help with '76 CB750F Frankenbike

[riffman12]

I was hoping some of you on this forum might be able to help with my build, especially TOOLS1 who seems to own most of the bikes that donated parts to my bike...

A little background:

My bike is a '76 CB750F

The previous owner installed the front end from a '77 CB750F to get dual front disks

He also installed a rear swingarm from the same '77 CB750F. I'm not really sure why...just to get matching comstars I suppose.


This is what created my first problem. As I'm sure many of you know, the '77 CB750F swingarm is a little bit wider than the '76, so my rear shocks don't quite line up. Do you know what the best way to make this work is? Is it even that big of a deal? I won't be running the rear grab bar, so I do have a bit of wiggle room in the top shock mount.


The next issue is that when I bought the bike, both comstars were bent. Because of this, I bought wheels from a '79 CB750F / CBX (same wheels). They seem to fit fine in the '77 swingarm when I mocked it up, but some people have told me that it might not line up properly / center in the swingarm. Can anyone shed some light on this?


Am I stupid in trying to make this all work with my '76F? Is it even possible?

My last resort will be to revert back to a '76 swingarm, but then I will have a spoke rear wheel and will need to find a front to match, but I just spent a lot of money powder coating my comstars and swingarm :(


Thanks a lot in advance for your help. I just really want to get this thing riding soon!

[TOOLS1]

I don't understand why the previous owner had to switch the front end to get duel disc brakes, you can easily bolt on another disc and caliper to the 76 F front wheel and forks to have duel disc brakes. There is even a write up on here somewhere that shows how to do it. As for the DOHC rear wheel, it will go right into the 77-78 swingarm. However you will need to use the 76 cush drive in order for the chain to line up with the engine sprocket. Also you will want to make sure you have a "530" chain and sprockets. For the shocks, how far are they off? Can you post a picture?
TOOLS
P.S. Do you have the 76 F rear wheel and brake?

[riffman12]

Yeah I'm also a bit confused as to why he went through all the trouble to change the parts. For all I know, he could have damaged them. Who knows. The weird thing is: how was he riding this thing with misaligned shocks and a crooked chain line??

As for the DOHC Wheel, I wish I could change the cush drive, but the wheel has 5 rubber bushings, where is seems like every other bike including the '76 has 4, so I can't just put on a different drive.

I already have 530 chain and sprockets. Worst case I guess I can buy an offset front sprocket to help with chain line.

I don't have any of the swingarm or rear wheel parts from the '76F unfortunately.

Here are some pictures of the rear shock alignment. Keep in mind there is a bit of wiggle room up top as I will not be installing the rear grab bar which adds some thickness to the top shock mount.







Thanks again

[TOOLS1]

I cannot understand the cush drive being different. I have a shed full of CB 750 wheels, and the cush drives will swap between all of them. On the shocks, that is not enough of an offset to hurt anything, they are mounted in rubber, so they will be able to give as the swingarm moves.
TOOLS

[riffman12]

Hmm well that's very good news on the shocks. I've had multiple people tell me that it will lead to problems due to the left side of the swingarm being offset by 10mm.

Here's what I mean about the different cush drive. This is my cush drive off the CBX wheel:




Here is what the '76 cush drive looks like>




I guess because my wheel has a 5 bolt drive and the '76 bikes have a 4 bolt?

[shinyribs]

My '76F had the same Comstar with the same 5 spoked cush. Best I can tell, the SOHC CB500 & 550, and the DOHC 750 & 650 (and apparently, now the CBX as well- quite possibly all DOHC's had 5 cush spokes) all used the 5 spoked cush.

I think I still have a ultra gloss black powdercoated s-arm for a 76F here somewhere that I never did sell.

That shock alignment looks a lil funny, but I can't see it hurting anything. Can you verify that your chain run is aligned?

[riffman12]

Hm that's interesting. You just became my best friend since you're the only person I've heard get this all to work haha.

So were you able to mount that 5 cush rear wheel to your '76 swingarm? Or were you running a different arm? What did you do to get your chain lined up?

Or are you saying you used a narrower drive from a 500 / 550?

I've taken the rear end apart now, but last I remember, my front sprocket was about 10mm to the right of the rear sprocket, which is why I was looking into custom offset front sprockets. I figured this was due to my newer swingarm and newer wheel. I made a mistake with some wheel bearings and things though, so I will take a picture once I've got it all back together properly.

thanks again for all the help guys

[TOOLS1]

That bottom cush drive is not off a CB 750. I think it is a CB 500/550 cush drive. What you need is an actual 69-76 CB 750 cush drive. The only difference between the 69-76 cush drive and the 77-83 is the sprocket offset. The 69-76 cush drive will solve your chain alignment problems.
TOOLS

[shinyribs]

riffman12 wrote

Hm that's interesting. You just became my best friend since you're the only person I've heard get this all to work haha.

So were you able to mount that 5 cush rear wheel to your '76 swingarm? Or were you running a different arm? What did you do to get your chain lined up?

Or are you saying you used a narrower drive from a 500 / 550?

When I got my '76F it already had the Comstars on it. It was working in the original s-arm, but I wasn't the one that put it there, so I don't know what had been done with spacers or whatnot.

As far as chain run, I honestly can't vouch for it as I promptly put spoked rims on the bike, but the guy I bought the bike from had been riding it for years, so I can't imagine the chain run could have been an issue.

I wish I had more info. I just recall running in to the 5 spoked cush issue and what I found out by researching it. I remember that some guys swore up and down that the 1st year model DOHC used a 4 spoked cush, but others were convinced that all DOHC's had 5 spokes. I do remember looking up part numbers for '79 CB750 and '80 CB750 and there were two different part numbers, so I'm inclined to believe there were some differences there. I just don't have enough solid, or hands-on personal experience to really say.

[shinyribs]

TOOLS1 wrote

That bottom cush drive is not off a CB 750. I think it is a CB 500/550 cush drive. What you need is an actual 69-76 CB 750 cush drive. The only difference between the 69-76 cush drive and the 77-83 is the sprocket offset. The 69-76 cush drive will solve your chain alignment problems.
TOOLS

But a 4 spoked cush won't go in his wheel.


Did 500's and 550's ever come with Comstars? I know 450's, 77/78 750's and  DOHC's had them. Not really sure about anything else.

[TOOLS1]

You know after thinking about it, I think that bottom cush drive is not from a CB 500/550. The CB 500/550 cush drive is held in with with a threaded ring. If it came off a comstar wheel, it might have been from a CB/CM 400/450. However he still needs a cush drive from a 69-76 CB 750 to go with his DOHC rear comstar to make the chain line up. If it were not so cold and nasty out today, and I had some ambition, I would go out to the shed and dig some out to take pictures.
TOOLS

[riffman12]

sorry for the confusion,

my wheels are from a '79 CBX, and for that year only, it was the same wheel as the CB750F / Super Sport. The K bikes had different wheels and thus different cush drives.

So it sonds like my options are:

a) try and find a narrower cush drive that perhaps mounts to the CBX wheel

b) get a custom offset front sprocket from CycleX

c) go back to a '76F swingarm...but then the rear wheel won't fit

hmmmmmmmm

[shinyribs]

Swingarm won't matter. The distance from the center of the chain to the center of the rear tire is a constant, unless you can find a slimmer cush.

If I have a spare 5 spoke cush I will inspect it. There may be enough material to turn it down in the lathe to do what you need. What State are you located in?

[shinyribs]

TOOLS1 wrote

 However he still needs a cush drive from a 69-76 CB 750 to go with his DOHC rear comstar to make the chain line up. If it were not so cold and nasty out today, and I had some ambition, I would go out to the shed and dig some out to take pictures.
TOOLS

Still cant make a 4 spoked cush go in to a 5 spoked rim.  Or am I nuts. Don't the SOHC 750's have a 4 spoked cush?

[TOOLS1]

OK, you got me. Where in the heck does the 5 spoke cush drive come from? I have seen it somewhere, and not long ago.
TOOLS

[shinyribs]

Well, I went on a Google hunt and decided to do an image search to see what I could find.

Found my old thread on the topic.

http://honda-cb750-s.456789.n3.nabble.com/Need-help-finding-a-cush-drive-please-td4027172.html


As far as DOHC cush drives, I've never had a stock rim on my '80 so I don't know what's s'posed to be there. But my 1100F does have 5 studs on it. So I looked in the 79-83 CB750 FSM and it does only show 4 spoked cush drives. So like Tools said, it seems 750's never had this 5 spoke cush. I'm guessing they must be for the bigger engines? CBX's and 1100's. Maybe even 900's? Definitely a mystery.

[riffman12]

yep just the '79 CB750F. you can see it here:

http://www.houseofhondaparts.com/fiche_section_detail.asp?section=2564260&category=Motorcycles&make=Honda&year=1979&fveh=132912

Now the question is, if I can't find a narrower cush drive...can I run the one I have with a 10mm offset front sprocket? You can see them on this page:

http://www.cyclexchange.net/Chain%20Sprocket%20Page.htm

Or is that likely to rub my frame on the outside?

This is complicated...

[TOOLS1]

I have used a 69-76 (narrow) cush drive in a DOHC comstar before without any problems, but it was from a "C" model. I guess the "F" wheel must be different. As for the offset sprocket, I would go on and try it. I doubt it will be enough offset to cause the chain to rub.
TOOLS

[TOOLS1]

Hey, I just noticed the offset rear sprocket. You could use it instead of an offset front. And it is cheaper too.
TOOLS

[riffman12]

I thought about that, But I'd have to mount it backwards and I'm not sure if there would be room.

I believe they are designed to make the chain stick out further, where I would need it closer to the wheel