Changes made to the CB750K in 77-78

[Qaxe]

This is especially for guys and gals who have a 1977 or 78 CB750K, who need to buy a part, and wonder if this CB750 doo-dad is going to fit on YOUR bike. This site has a 10k word limit, so I have to break it up in sections.

As you know, the CB750 started in 1969 (68' if you count the 4 prototypes). The 69' is simply a "CB750" there was no K designation at first. The "K0" isn't a Honda thing, it was made up by us. That's because the "K" stands for 改良 (Kairyou) which is akin to "Upgrade" or "Improvement". "K1" was the first major upgrade, which conveniently came out in 1971, and K8 was the last upgrade of the SOHC CB750 which came out in 1978. From 1971 to 1976 there were changes each year, but the majority of parts are interchangeable between bikes from those years. That isn't necessarily so with our K7 & K8s.

In 1975, Honda came out with the Supersport version of this bike and dubbed it the CB750Four ("F"). There were obvious aesthetic changes, but most importantly it had a bigger cam, new carburetors with a fuel pump, 4-into-1 exhaust, and a rear disc brake. When 1977 rolled around, Honda made sweeping changes to both the standard model, and the F model. Honda chose to use the older 1975 F model engine and carburetor in the standard CB750, which is why the K7 and K8 have more pep than the K1-K6. (But not 69-70; those were real zingers). The K8 (1978) is almost the same bike as the K7, with a few changes of its own. (You can find those in the service manual)

When working on a CB750K we can often mix-n-match parts from different years. That's part of the reason why I have bought so many CB750s. It's a forgiving bike for aspiring gear-heads. But things become a bit more challenging when we start swapping parts between K1-K6 and K7 & K8. To make it easier on myself, I started making a list of changes. Some of these changes are in the service manual, but not everything. You can save yourself some stress if you learn the differences for yourself My list may also not be complete, but I think it will help you guys who want to know... "WILL IT FIT?!?"

(CONTINUED BELOW)

 

[Qaxe]

"All" CB750K and F engines from 69-78 will mount up into a frame from any of those years, but there are caveats.
77/78 frame differs from older models in a few spots.
- Engine mounting brackets added to the left side, so you can remove/install the engine from either side easier.
- Oil tank mounting points changed. Older style oil tank WILL NOT bolt on to a K7/K8 frame, and vice versa.
- Rear end of frame is slightly (an inch?) longer. This will be relevant further along.
- Mounting for rear fender and bumper/grab-bar is further apart (old fenders must be re-drilled in order to fit, and grab-bars, handles, racks, and sissy-bars from K1-K6 won't fit)
77/78 Oil tank is different in both shape and mounting points (as mentioned above).
- Cheap, plastic dipstick replaced the stylish chrome one.
- Dipstick is now hidden under the side-cover, so you actually have to remove the side-cover to add oil.
- Oil insert port is MUCH smaller making it harder to add oil. From K7 you need a funnel.
- Hoses. While "Hose A" (lower one) is the same between K6 and 7, "Hose B" (the upper one) is a different part and slightly longer.
77/78 side cover shape and mounting ports changed.
- Older side covers will NOT fit on K7/K8 frame, and vice-versa
- Side cover badges are now reflective gold and cursive, and read "CB750K" instead of "750Four". The Honda wing in the orange diamond is gone.
77/78 Triple clamp
- has a new hole to mount the new choke cable stay
- gauge-cluster bracket holes changed (more on this below)
- Triple clamp bearings changed from individual ball bearings to a single unit that must be pressed on with a special tool.
- Rake angle has changed (not sure if this is at the frame or dictated by the triple-clamp)
77/78 Swing-arm is longer and 1cm wider. This one change affects A LOT of other parts!
77/78 Chain guard is a different shape and mounting points moved, meaning that a K1-K6 chain-guard cannot be mounted on this frame without major alterations. (The Supersports' chrome chain guard mounts right up.)
77/78 rear wheel changed to 17". Wheel itself is 1" smaller, but tire is wider and taller.
77/78 Rear wheel axle and spacers differ (to fit the 77/78 swing-arm). Cannot use K6 axle on a K7. Wheel HUB is the same however, and brake shoes are interchangeable from K1-K8.
77/78 Sprocket carrier on hub may be different in size, perhaps wider (need confirmation)
77/78 Drive chain adjusters differ in length by 1/4" (1cm), and they are more narrow where they grab the frame.
77/78 Rear brake pedal is longer, rounded, and bend is different.
77/78 Rear brake arm (lever) is slightly shorter, and has a groove down the length of it.
77/78 Rider peg/pedal/step have metal "Stopper" nibs on the bottoms.
- Right side rider step is about 1cm shorter than previous year.
- Right now has a bolt hole for a 10mm bolt and lock nut, for making minor adjustments to brake pedal height.

 

[Qaxe]

77/78 Both front and rear sprockets, and the drive chain changed.
K7 Front Sprocket 15T x 630. Rear Sprocket 41T x 630. Chain 630 x 88
(~ K6: Front Sprocket, 18T x 530. Rear Sprocket, 48T x 530. Chain 530 x 100.)
- K7/K8 front sprocket connects with a single bolt in the center.
K1-K6 front sprocket bolts on with 2 bolts and a clamp. In other words, you couldn't use an older sprocket if you tried.
- Sprockets and chain should match the engine. Left-right alignment between front sprocket and rear wheel is a full centimeter different (10mm). It has been suggested, that if using a K7 frame with an older engine, that should use the rear sprocket carrier from the older K bike. (Did I do this?!?)
77/78 Rear fender is longer. Fender now has threaded holes where the frame, fender and grab-bar all meet.
- Contoured where it meets inner, plastic fender, mounting bolts are actually part of the fender now (K6 and older had holes, and bolts were separate)
77/78 Taillight bracket now much smaller, sits lower on the fender. (No change to the actual taillight lens itself.) (Tail-light bracket mirrors the 69' style.)
- With some minor effort, an older rear fender will fit on a K7/K8, HOWEVER the stock K7 passenger grab bar will no longer fit, because of the longer K1-K6 taillight bracket will get in the way. (Trade-off)
77/78 Rear bumper/grab-bar is longer and the mounting points are further apart. It is solely there as a handle, and for a touch of chrome, whereas K6 and older had turn signals mounted to the grab-bar.
77/78 Rear turn signals have their own separate mounting hardware, which mount between the frame and grab-bar. (One nut holds the winker stay, grab-bar and shock absorber.)
75-78 turn signals are much bigger than 1969-74 models. (Size changed in 1975)
77/78 Front fender is shorter, sitting back slightly. Front fender now only has one chrome bracket in back.
- K1-K6 fenders mount up nicely on K7/K8 and vice versa.
- Front brake-line stay on fender is more recessed. The older stay was closer to tire.
- Speedo cable stay is similar, but the rubber is light gray (was black on older models)
77/78 tank is longer, wider and holds more gas. Chrome gas cap is gone. Gas cap is dull metal, concealed under lid with a false lock. Drain hole for spilled gas. Gas cap is secured with a bit of chain.
- Tanks are not exactly interchangeable due to petcock location and seat lengths, but can be done.
77/78 Seat is longer in front AND back. Hinges on LEFT now. Passenger sits higher. If you want an older tank, be prepared to make a custom seat.
77/78 Ignition moved up between the gauges and doubles as a steering lock. (K6 and older had a separate lock point from the ignition)
77/78 Idiot-lights are now between gauges housed in plastic that wraps around the ignition.
77/78 Gauge bracket plate changed and gauges sit further apart to make room for ignition and idiot lights
77/78 Different background color on gauges (darker)
77/78 Handlebar mounts serve only one purpose. (no longer part of idiot lights)
77/78 Odometer knob is on the side instead of back of speedometer. (Started with K6).
This means that the chrome speedometer housing has one less hole. One could use the chrome backing plate from an older model speedo on a K7/K8, but not the other way around.
77/78 Handlebar rise, rake and length differs between various years. K7/K8 are identical.
- Note that handlebar size dictates the length of your throttle cables.
77/78 Handlebar switches are different. No option to turn off headlight (Uncertain of year that went away)

 

[Qaxe]

77/78 Headlight brackets changed quite a bit.
- Front turn signals now have designated posts welded low on headlight brackets. (K0-K1 turn signals held the headlight on. K2-K4 turn signals attached to the bracket mid-height. K5-K6 turn signals larger but also attached at mid-height.)
- Fork gaiters are gone, so the headlight bracket end with rubber gaskets, and have no lip to add old-school gaiters even if you wanted them. In order to have gaiters, you need K6 or older headlight brackets, and therefore, you will lose your K7/K8 front winker mounts. In this case, buy K6 front winker mounts.
- Distance from forks to headlight mounting bolts is longer, probably to make room for the new ignition switch up top.
This creates a bit of an annoyance if you want to use the K7 ignition switch with K6 or older headlight brackets.
In that case, be ready to have to force the headlight on, because the back of the headlight will hit the front brake line.
This becomes more difficult if you change your front brake line to stainless braided lines, because they don't bend as much.
(I had to put my lines on loose, mount the headlight, then tighten the brake lines last.
77/78 Forks are longer with simple dust-caps instead of gaiters.
77/78 Front BRAKE changed completely. All year calipers will mount up and can be used on the K7/8, but caliper parts themselves are not interchangeable
- Caliper bolts on the outside now with 12mm bolt heads. Older models bolt from the INSIDE using 8mm hex-heads.
- Caliper piston is different. 42.8mm Diameter x 34mm Length (69-K6 is 38.15mm Diameter x 34mm Length)
- Bleeder bolt is smaller (which kind of sucks) with a 9mm head.
- Brake pads changed (K1-K6 pads have a bolt, and a metal piece that sticks into the small half of the caliper, held in place with a cotter pin)
- Front brake switch is slightly SHORTER (.75cm or so?) and is held on with only ONE bolt.
- Front Brake Hose "B" is 1/2" longer at 16". Front Brake Hose "A" is 13" long. (K6 was 12.75")
77/78 Engine is externally the same (will mount into older frame), but has larger crank (check my terminology)
77/78 Carburetor is totally different, with accelerator pumps and shorter manifold boots. (Same carbs as "F" model from previous year)
- Choke lever is replaced by a cable that reaches up to the handlebars. (One more part to have to buy)
77/78 Air-box changed.
- Has larger boots (and holes) to match the carburetor.
- Airbox opens with wing-nuts on the bottom, easing the access to the air filter.
77/78 Kick-start lever longer and angled. Older kickstart levers have a preset position on the shaft, and went on one-way and one-way only.
77/78 Muffler/silencer design changed
- Older 4-into-one pipes had an outer heat-shield. K7/K8 pipes were simpler. Easier to keep clean...less retro.

 

[Qaxe]

77/78 Exhaust pipes mount differently on BOTH ends.
- Exhaust manifold has threaded bolts that stay on the engine.
- Exhaust clamps are mounted with nuts instead of Philips-head bolts, and go on at the same time as the pipes (~K6 the clamp went on last)
- No "Frange joint" on 77-78. (The part that was attached to the motor with cross-point head bolts on ~K6, and VERY hard to remove when the bike is crusty.
- New interior "collars" come in 2 pieces that are VERY annoying to install, because they can fall out while you're trying to bolt on the clamps.
- Exhaust gaskets are now crush-rings instead of fragile asbestos type. (this may have been a "brand" thing.)
77/78 Clutch lever rubber step is squared. K6 and older are rounded.
77/78 Missioncase Cover (Gearbox cover) now reads "5-4-3-2-1". K6 and older had no writing.
K7 crankcase changed somehow, but I cannot see the difference.
Engine covers for the Dynamo/Stator, clutch box, and valve cover are all unchanged.
77/78 output shaft, which holds the front sprocket, extends an additional 10mm (1cm).
In order for the 2 sprockets to line up, both the swing-arm and the rear sprocket carrier were made 10mm wider.
This is fine unless you put a K6 or older engine in a K7/8 frame (or vice versa, which is much more common).

US paint choices for 1977 - 1978 - Candy Alpha Red or Excell Black.

 

[gearsoup]

Thanks for this thread, helpful information. As someone who's going to be diving into a basket case K8, I'm sure I'll reference this thread more than a few times!

 

[madmtnmotors]

Having owned six K8's I've learned a few things as well:

>All of the K8's (and F2's) I have owned all still used the individual (loose) ball bearings in the steering neck/triple tree.
>There are at least three different aluminum case clutch covers. The outer tin covers are the same, but there are the real early aluminum clutch case covers that had one fewer mounting screws, then there was the "in between" aluminum clutch covers that had a little extra material behind one of the screw bosses for the outer tin cover that will interfere with the later (thicker) clutch hub, and then the later aluminum clutch cover to clear the taller clutch pack of the later clutches that used the double steel riveted clutch plate in the stack.
>The front sprocket covers are different on the K7/K8/F1/F2 models to accommodate the 10mm outboard offset of the later models. I've attached picts showing this difference. I realized this when I received a K8 that had an early style cover and the chain had "self clearanced" the early style cover.

 

[Nightshift]

Wow Qaxe, really appreciate you making this massive summary of changes. I have a K7 sitting in the shop waiting for a restoration and I'm in the process of collecting all the parts I will need to do it. This summary really helps me avoid buying stuff that won't fit from fleaBay, etc.. Awesome job! Cheers, Bill

 

[JB_7]

Love the detailed information, thank you

 

[Star_circle]

This is awesome! Thanks for the information.
I just bought a 78k and about to rebuild it.
I found a front brake rebuild kit on website for 73-76k:
Front brake rebuild fit

Do you think I can use this kit on my 78k?
Can it be mounted on my 78k’s caliper holder?
Or I need a 73-76’s holder?
And if i use 73-76’s holder. Do I need a set of 73-76’s forks?
Thank you!

 

[Ducklife]

Great info thankyou so much for posting

 

[pickleknuckles]

"77/78 Forks are longer with simple dust-caps instead of gaiters."

The 77-78 forks are actually the same overall length but the lowers are taller by 1" and the tubes are hard chromed.

I've got K7 legs in K4 trees w K4 ear tins and dust boots, a K2-6 fender, '75 F1 wheel, and K4 dual brake setup. I went w the K7 fork for the corrosion resistance and increased rigidity of the taller lowers as well as the increased fender clearance to use a period correct brace.

 

[Armstrong17]

Hi,
I'm just curious if any 750K's came with factory DID wheels. I just noticed that my '78 has them.
Thanks.

 

[pickleknuckles]

Yup. My K4 has DID stamping on both.

 

[Armstrong17]

Hey,
I'd like to put some rubber fork boots/gaiters on my '78K. Will gaiters for an earlier model K work for me?
Thanks

 

[madmtnmotors]

This is awesome! Thanks for the information.
I just bought a 78k and about to rebuild it.
I found a front brake rebuild kit on website for 73-76k:
Front brake rebuild fit

Do you think I can use this kit on my 78k?
Can it be mounted on my 78k’s caliper holder?
Or I need a 73-76’s holder?
And if i use 73-76’s holder. Do I need a set of 73-76’s forks?
Thank you!

The kit you linked to would use the earlier style inline pressure switch for the brake light (typically mounted on the tree) while the 78K has a mechanical brake light switch at the lever. There may be other subtle differences.

 

[pickleknuckles]

Hey,
I'd like to put some rubber fork boots/gaiters on my '78K. Will gaiters for an earlier model K work for me?
Thanks

Kinda. You'll need to use the earlier fork/headlight ears as the boots attach to them. I'm using '74 ears, on '77 tubes and lowers. It all fit like it should. The only difference is the later lowers are an inch taller and the tubes are hard chromed.
Also using a '74 fender and all '74 parts for dual front brakes.

They do make aftermarket boots that fit the lowers and have a narrower top opening that uses a zip tie to secure it on the tube, but imo they don't look right.

 

[Armstrong17]

Kinda. You'll need to use the earlier fork/headlight ears as the boots attach to them. I'm using '74 ears, on '77 tubes and lowers. It all fit like it should. The only difference is the later lowers are an inch taller and the tubes are hard chromed.
Also using a '74 fender and all '74 parts for dual front brakes.

They do make aftermarket boots that fit the lowers and have a narrower top opening that uses a zip tie to secure it on the tube, but imo they don't look right.

Thank you for the info. I removed my headlight ears in lieu of some smaller aftermarket ones so I'm fully prepared to ziptie them. I'm still having a hard time finding gaiters that will fit top and bottom tubes.

 

[cdesigner]

Thanks for the info!

 

[Hank99]

How about pistons? I'm finding conflicting information. Best I can find is '76 pistons will work in a '78, but slightly lower compression ratio?