Contributed by Richard612

(Click on an image for a larger version..)

See that cover? It's comin' off! Remove the four small bolts and the inner shift lever.  Note - You only need to remove the inner crank, not the entire shift lever assembly.  I removed the pinch-bolt and just wiggled the inner shift-lever until it came off the shaft. No need to remove the foot-peg or any of the shift-lever linkages.

See that little bolt hole? That's the 'hidden' one.

Here's what the four cover bolts look like, plus the shift lever clamping bolt.

Inside the countershaft cover

The unmolested OEM countershaft sprocket.

See how the bendy washer is, uh, bent? Gotta pound it flat. Be careful using a screwdriver. The washer can actually tear.

Use a 27mm six-point or a 1 1/16 inch six-point socket to take the bolt off. 1 1/16 inch is VERY close to 27mm and may be easier to find. Regular lefty-loosey threads. You gotta stop everything from turning somehow. Put the engine in 1st and have an assistant hold the rear brake. You'll need a good 1/2' flex-handle (like the one shown here) as this puppy is TIGHT and the force required will probably ruin a 3/8' ratchet. Impact guns are your friend...

The axle nut. Remove the cotter pin and loosen the axle nut with a 27mm wrench(?). No need to remove it. You will probably need to stop the axle itself from turning - use a 22mm wrench. Both of these wrenches are in your ZR's toolkit.

Loosen the adjuster locknut with a 12mm wrench. No need to remove this one either...

Notice that the ends of these 'studs' are square. Use an adjustable/6mm (?) wrench or the wrench in your ZR's toolkit to turn these studs so that the rear wheel moves as far forward as possible. The goal is to get the maximum possible chain slack.

Slide the old sprocket off of the countershaft. It may or may not be easier to first de-rail the chain.

The new 17T sprocket. This same part also fits the ZR750 (C1-C2), the ZX-7R (96-02), the W650, Gixxer 600 (97-02), the SV650 (99-02), and the Gixxer 750 (98-02).

This is the notch that I made with a small grinder in order to gain clearance for the sprocket. The top of the teeth on a 17T sprocket will whack into this piece of aluminum. Note that the chain can be installed and the cover WILL go back onto the engine without having this clearance. The fun part is when you start the engine and ride away. I'd love to see a photo of this... The notch I made here is MUCH wider than it needs to be. Use a piece of chewing gum of something to see where the sprocket hits the cover. Install the sprocket and the cover, then reach around behind the cover and try to twirl the sprocket so that it contacts the cover. Look for the impression in the gum.

The new sprocket (installed).