Topic: Swapping CB750 engine cases

[Teddyhoeg1982]

I lack an engine for a CB750K2, but have several good K7 engines. I managed to buy a good pair of empty K2 engine cases as seen in the picture. Does anyone have experience or reference to what it would take to move the internals from a K7 engine case to a K2, if possible? I think the output shaft and sprocket will need different alignment, but is there anything else?

[scottly]

The output shaft is the only difference that I can think of, and I don't know for sure if the early output shaft will work with the rest of the K7 trans?

[bryanj]

Output shaft complete plus bearings and new main shells definately, the rest is suck it and see, try comparing part numbers but you would have to check early numbers to see if they were superceeded

[Teddyhoeg1982]

Bumping to see if anyone else could have more detailed information - seems hard to find clear answers before I break the cases apart.

[HondaMan]

The distance between the mainshaft in the transmission is shorter in the K7/8 and F2/3 engines than in the K0-K6 engines. There is an extra tooth on the mainshaft's primary sprocket but the chains were the same, so the mainshaft had to move 1/2 chain link closer to the crankshaft in order to fit it.

Also, the K7/8 transmission (besides having poorer bearings in it than the K2) is different in 1st and 2nd gear ratios (teeth), like the F0/1 bikes.

I have successfully used K7/8 cranks in K4 engine cases (all else being K4) and K3 cranks in K7 cases, but the transmissions were native to the cases.

The K7/8 is essentially the F2/3 engine with higher-compression pistons and superior cooling (with a lesser cam and better valve train). To "follow" which parts are which, print out a list of the transmission parts of the K7 and of the K2, then look at the part numbers. The K7 gears and inner bearings that are different will not have "-300-" for the type definition (e.g., nnnnn-300-nnn is pre-F0, while nnnn-410-nnn is F2/3 and nnnnn-392-nnn is K7/8 parts).

Finally: the F2 and later engines used a special case-width shim 'system' on the mainshaft that today escapes many of the parts fiche (South Sound and CMSNL both miss it) that pushes the mainshaft's gears toward the clutch side of the transmission by a certain amount, depending on how far "off" the chain-drive-side of the engine case was after casting and machining. There are [up to 3] shims on that end of the mainshaft, of 0.1, 0.3 and 0.5mm thickness (and I have also seen 1mm there, in an F3 engine that lives in Alaska now) that pushes the mainshaft's gears toward the clutch side to ensure at least 3mm of gear dog depth between gears 4 and 5 on that shaft. It was a quick-and-dirty way to reduce machining costs of the K7/8 engines (and the F3 sometimes has this 'feature', too) if the injection-mold machine came up a little short of aluminum on the cases (this area is farthest from the injection site). The earlier engines were cast with lots more handwork than the later ones, so the transmissions seldom come even close to swapping between them.

[scottly]

The distance between the mainshaft in the transmission is shorter in the K7/8 and F2/3 engines than in the K0-K6 engines.
Are you sure about that?


The K7/8 is essentially the F2/3 engine with higher-compression pistons and superior cooling (with a lesser cam and better valve train).
The K7/K8 is essentially the same as the F0/F1 engine with slightly lower compression pistons

[HondaMan]

The distance between the mainshaft in the transmission is shorter in the K7/8 and F2/3 engines than in the K0-K6 engines.

Are you sure about that?

Yep. The larger sprocket on the mainshaft (behind the clutch) has an extra tooth, and the crankshaft and primary chains are both the same as earlier bikes. So, the mainshaft has to be closer to the crankshaft by 1/2 the OD difference of the mainshaft's sprocket.

This isn't to say it hasn't been tried: it is to say that the primary chains must be VERY worn before the mainshaft will drop in place. In practice, it was done with drag bikes here in Colorado circa 1990: I only happened to come across it because someone here wanted some "very used primary chains for a CB750" for a dragster bike engine he was building, and had already boxed himself into the project. I had some from a 60k+ mile engine and he used them: then the mainshaft would just barely drop into the bearing seat. I have no idea how that bike/engine turned out, though.

The K7/8 is essentially the F2/3 engine with higher-compression pistons and superior cooling (with a lesser cam and better valve train).
The K7/K8 is essentially the same as the F0/F1 engine with slightly lower compression pistons

The K7/8 has 9.5:1 compression and the F2/3 has 9.2:1 by direct measurements. Since their cams differ, the pneumatic compression test does not indicate the actual compression ratios (and no one seems to have Honda's magic compression tester gage numbers, either). Some folks think that cams alter the compression ratio and while at running speed they can, but at footcrank speeds they don't.
It may depend on which advertisement, Honda engineer, or American Honda personnel one chose to believe in the late 1970s, but by direct CC measurements, this is what is true in those engines I measured, many years ago. It has often erupted with fights and arguments, which is the primary reason I left the F2/3 and K7/8 bikes out of my book, specifically.

And, more than once I have been asked "why won't this mainshaft fit in my [750K0-K6] engine" when the replacement mainshaft and clutch came from a K7/8 engine. With new primary chains, they won't interchange, but with worn parts, they can at least [sometimes] drop into place, as noted above. I suppose this has led to even more confusion about it all. It was this ratio change that made the 750 suddenly less popular for touring in the F and later models because of the increased revs & engine noise behind the fairings in those days: by 1975 the go-to touring bike was a CB750 with a Vetter or at least a Bates handlebar fairing, and every 750 we sold had one of those 2 fairings on it. All of the bike magazines' test riders noted the noise, too, when evaluating the new models.

[scottly]

The K7 had the same 1.708 primary as the earlier Ks. The Fs as well as the K8 had a 1.985 primary.

[Don R]

 This is rather disturbing news for a guy with an F2 backcut transmission sitting in a K6 case waiting for a set of Rods and a big bore top end or a turbocharger.
  Previously, I believed the F2 output shaft only needed the bearing to match the case and the same sprocket drive as the trans output for the offset sprockets.

[scottly]

Don, is the crank with the primary chains installed yet?

[Don R]

Don, is the crank with the primary chains installed yet?

No.  When I mocked it up, I hadn't yet received the German chains and the case studs. I have them now, the motor has an ATP turbo kit also mocked up so it will take a minute to move bikes and dig back down to it.
 
 Edit: Either way, no worries I guess, I also have an F2 motor rat holed that had under 9K miles when the valve guides went south.

[Don R]

 Revzillla shows the same part numbers for 76K and 77/78F for the primary driven sprocket and the chain. Final driven gears are of course different, shaft numbers different, final bearing is the same.
 I believe that I moved the F gears to the K shaft. It matters not, I can make it work.    I even own a set of very experienced K2 primary chains with 70mm of slack. L0L!

[Don R]

 I believe that the K2 output shaft can fit the later gears, it will probably need the K2 output bearing to fit the case. The primary cush drive has a different part number K2 to K7, You should be able to retain the K2 part if the K7 is indeed bigger. I can't say for sure. Use the shift drum that matches the forks and gears.
 Check the crank bearings for color, you probably won't see the color but look at the case for the stamped codes and the crank throw for the lightly etched main codes. Check the charts for both engines, you might be lucky enough to get a match such as all green on both like mine was.
 Sorry about hijacking your question, hopefully we all sorted out some info.

[scottly]

The K7 used the same primary ratio as the early bikes, so that isn't an issue here anyway.
The K7/K8 F2/F3 cases protruded out at the counter-shaft, while the earlier cases were flat; this is why an early output shaft and it's matching bearing are required. I thought I had a pic of the early case for comparison, but I can't find it..