How do I measure chain slack?

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.

As a chain stretches the most at the beginning and end of its life, it should be checked a handful of times in the first thousand miles.

Tools

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 24mm socket to fit on the torque wrench
• a breaker bar with the same drive size (3/8 or 1/2") as the socket
• a new cotter pin (or R-clip, which can be reused; see below)
• a small piece of cardstock (perhaps 1 by 4 inches or 3 by 10 cm) or similar material, or a short ruler, with your preferred chain slack measured on it. On the Ninja, that's between 35 and 45mm.

Measurement

Put the bike on its centerstand. Rotate the rear wheel until the chain is at its tightest, and measure the chain slack before making any adjustments. Grab the card/ruler in one hand and the chain in the other. Pull the chain down, and line up the "0" mark on the card or ruler with one edge of the chain (either edge of the chain works, as long as you always use the same edge). This is "0".

Holding the card still (at the bottom "0" point), 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. This is your chain slack measurement. You're looking for the distance between the low and high points to be between 35 & 45mm.

This sounds a little complicated, but it's really as simple as this illustration shows. "A" is the distance between pulling down and pushing up.

Measure as close to the center of the lower chain run as possible. The back edge of the chain rub pad on the swingarm is the correct area. 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.

Adjustment

If your adjustment is out of spec, take out the cotter pin and loosen the axle nut with the breaker bar (or a ratchet, but the breaker is less apt to break) and 24mm 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. Loosen the nut on each end of the brake torque link. You may need an open end wrench instead of a socket.

Then, using the 12mm wrench, loosen the locknut on each adjuster bolt (one on each side of the swingarm). Right side is shown.

If the chain is too loose, screw in (clockwise/right) each adjuster bolt one half turn (thereby lengthening the distance from the front 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. Using half-turns makes the adjustment really easy to see (wrench straight up to wrench straight down, or vice versa).

Likewise, if the chain is too tight, loosen (counterclockwise/left) each adjuster bolt the same amount. In cases where the chain is 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. 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 running from the centerstand to the front wheel, so the centerstand 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.) 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. Re-tighten the nuts on the torque link, and torque to 24 ft-lbs. Install a new cotter pin for the axle (you may have to move the crown nut a little bit to get a cotter pin hole lined up). You can get an assortment of cotter pins from an auto parts store, or look for reusable alternatives such as these:

You're done.

And for the visually oriented, we have photos

Measurement is always done with the bike in neutral. It's also always done on the bottom run of the chain, and as close to the middle as possible. Simply pull down (not hard, just firmly) and set your ruler by holding it against the swingarm:

Note that this rider is measuring the bottom (outside) of the chain (“0” is at the bottom), but you can use the top (inside) if you so desire. You just have to be consistent.

Then, without moving the ruler or your point of view, pull up on the chain and look at the difference. Here, it's 30mm, which is smack dab in the middle of the specs for our test bike, a Bandit 400. Note from above that the range for a Ninja 250 is 35-45 mm. From the Kawasaki service manual:

"Too tight: Less than 35mm" "Too loose: More than 45mm"

Tool for checking slack

Here is a quick and easy-to-make jig that makes for quick and accurate slack checks.

This simple tool uses a scrap of 2x4, a dowel, and a drinking straw. Flatten an end of the straw so it grips the dowel and doesn't fall down, and mark it at your minimum and maximum settings. Since it slides on the dowel, it can be easily zeroed. It's very useful; just set it on the ground behind the chain. Check, rotate wheel, check, rotate wheel; very efficient and accurate, since it stays put, whereas a piece of cardboard held against the swingarm can move up and down.