able to take it off completely), dismantle the one piece crank and unscrew the bearing race that clamps-down the chainring. In all likelihood, your chainring will have a wobble against the axle shank. What you want here is a snug-fit or better yet - a slight press-fit. If the wobble is very slight, you may have to go to an auto-engineering shop that sells extra thin sheets of brass. You want to cut a collar-spacer to wrap around the exact circumference of the shank, to get rid of the "gap wobble" when you locate the chainring back in it's place. In some instances when the wobble is pretty big, I've had success in cutting this collar-spacer from a tin can, or some steel or aluminum sheet I've got lying around. You have to try various thickness until you find the right sheet metal that gives you a snug fit.
Then screw the bearing race back on, but make sure it's really tight. Reassemble the rest of the guff and hope for the best!