I haven't found a thread that specifically lists the differences between the k and f model 750s and 550s.
Put the claims to rest knowitalls!
Frames
Bodys
Brakes
Wheels
Lights
Suspension
Engine specs
Everything else...
Boy, that would be a long post!
Yep, i didn't cover everything..... 
I think I'll "do" the 750 engines (not the carbs, airboxes, pipes, etc....):
Starting from CB750K0 sandcast before S/N 4000 (approximately):
Cams lost 2 degrees duration and 0.25mm lift before sandcast stopped. In K1, the lift dropped again by another 0.1mm, to 0.35mm less than sandcast, before 3/71 production. The cam lost 3 more degrees duration in the late K2, another 2+ degrees in the late K3. In the F0, the lift & duration came back to K0 sandcast spec, but was delayed 5 degrees overall. In the F2/3, another 0.5mm of lift was added above the K0 sandcast values, still with the 5 degree overall delay.
Spark advancers: the K0 sandcast and diecast until about engine #9000 had 41 degrees full advance, idle was 5 degrees. These bikes idled like silk and ran like wet cats. The K1 until about 3/71 had the same idle degrees, but only 38 degrees advance. The K3 dropped this to 35 degrees advance. The K4 increased the idle advance to 7 degrees, left total at 35 degrees. The K5 and K6 had 2 different advancers (TEC & Hitachi) with one having stronger springs (Hitachi), intermixed: the K6 sometimes had F0 advancers. The F0/F1/K7/K8 advancers are all similar, with 6 degrees idle (typically) to 36 degrees advance, while the F2/3 reach up to 38 degrees advance, but with stiffer springs.
The rods were the same until the (heavier) F0, got stronger again in the F2/3.
The sandcast-thru-3/71 K1 engines had hand-finished head ports,inlet and exhaust. The sandcast and K0 diecast ones looked like Branch Flowmetrics did them. Starting in late K1, this gave way to careless castings and rushed one-cut swipes on the inlet sides to try to correct some of the flow problems. By the K4, the casting were improving: by the K6 they were very nice and uniform, but with large valve guide bosses and overly-sunken valve seats. The F0/F1 head ports are very well done, and the K7/8 flow well: the F2/3 are the highest-flowing since the K1 late, and have bigger inlet valves.
The valve guides changed to have seals on the shorter exhaust guides after 3/71 production from both Old and New factories: after 10/70 from the New Factory. The material in the exhaust guides changed to cast iron during late K4. With the leaner-burn approach in carb calibrations after that, the valve guides wear very quickly in the later engines, compared to early ones.
The valve surface quality in the pre-1973 engines was excellent, and the valves were very hard. This changed during the K3. During the K4, the valves came with unfinished, as-cast upper surfaces, often needing polishing, until 1979.
The pistons changed in the K3 to use 3-piece oil rings, which persisted until 1979. The F0/1/K7/K8 pistons are about 9.2:1 compression, while all earlier ones were about 9.0:1. The F2/F3 are about 9.6:1, with different ring lands and rings to suit the higher pressures.
The gears 1 & 2 changed in the F0 to be closer ratios than the earlier bikes. In the K5, some oil holes in the countershaft disappeared in favor of moving that oil to the mainshaft's gears.
The final drive shaft changed during K3 to use a power-robbing dual-row ball bearing.
The countersprocket changed to 18T in the K2.
The rocker shafts got locked down with little 5mm bolts in the K3, about mid-year. This quieted the engine mechanically behind the (incredibly) popular Vetter frame-mounted fairings, but cost HP and shaft wear issues. The cylinders received 8 new long rubber seals at this time, to reduce the front-engine oil seepage that could occur during prolonged 80+ MPH touring. (A little too late: the U.S. went to 55 MPH speeds before this came to market!).
The clutch: there is the K0 sandcast version with little hairspring plate retainers, the sandcast later version without but the top one, the K1 with and without the top retainer wire, and the K1/K2 version with 7 plates instead of 6, but with the steel-reinforced hub. The post-K2 era clutches all held 7 plates until the F0 (and the few K6 that also inherited F0 parts) when the 3rd steel plate from the engine side became a dual-steel-plate-with-springs-between affair, and the top plate had slanted cork blocks for some extra startup slip (some K6 have this arrangement as well). The clutch cover came in the sandcast 9-hole and diecast K0 10-hole versions, the K1-K5 version, and the F0/1/K7/8 version, and the F2/3 version. They can all (with the same number of screw holes) be made to work with any, but the lifter itself must change to meet the clutch pack in use because the thickness changes per plate types and numbers.
Clutch springs also changed 3 times: the K0-K6 version, the F0/1/K7/8 version, and the F2/3 version.
Valve springs: there is the K0-K6 version, the F0/1/K7/8 version, and the F2/3 version. Don't mix these up! And, don't install the F2/3 version upside down. I find lots of these...
There are 4 different collections of engine/cylinder/head alignment dowels. If you just buy the long ones, you can always cut them back to fit...
The carb boots changed with the F0/F1. Some F0 actually have the K1-K6 type, with the K6 airbox and the related 7A carbs (but, I promised not to go into carbs...)
The cam sprocket bolts came with one shouldered and one not until the K6, then both are shouldered. All are hardened. The "holey" cam sprockets of the K0 disappeared during the K3, came back in the F0. They all interchange, directly. Only the cams differ in timing.
I'm sure I've missed something...
(Maybe this should go into the Thoughts of Hondaman archive?).
