Mark,
What was the issue with that clutch cover? I know they changed them/updated them but that was 2 years later for the 77 when they went with the longer basket. They did a relief in the ribs, changed the lifter piece which really doesn't seem to matter and a minimally longer clutch center. All to accomodate the 'noise update' that included the GL double center metal disc. Perhaps they did the update but used the earlier cover.
My bike was purchased during the first month/weeks of availability in the US at what was advertised as the world's largest Honda dealer, Pug Vickers in Huntingdon, TN. I picked it from probably 20 they had assembled and everyone of this very first batch had the rear disc. This is indeed a mystery.
The 'vintage shop' in Ohio had installed a chromed K0 diecast cover on it, which has a shorter distance between the plunger's hole and the inner face. The F0 has the 7-plate-deep clutch basket, while the K0 had the 6. This caused the adjuster to always be pressing in the clutch a little bit, even when backed all the way out. The top plate (the slant-cut cork one) kept wearing out in about 1000 miles, burned up. I had to replace the burned top lifter plate, too, as it had become warped from the abuse.
His OEM cover was sorely pitted from being parked where water dripped on it for 3 years during his first 2 deployments (it was under a tarp by his garage, crack in the tarp right over the side of the bike). The shop was "improving" it for him, obviously didn't know their SOHC4 parts very well, I guess? They went out of business, he got mad, then drove the bike clear to here from Ohio for me to fix it, in February(!). And, he dropped it off after a 12-hour driving day, turned around and got back in his truck - said he made it to Lincoln NE that night before stopping, to boot. Tough guy!
I don't get it either. The K0 fiche shows 7 friction plates...same # as the F. The F has the outside plate with the different tabs....the F clutch basket outer slot is different from the early K to accommodate the outer most friction plate.
CB750 clutch baskets part #'s (1) up to serial # 1056079 22100-300-010 For the K
(2) From CB750E 1056080 22100-300-020 For the K
(3) up to serial # 2470426 22100-392-000 For the K
(4) from CB750E 2470427 22100-392-010 For the K
(1) up to serial # 2558988 22100-392-000 For the F
(2) from CB750F 2558988 22100-392-010 This is the final basket # for the F/F2 as well as the K that used the double steel plate.
I cannot find any CB750 clutch basket (starting from CB7501000001) that holds only 6 total friction plates or 6 total steel plates for that matter. Everything is 7.
Yep, the Honda fiche show exactly that, and they are wrong. This happened in about 1998 when they "unitized" the fiche into the current parts sets. At that time the 6-plate inner hub was also discontinued. To use the longer, later 7-plate inner hub on those engines, the outer must also be changed.
The K0 diecast hubs, found on K1 up to about 3/71 builds and occasionally even later (like in my K2, 11/71 build, that came with a leftover K1 reworked engine) have a steel plate in first. These hubs hold just 6 corks. Once in a while you may find, in a virgin K0 or early K1 that has the little wire retainer on the inner steel plate (which is the first plate in) that the top plate is also steel, with the pressure plate surface not made to contact the corks. On these, the inner plate is a normal-thickness steel plate. These other earlier ones had a thicker inner plate with one side of it not smoothly finished to receive a cork plate - that side went in first, and the first cork plate rides against steel that way. On these, the top cork rides against the aluminum of the pressure plate. These particular early ones have a steel band around the spline boss on the mainshaft and are the strongest hubs they made. Later versions of this included the wire retainer on the top steel plate (so you find the wire retainer hole at the top of the hub stack instead of the bottom), with the [longer] inner hub receiving the first cork plate on aluminum, instead. This type also has a steel band around the spline boss, and is very strong, coveted by dragracers for having both this band and the extra friction plate. These are the ones where the thicker Barnett clutches can only fit 6 plates each, though.
Starting about in the K2, the hub became a bit longer and the inner face was machined to receive the cork plate against its aluminum surface, instead of a steel plate. On these, there are 7 cork plates, and the top pressure plate rides aluminum against the cork, also. This was the first change to the clutch lifter (engine) cover, because the whole "stack" became a little taller. One way to "find" these clutch cover plates is to look for the ones with the recessed holes for the screws, on the K3-4-5 engines. Most of the K2 engines like this did not have the recessed holes, though, and their surface finish is a little grainy.
Sometime during the late K5, the top plate became the 'slanted cork' shape and the basket got wider slots to hold this different top plate. This design included the dual-springy steel plate as the #2 steel plate from the top of the stack (outside). Somewhere during the F1, the springy plate moved inward to be the 2nd one from the back instead. In this position, it tends to produce the infamous 'clutch rattle' issue after the rivets wear a little loose in the 2 plates. I have sometimes stopped this noise by just moving it back out to the #2 position from the outside, don't know why this works, exactly...but, it also does not ALWAYS fix it. On the K6/F0 bikes the clutch lifters are sometimes mighty "tight", and I think it is because the distance between the lifter and the older clutch case cover was still the "old" distance, while the dual-steel plate added some extra height to the pack, namely the spring thickness. The clutch lifters are sometimes hand-modified in these bikes, with a thinner backstop plate that looks like it has been set on a mill and thinned manually by a tech. I've seen several of these, always the 1975-76 genre. I also saw just one that had the cover milled back to move the adjuster further back, too, but the bike was not virgin and I suspect it was a racer at work, adding an extra plate. It had 7 Barnett plates in it, which was a clue.
Then, the cover changed to add some extra space late in the F0 bikes. I first noticed it because the outside has some minor details that are different from the earlier ones, and the adjustment "feels" like on the earlier bikes (although I admit that sounds like a totally subjective tuning tip...). When these are adjusted, the clutch overcenter feel during pull-in makes them feel just like the old K0 clutch: there is a definite "knee" where the engagement begins, and the force on your fingers is highest right at that point. This gave the later bikes the same sort of "in-out box" feeling of the earliest bikes' clutches, with a somewhat more sudden engagement. It also tends to shift the engagement point more with heating than the K2-K5 bikes do.
Most of this stuff is also in my book, but it is sort of scattered between Chapters I and II.
