Topic: 1978 Honda CB 750 Frame Size...any difference?

[Jaydem]

Hello everyone!

I am currently looking into purchased a 1978 CB 750. Does anyone know if the frame is any different (bigger...etc) then lets say a 1969-1974 frame? I know the noticeable differences like the tank and the seat/tail...and small engine differences? But, is there more? I want to buy a cb 750 and transform it into a cafe racer and want to know that I have the right foundation to start with. I see alot of cafes and all seem to be 1969-1974 years.....why?

Thanks in advance!

Jakub

[MCRider]

Hello everyone!

I am currently looking into purchased a 1978 CB 750. Does anyone know if the frame is any different (bigger...etc) then lets say a 1969-1974 frame? I know the noticeable differences like the tank and the seat/tail...and small engine differences? But, is there more? I want to buy a cb 750 and transform it into a cafe racer and want to know that I have the right foundation to start with. I see alot of cafes and all seem to be 1969-1974 years.....why?

Thanks in advance!

Jakub

The raw frame (K) is the same size thoughout. The later bikes had longer tanks, seats ( and the part of the frame that suports the seats) swingarms (maybe). The later Fs may have had some different fork geometry I think.

I'd say you see more 69-74 rebuilds as that's time frame the most used bikes are. Sales after 1975 dropped off pretty quick. It's all relative.

[Stev-o]

welcome!  Yes, you are correct, the motors of the K models are pretty much the same 71-78.

[Jaydem]

so is there a website for specific details on changes for these years? I just want to be 100% sure!

[MCRider]

so is there a website for specific details on changes for these years? I just want to be 100% sure!

I think if you go to the "website" button in the top blue row, you'll go to the area where you can find a "timeline" of changes. These timelines exist in other places too, none i can dirct you too at the moment.

Here:
http://www.sohc4.net/index.php/cb750/model-guide

and elsewhere.

[GammaFlat]

From a ways back:
http://forums.sohc4.net/index.php?topic=66151.msg726935#msg726935

What are glaring differences between the 77's & 78's  vs the
older ones that I'm more familiar with ?

The 77/78's have a 17" rear wheel vs. 18".
77/78 has a 76F motor - higher compression than 76K
77/78 has caps instead of gators on the shocks
77/78 has a longer grab bar
77/78 has a hidden fuel cap with door - tank is bigger/wider
77/78 has a saddle with hinge (and latch) on the wrong side?! 78 saddle is "stepped"
77/78 has HM401s instead of HM340s or HM300s
77/78 has a slightly longer swing arm (I think) - overall wheelbase is longer
77/78 has a 630 chain as opposed to 530
77/78 has a new style side cover
77/78 has blue background gauges
77/78 has ignition key in the console (diff console as well) that locks the fork
77/78 has accelerator pumps and shorter manifolds (rubber connectors to head)
77/78 has a different airbox with larger openings for the carb/airbox connector
77/78 fender is shorter at the top with no stays on top side

The look of a 77/78 is decidedly more modern.  I have a 74, 76, 77, 78 and I like the K0 look best ?!

I'm sure there's more

[Toxic]

If you want to build a pre-77 cafe stick with a pre 77 donor bike.  Don't try and mix and match.  It can be done but there are allot of small parts you will be hunting for.  It's much easier if you have lots of experience with all of the SOHC models.

There is nothing wrong with a 77/78 cafe.  There are lots of great examples on this site.

[Nikkisixx]

Some reasons why I would recommend a 77-78 to modify (please note the lack of the "C" word):

>less desireable and therefore cheaper (usually)
>wires from the switches on the bars (horn, start, turn, kill) run outside - easy to change bars
>brake light switch is in the lever/MC *(on a '78) - one piece brake line
>big ass 630 chain looks bad ass
>longer bike handles better at ton-speed in straight lines on the highway (where do you run flat out?)
>longer fork sliders mean that a fork stabilizer is available from tarozzi
>prefer the accelerator pump carbs IMHO
>choke is cable opp erated and the fuel valve is on the left - nice to be able to move those without standing on your head
>ignition switch up on the bars and it also locks the bars - older CBs have a separate lock for the bars
(again standing on your head) and the ignition switch is down under as well
>the tank is ugly as homemade sin, but older cb tanks fit and if you are changing the seat anyways this is an easy swap
*Just trying to keep cooking daddy happy

[Spanner 1]

+1 agreement with Nikki ( Hi, Nikki ! )..... except I like the big-ass K7, K8 tanks 

[cookindaddy]

I like the big ass tank too! And the recessed filler cap. All the items listed by the others I agree with except:

The brake light switch on the 77 is down on the triple tree and it has a two piece brake line, the 78 has the brake light switch in the master cylinder and the one piece brake line.

I thought I "knew" that the frame was about 1-1/2 inch longer on the 77-78 than the earlier bikes. This is why the muffler from the earlier models don't fit?

Hondaman's book is a bit light on the 77-78 model specifics if that makes a difference. (not a criticism, otherwise his book is great!)

[Spanner 1]

We lined-up my K1 against F-Bombs '77 F and I think his F was 3 inches longer in the wheelbase, the swingarm is longer for sure, but maybe the frame is too? Actually, maybe the steering head angle is more shallow which would increase the wheelbase too.... the same would apply to the '77-'78 K bikes.
Overall, the later K's and F's are bigger bikes, make the 'ol K1 look small 

[Jaydem]

this is great information guys...thanks for all your help!

[GammaFlat]

The swing arm is longer on the K7/K8 (held them side by side).  Also, the triples are different.  The K7/K8 triple has the forks sitting more forward relative to the stem (learned this one the other day).  The geometry between the engine and swing arm mounting location is the same.  The sprocket on the K7/K8 is moved outward a bit.  I think the frames are otherwise the same length. 

[Stev-o]

... except I like the big-ass K7, K8 tanks 

I like the big-ass K7 tanks too! Getting ready to paint mine.

[lwitchey]

One big difference is the oil tank. You will not be able to switch them. Also you will not be able to run clubman bars on the 77-78. The tank is too wide.

[754]

 I suspect rear of frame is longer by same amount as swingarm.
 But I think the trees have LESS offset..the tubes hit the tabnk on my 76&down K frames.

 If you desire rearsets the 77/78 have them altho a bit fugly PLUS sidestand is moved back and up a bit.. that may look better with rearsets. Also the F rake may be better for cornering.

[Markcb750]

Some reasons why I would recommend a 77-78 to modify (please note the lack of the "C" word):

>less desireable and therefore cheaper (usually)
>wires from the switches on the bars (horn, start, turn, kill) run outside - easy to change bars
>brake light switch is in the lever/MC *(on a '78) - one piece brake line
>big ass 630 chain looks bad ass
>longer bike handles better at ton-speed in straight lines on the highway (where do you run flat out?)
>longer fork sliders mean that a fork stabilizer is available from tarozzi
>prefer the accelerator pump carbs IMHO
>choke is cable opp erated and the fuel valve is on the left - nice to be able to move those without standing on your head
>ignition switch up on the bars and it also locks the bars - older CBs have a separate lock for the bars
(again standing on your head) and the ignition switch is down under as well
>the tank is ugly as homemade sin, but older cb tanks fit and if you are changing the seat anyways this is an easy swap
*Just trying to keep cooking daddy happy

You have given this way too much thought, go with the flow man, relax it's all good.

It does handle nice at spped, and the mod's Nikki has on it make his handle the twisties in western NC real nice.

[climbingaz]

One big difference is the oil tank. You will not be able to switch them. Also you will not be able to run clubman bars on the 77-78. The tank is too wide.

Sure you can.  Just gotta weld in some small nuts to increase the steering lock.  

[climbingaz]

If I had to do it all over again, I probably would have bought a pre-'76 instead of my '78.  I'm starting to see the light at the end of the tunnel on my cafe project (see picture in post above), but its just that finding parts for the '77 and '78 is much more challenging than the early years.

[greyghost18t]

Can you swap out earlier tanks onto the 1978 K bikes? Anything need to be custom or is it as they say plug and play?

[750K]

Swing arm is 10mm wider as is the upper shock mounting bolt and sprocket carrier on the left side of the 77-78 750's, the rear wheel is the same but the axle spacers are different on the 77-78 years to account for the difference in width.

[Retro Rocket]

Swing arm is 10mm wider as is the upper shock mounting bolt and sprocket carrier on the left side of the 77-78 750's, the rear wheel is the same but the axle spacers are different on the 77-78 years to account for the difference in width.

And the frame is about 2 inches longer at the rear, same as the F models...

[750K]

Correct me if I'm wrong Retro, but didn't the rake and trail increase a bit as well with the 77-78 750's also?

[Retro Rocket]

Correct me if I'm wrong Retro, but didn't the rake and trail increase a bit as well with the 77-78 750's also?

Yes, 1 degree extra rake, 10mm less offset on the triple clamps, which resulted in a fraction over 4 inches of trail...

[SKTP]

I have a 78K and I have physically put all 3 gas tanks on it. 76-and-earlier tanks will be too short for the stock seat, and the F is too long...they all fit the mounts but the seat has to be modified (well I guess you could run with a big gap in between if you like with the older K tanks). I have had my 78 beside a 76 and for sure the rack is more "chopperish" on the 78

There are no d-ring washer/safety-spacers on the top triple tree.

[EvLoutonian]

I have put a K2 tank on my K7 frame, and I had to change the position of the two mounting posts up front, near the steering stem.

Nobody else encountered this discrepancy?

[EvLoutonian]

One more question .. (re. oil tanks)

Is there any reason not to use the older style oil tank on the 77/78 models, if I'm prepared to change the mounting points on the frame?
(my bike was already "trimmed" of tabs when i got it, anyway)

I have mounted tabs to the K7 frame to suit the older style oil tank, and the oil hoses seem to match up to my (F1) engine fine.

Anything else I should triple check for compatibility?   THANKS!

[Don R]

I have put a K2 tank on my K7 frame, and I had to change the position of the two mounting posts up front, near the steering stem.

Nobody else encountered this discre

I have put a K2 tank on my K7 frame, and I had to change the position of the two mounting posts up front, near the steering stem.

Nobody else encountered this discrepancy?

 The 77/78 seat is further back. I've used early/ late/ F  tanks on a 77 with no changes except to the seat.