Yep, the K6 sometimes ended up with the F0 clutch plates.
The late K5 bikes were going to be Honda's last of the "K", but American Honda said otherwise, after Honda's plans had been laid. So, the K6 was resurrected after it was stopped (at the New Factory), so it sometimes got a mix of parts. I often see them with F0 wiring harnesses (with little jumpers using collars-on-colors to join dissimilar wire colors together) and sometimes with 1/2 K, 1/2 F clutches. - Among other things ---
Here's the skinny on the clutch: if it has a fully "F" hub, you will find a double-steel plate in the stack (either as the first steel one next to the top slanted cork plate, or else about 2nd from the bottom of the stack), which has tiny springs in between those 2 plates and rivets that hold the 2 plates together about 0.1mm apart from each other. This was the "slipper" plate. In these engines, the gearset is usually the same as is found in the F0 engines, and the bushings (in those tranny gears with separate bushings) will likely be Oilite dimpled bronze types (intended to carry extra power, as they are more efficient, like in the K0-K3 bikes). This hub setup will include the "F" clutch cover, which has a slightly greater distance (about 0.8mm) between the lifter and the hub, to allow room for this extra-thick steel plate. The top clutch plate will have wider outer tangs on it, and will have the slanted corks, and will therefore only fit on the top of the stack to match the "F" basket. Some of these only had cork on one side (the engine side) as well. The intent of this arrangement is: with the taller 1st gear, this lets less impact occur if the rider lets the clutch out too hard, reducing the shock loads on the countershaft and drive chain. This became the post-1975 design. This stack-up uses the longer clutch springs of the F0 clutch, and there may be 6 or 7 cork plates, depending on which hub got used. If it has the short hub with 7 plates, the slant-cut plate will have cork on just one side: these usually wear out pretty quickly. Most of these had 6 plates with the double-spring steel plate and a 2-sided top slant-cut cork plate, but I have seen at least 2 of them with 7-plate clutches (although, these may not have been Honda plates...).
The earlier K5 gearbox might be in the K6 engine, but without the F0 clutch hub. If this happened, then the "mystery" steel double-plate does not appear and the hub is the standard "K" length, but the fingers of the clutch basket may have been the "F" model. In this arrangement, the slanted top plate was used so the wider fingers of those plates will meet the wider top slot of the new basket: this will always be a 2-sided slant-cut cork plate. This stops the clutch from "snapping" when released during hot-oil shifts (i.e., engine is well warmed up). If a standard narrow-finger, square-cut cork plate is installed, the clutch will still work, but will tend to wear a small notch against the wider slots of the fingertips of the basket if the rider sits at traffic lights with the clutch held in, in gear. This clutch will use the K5-type clutch cover on the engine, too, as the stack is the same height as in the K5. This clutch uses the K5 springs, and there will be 7 cork plates.
Already you may begin to see that there is a 4-way possibility, here...
The K6 sometimes came with the F0 gearbox, but with the K5 clutch. In this case, the standard 7-cork-plate clutch and clutch cover was used, and the parts are the K5 parts. This gearbox also has the early 1st and 2nd gears, but the bushings in the countershaft and mainshaft gears are sintered iron (not bronze). These engines tend to be fussy about the oil being used, because today we don't have much zinc in our oils: the sintered iron bushings drag A LOT without zinc, making shifting sticky. One way to help reduce this problem is to go back to the original 1000-mile oil change intervals of the K0/K1 bikes, which will help (it was sometime during the K3 that the oil change interval magically became 1500 miles, corresponding with the addition of exhaust-valve guide seals). These hubs especially appreciate the "hub mods" I talk about in my book and other posts.
The 'complicated' combination of parts comes when the gearbox is the F0 version (extra-tall 1st-2nd gears) and the clutch is the standard K5 type, but with F0 springs. These bikes have "grabby" clutches because they do not have the slant-cut top cork plate, and they were the ones that got dubbed "...that in-out box of a clutch..." by Motorcyclist Magazine in their K6 review, hurting the CB750F0 sales in Britain quite a bit, then (even though they were writing about the "K"). That's one of the big reasons later blamed on the poor F0 sales there... But, the easy fix for this one was to either add some 4 extra oil holes to the hub, or modify the existing holes by adding oil sipes, or change the clutch springs to be the slightly weaker K5 springs, all of which made the clutch happier.
These were some of the things that made the K6 such an interesting (and sometimes messed-up, if mis-repaired) bike! They sometimes got F0 pistons with or without F0 heads, sometimes got K5 or F0 cams, spark advancers, wiring harnesses, or trannies - or clutches! If the "wrong" combination of parts happened, the K6 was kind of doggy. At the other end of the spectrum, though, it was a hotrod overshadowing the F0, while almost no magazine ever tested it on the [typical] dragstrip, quoting instead the (last year's) K5 specs they had against the new F0 when doing their test rides. The "doggy" ones are 100k mile bikes, though, much like a Beemer, while the "hotrod" versions continue today to add to the mystique of this history-making (yes, I said that...) bike.
