How to adjust chain slack

Posted January 21, 2005
Updated January 25, 2005

Adjusting chain slack is a simple job that every motorcyclist should know how to do. It takes about 10 minutes every 400-500 miles (that is, every time you lubricate the chain), and is an excellent investment in keeping your drivechain healthy. The procedure differs in details from bike to bike, but the general principles described here apply to all bikes. The details in the article are specific to the Ninja 250. Consult your bike's owner's manual for specifics on your motorcycle.

On the Ninja 250, you'll need the following tools:

  • 12mm open-end wrench
  • 17mm open- or box-end wrench
  • a torque wrench that goes up to 80 ft-lb (108.4 Nm)
  • a 27mm socket to fit on the torque wrench

You'll also want a new cotter pin, and a small piece of cardstock (perhaps 1 by 4 inches or 3 by 10 cm) or similar material with your preferred chain slack measured on it. On the Ninja, that's between 30 and 40mm.

First thing, rotate the rear wheel until the chain is at its tightest, and measure the chain slack before making any adjustments. Grab the card in one hand, and the chain in the other. Pull the chain down, and line up the "0" mark on the card with one edge of the chain (I usually use the top edge, but either one works). Holding the card still, pull the chain up so it's as tight going up as it was going down, and look at the same edge of the chain against the card. The following diagram illustrates how to measure chain slack:

Note that you're measuring from one fixed point on the chain. If you measure any other way, you will get an incorrect reading. You should be pulling up and pushing down against the chain with about 10 pounds of force. Not with all your might, and not just a little bit -- you're trying to take up most of the slack in the chain.

Take out the cotter pin, and loosen the axle nut with the torque wrench and 27mm socket; you may need the 17mm wrench to keep the axle bolt from rotating. Don't remove the nut, just loosen it so it spins freely. Using the 12mm wrench, loosen the locknut on each adjuster bolt (one on each side of the swingarm).

If the chain was too loose, screw in each adjuster bolt one half turn (thereby lengthening the distance from the driven sprocket to the wheel sprocket, taking up slack in the chain). Always move each adjuster bolt the same amount, and you'll never have to worry about whether the rear wheel is straight or not. Check the slack again. If it's not tight enough, screw in each adjuster bolt again, always the same amount on each. I usually use half-turns as being really easy to see (wrench straight up to wrench straight down or vice versa).

Likewise, if the chain was too tight, loosen each adjuster bolt the same amount. In cases where the chain was too tight (which is an unusual circumstance, and will probably only ever happen if you tightened things up too far at some point), you'll need a helper, some coordination or a rope right now. After each adjustment, you need to give the rear tire a solid kick to slide it forward, as just loosening the adjuster bolts won't do it. Make sure someone (you or a helper) is holding onto the bike, or it could rock forward off its stand! You can also use a length of rope or a tie-down to the front wheel, to hold the centerstand forward, so it can't collapse.

Once the chain is at the correct slack, check that the pointers on the axle holders point at the same position on each side. Make any adjustments necessary to get them even again. This is usually sufficient alignment for most people. (If the rear tire is seriously out of whack, you'll probably want to check and adjust the alignment using a more precise method, which is beyond the scope of this article.) Remember to kick the tire forward if you have to loosen either of the adjusters. Then, holding the adjuster bolt so it doesn't move, tighten down the locknut. It doesn't have to be really tight, just enough to keep the adjuster from moving. If you tighten it too much, you'll stretch out the adjuster bolt.

Once the adjusters are locked down, check the chain slack one more time to make sure it didn't change, then tighten the axle bolt to 80 ft-lb (108.4 Nm) with the torque wrench on the crown nut, and the 17mm wrench on the bolt head on the right-hand side of the bike. Check the chain slack again, and re-adjust if it changed when the axle was tightened. Install a new cotter pin (you may have to move the crown nut a little bit to get a cotter pin hole lined up). You're done.

Copyright 2005 by Ian Johnston. Questions? Please mail me at reaper at obairlann dot net.