Topic: '77 & '76 750 Build...help!

[Radski]

Hey all, yes I'm a newb to this site & forum and wanted some hints, strategies, and positive opinions about a project I'm about to undertake. A friend just had a baby and is basically giving me a '77 CB750K (near complete) and a '76 CB750K frame which has a '74 engine which is stuck. My first question is this..

Due to some BOS and registration problems with the '77 frame... I am going to attempt to put most of everything from the '77 into the '76 frame. I realize I will need to make some different mounts for items but the bigger issue I have is what to do about the swingarm and how the sprockets are offset on the 77' engine and rear hub I will be using. Should I use the '77 swingarm on the '76 frame, or should I use the '76 swingarm with the '77 parts. Does this matter at all? I know I will need to run the 630 chain but are the differences in the swingarm length and width different enough to need to change that chain too?

All positive suggestions and hints are greatly appreciated.

[seanbarney41]

OK...this can get complicated and difficult to explain.  Hopefully, when you get around to actually comparing parts, you will understand what I am talking about.

To fix your chain alignment with 77 engine, 76 frame, 76 swingarm you will need  to
     1.  CycleX used to sell a 10mm offset rear sprocket.  This how I did it, but that was several years ago.  No idea if this is still available.  The sprocket was for 530 chain and YES front, rear and chain all HAVE to be for 530 size chain.  So will have to source 530 100 link chain, 530 front sprocket(more about this further down), and the CycleX 10mm offset rear sprocket.
OR
     2.  Figure a way to space your rear sprocket out 10mm by means of a custom machined spacer or even washers on the sprocket studs under the rear sprocket...problem is the studs may not be long enough and there is a shoulder on the sprocket carrier that may no longer engage the sprocket correctly.  You would be able to use 530 chain or 630 chain.
OR
     3.  Run your 77 wheel in the 76 swingarm by trimming a spacer to fit.  You may even be able to find an oem spacer combination that fits...just gotta know that the rear wheel needs to stay in line with the front, the sprockets need to line up, and you are gonna have to mess with the wheels spacers whether it is some mix and match of stock parts, trimming stock ones down, or turning new ones on a lathe.

...and then your chain is gonna be very close to your rear shock, so probably no stock chain guard is gonna fit, and fatter aftermarket shocks will not fit.

Some have used the 77 swingarm to get away from having to mess with wheel spacers, but then your chain side shock will be 10mm out at the swingarm unless you mod the top shock mount.

Also finding correct 530 conversion front sprockets can be difficult as the overall thickness of the front sprocket needs to match the stock thickness of a 77 630 sprocket and sellers don't seem to realize this. 

[Radski]

Thanks so much for your info!

Maybe I failed to mention the hub on the rear wheel is off a 77/78 so the rear sprocket is already spaced that 10mm I believe.

I dont mind running a 630 chain, however I wasnt sure how the swingarms will affect this configuration. whether I should just use the 77 swingarm and not worry about the length being longer (which I dont know how much longer it will be and IF this really matters?)

So again, using the '77 front engine and rear hub sprockets...just need some advice on which swingarm to use. and maybe pros/cons.

[seanbarney41]

I would try to use the 76 swingarm, as it is easier for me to fix the wheel spacers.

Perhaps, for you, modifieing shock mounts is easier, then use the 77 swingarm.

I don't think the different lengths will cause a large handling problem.  The bike will turn in slightly slower with the longer 77.  This can be adjusted with front and rear ride height.  Many feel that k0-k6 turn in too quickly anyway.