Topic: 1971 CB750 Motor to 1977 CB750K Frame-Need Help

[scott2011]

I have a 1971 CB750 and need to switch out the frame.  I came across a great deal on a 1977 cb750k frame.  I see that the seat hinges are different which is OK because I need a new seat anyway.  And also the side covers are different.  The only thing I can see different is one front left motor mount on 71 is welded so I'll have to grind that off and re-weld to the new frame.  Other than that it all looks interchangable.
Am I right on this?  Or are there other things to be aware of?  Perhaps 1977 parts I might need or anything to fabricate?
Any help would be greaty appreciated!

thanks!

[GammaFlat]

There should not be any difference in how the engine mounts to the frame.  The 71 engine would have had a sprocket that was further toward the center of the engine.  Also, the sprockets mount differently to the engine.  I'll go find my pictures of the differences in the way the sprockets mount.  So... You may need to run a custom rear sprocket from www.cyclexchange.net. Not sure if this would work... I think it is the intention to offset the difference between K7/K8 vs. K6 and prior sprocket alignment (this issue sorta came up just recently...)

[scott2011]

OK great!  Sorry, new to the forum.  I did a search and thought I'd post.

Can you tell me the part number or what I need exactly from the site?

thanks!

[GammaFlat]

http://www.cyclexchange.net/Cycle%20X%20Exclusive.htm

Scroll about 2/3rds down.  This is 10mm offset.  You may want to talk to Ken at CycleX to make sure that's a good application of the part.  It seems that this sprocket is intended for moving the chain outward and not inward.  I'm thinking this is not a good option. 

Another option is to change to a K6 or prior swing arm - you probably have on on your 71 frame - and use the 18" wheel from the '71.  This all will serve well to confuse the crap out of the subsequent owner and align your chain.  If you could pull that trick off, use a 530 chain and sprockets.  Then you don't need to worry about monkeying around with the mounting of a 630 sprocket where a 530 sprocket belongs (on the 71 engine). 

Good luck!

[scott2011]

Actually I intend to use the old swing arm and everything from the 1971... just using the 77 cb750k frame.  In this case will I still need a differnt chain and/or other things you described?

thanks much!!!

[GammaFlat]

I'm not positive, but it seems to me that you will be fine with a 530 chain and sprockets as well as be aligned (the chain).  I just shipped off my K6 swing arm to HondaMan for his swing arm magic otherwise I'd be able to look closer at the issue.  The K7 and K8 is definitely wider... apparently to accommodate a wider 17" tire and possibly to make more room for a 630 chain setup.  I believe you'll be fine with a K1 swing arm and wheel as well as spacers, axle and stuff.  You'll need the K1 chain guard as it attaches directly to the swing arm.  Now I'm scratching my head over whether the shock mounts on the swing arm are at a different width.... I'm betting they are different but I'm wondering if it matters.... The shocks may no longer be perfectly vertical....  anyone?

[scott2011]

It sure is tough to tell by looking... I have a 1971 frame and this one side by side and they look identical.  I suppose I should just dive in and give it a whirl.
Thanks for all your help

ANyone else have any suggestions?

thanks!

[scott2011]

Well I started it and so far the things that will definately need to be replaced are:
Seat... 71 seat doesn't fit due to brackets being on opposite side.
Side covers... different sizes
Left front motor mount... had to grind it off and weld to 77 frame.

I do see the 77 swing arm is slightly different.  So to avoid having to do the custom sprocket/chain, I am going to try the 71 swing arm for now.

Otherwise everything else looks like bolt and go.

Anyone know where I can get a 1977 seat?

[scott2011]

I decided to go with the 77 swing arm and will figure out the sprocket adn chain when I get to it.

A few other things that don't line up... battery box from 71... had to drill new mounting holes
Oil tank... will not work, had to order a 1977
Chain guard... will not work had to order a 1977
The tensioners on the back of the swing arm... had to order 1977
Rear fender, have to order 1977

Last but not least... I may have to get a 1977 rear wheel, not sure yet.

Got the engine all botled on the frame and she's coming along!