This response is late to the original date of the post, but for those who are reading this anyways:
I installed a 16 tooth counter shaft sprocket on a 2017 Z650. While the sprocket will fit, it did cause the chain to make very slight contact with the engine chain guard lip occasionally. Especially when chopping the throttle and the chain slaps from being driven from the engine to the wheel (and this was after adjusting chain slack tighter and looser than I'd normally run it). You will have better success getting the same amount of gearing alteration of a 16T countershaft sprocket by going to a 43T rear wheel sprocket. It is what I am currently using. Changes are, roughly speaking, 400 rpm reduction in top gear at same speed. There has been a slight improvement in fuel economy. For the argument that you could lose bottom end torque and acceleration; true. But I don't feel that loss, and not so much that it dismisses the advantage I feel in having a bike rev just a little lower at the same highway speeds I was running before. Generally, I now keep the bike running one gear lower than before for most sub-70mph speeds, but I like having a lower 6th gear ratio to handle 75-85 mph speeds a little calmer. Incidentally, if cruising at 75mph without undue load (like going up a grade or into a strong headwind), the little "eco" icon will stay on at 75mph. Prior to the gearing change, it would rarely do that.
I do wish the 16T could have worked without any clearance issues, as removing and installing a countershaft sprocket is far easier that swapping out a rear sprocket. I used an impact driver to remove the countershaft sprocket. You might be able to get away with using a pry bar. I set my torque for the countershaft sprocket at 92 ft/lbs. For those who do want to change out the countershaft sprocket, you could get away with reusing the countershaft sprocket washer, but I'd recommend replacing it if you do replace that sprocket (for a smaller one for those who want MORE torque or acceleration).
For reference, what is in the photos now is the original OEM 15T countershaft sprocket.
Pic1: There is a small gouge in the engine chain guard lip from when the 16T sprocket was used, showing where the outer edge of the chain roller plate hit the guard. I changed back to the 15T sprocket because on decel I could hear the "tick tick tick" of the chain just barely hitting the guard. I believe as the chain slack would grow over time this minor gouging could increase.
Pic2: 43T rear sprocket looks fine (to me) on the bike, and if you look at the chain adjustment index marks on the swingarm, I only had to move the rear axle back a little less than 1/2 inch. I don't feel any difference in turning and leans, and I still have adjustment left to account for roller pin wear on the chain over time. More importantly, I was able to use the stock OEM chain with the 43T rear sprocket, which meant no new chain (save $$$) and I didn't have to break the chain to shorten the link count.
Sorry if the pics are oversized to the forum screen space.
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