Ah, you guys are talking about what Lucasind and I have been working on lately! He was complaining that his ZN tachs at about 4000rpm at 70mph. He doesn't like that! OK, so I too have found myself stabbing for a sixth gear around 70-80mph. These cruisers just have a lot of torque. But, NOBODY makes, or ever will make, a different gear set ratio for these bikes since there were only about 4500 of them made, a lot of them are now gone and most owners don't care about the engine speed at highway speeds anyway! They aren't like cars where you can find dozens of different ratios for any rear end.
What to do?
Well, for one thing, the KZ's final drive has a ratio of 3.18-to-1. The ZN's final drive has a 3.40-to-1 ratio. So, that leads one to ask if the KZ drive would fit on the ZN. NO! At least not without a lot of cutting, aligning, welding and a little bit of adapter fabrication! A lot of work for such a small change in engine speed.
So, with a bit more research I found out that the Gold Wing 1200/1500 final drive has a 2.833 ratio. That would make the ZN's engine run at 3333rpm rather than at 4000rpm at 70mph. But, can it be done? I think so.
I bought a used swing arm and driveshaft from a ZN1300 so I wouldn't butcher my original ones on my bike, as well as making it easier to remove intact, and a final drive, driveshaft, rear wheel and swingarm from an '84 GW 1200. Grafting the final drive unit to the ZN's swingarm will be a piece of cake. A small adapter to attach the lower end of the ZN's right shock to the Honda drive case will need to be made. A GW 1200/1500wheel will have to be used since a wheel's interface to the final drive output is unique to almost each bike make and model. No big deal since it is a 16" wheel just like the ZN's. To mate the GW's drive box to the ZN's swingarm I will cut off the right end of the GW's swingarm and graft it onto the right side of the ZN's swingarm. The bolt flanges are quite different and not the same distance from the swingarm's pivot. This is where all of the careful fitting and alignment happens to place the wheel exactly where the ZN's wheel was. But, it's not that hard using a jig. The left arm of the swingarm has the axle boss to locate the wheel and drive unit. There's more to this, but it gets complicated.
The hardest part is grafting the two driveshafts together. The front part of the ZN's shaft has to be used to mate up to the ZN's tranny output flange, and the rear part of the GW's shaft has to be used to mate up to the GW's final drive input coupling. The difficult part will be getting the metallurgy restored after the welding and balancing is done. I have a local shop that makes custom driveshafts and can do the heat-treatment part of the job. Still, this will NOT be as strong as a stock shaft since they are forged units from the factory. I will be using a welded-on sleeve over the welded/ground area to help beef it up. I still don't believe it will repeatedly stand up to doing burnouts and side-slips of the clutch. But, on a ZN, this shouldn't be a problem since these heavy bikes weren't made for that kind of abuse anyway. On a KZ1300, that's a different story. I have no reservations about full throttle AFTER the clutch is engaged, in any gear, but just not for drag racing.
Since your KZ's start with a 3.18 ratio, they wouldn't gain as much advantage as the ZN's do by going to the GW's 2.833 drive, but it's the only game in town, as they say! Bigger tires are limited, but could also add to the advantage, IF, IF, IF they meet the speed requirements you are looking for. Also, by calculation, if you are now clocking 160mph on your KZ, then this new ratio would only increase you to 179.59mph if your engine could pull the same rpm.
I now have all of my used parts off of eBay to do this build and will have it ready for testing next spring. As Lucasind says, it will be a Hondasaki drive. Lucasind has done a lot of research into finding new gears, but has turned up nothing. I would have bet on that outcome! So, this is what I have come up with for his, and my bikes. Will let you know how it turns out! Not so sure I would want to try it on a 180mph bike, though!
