Topic: Comstar rim upgrade from 1980 CB750F for 1976 CB750F

[swashy]

Hi all, new member here.

I've got an inherited mess of a project bike for awhile now and I'm finally trying to figure it out, hoping I could get some guidance on this.

I've got a bike that *should be* mostly a 1976 SOHC CB750F and what is probably 1980 CB750C/F(?) comstar rims/parts. There's always been the question in my mind whether other parts on my CB were swapped at some point in it's past. But I digress, pictures:





I was told by the previous owner that the comstar rims, brakes, and forks should be a direct swap out to get me disc brakes and nice comstar rims over the stock drum brakes/spoked rims. The front swap out worked just fine, however the rear has been an issue.

With the provided (alleged) comstar axle, spacers, and rear brake caliper, it seems I don't have enough axle room to fit everything. The rear swingarm is simply not wide enough it seems.

Am I crazy here? Can I try bending out the rear swing arms(planning on doing this with a really big wrench) or do I need to swap out to a 1980 swingarm? It seems 1980 swingarms are cheap enough on ebay, however I don't know if it'll be the right width or even fit in the 1976 frame properly. One $40 ebay example:

https://www.ebay.com/itm/303426001611

There's also the question of whether bending out the current swingarm would be a problem for the shocks, since they wouldnt necessarily be in a straightline from the top of the frame down to the swingarms? Then again I feel like I only need maybe 5 mm at most to get everything fitted in.

Thanks for any advice anyone can provide.

[bryanj]

 1980 should be dohc notsohc and the swing arms should not be "bent"to make something fit, alter the spacers

[swashy]

Correct, the 1980 was DOHC, does that make it incompatible with my SOHC?

I think the DOHC was made slightly wider for the disc brake maybe?

If I alter the spaces, won't that affect how the caliper sits over the disc, or how the wheel clears the swingarms?

[scottly]

The 77/78 750's also had wider swingarms, with an additional 10mm offset on the left side. Perhaps the DOHC arm is the same? To use a 77/78 swing arm on your 75, you will need to space the upper shock mount out 10mm, which can be done with a special nut. Also, the 77/78 engines mounted the front sprocket 10mm further to the left, so that will need to be addressed to keep the sprockets in alignment.

[Shtonecb500]

You cant bend stuff and force it in there.
I know there are some guys on here who have swapped DOHC swing arms onto their SOHC. He used a cb900 DOHC and had to shave a little over an inch off and prolly some other stuff.

[newday777]

Hi all, new member here.

I've got an inherited mess of a project bike for awhile now and I'm finally trying to figure it out, hoping I could get some guidance on this.

I've got a bike that *should be* mostly a 1976 SOHC CB750F and what is probably 1980 CB750C/F(?) comstar rims/parts. There's always been the question in my mind whether other parts on my CB were swapped at some point in it's past.



I was told by the previous owner that the comstar rims, brakes, and forks should be a direct swap out to get me disc brakes and nice comstar rims over the stock drum brakes/spoked rims. The front swap out worked just fine, however the rear has been an issue.

With the provided (alleged) comstar axle, spacers, and rear brake caliper, it seems I don't have enough axle room to fit everything. The rear swingarm is simply not wide enough it seems.

Am I crazy here? Can I try bending out the rear swing arms(planning on doing this with a really big wrench) or do I need to swap out to a 1980 swingarm? It seems 1980 swingarms are cheap enough on ebay, however I don't know if it'll be the right width or even fit in the 1976 frame properly. One $40 ebay example:

https://www.ebay.com/itm/303426001611

There's also the question of whether bending out the current swingarm would be a problem for the shocks, since they wouldnt necessarily be in a straightline from the top of the frame down to the swingarms? Then again I feel like I only need maybe 5 mm at most to get everything fitted in.

Thanks for any advice anyone can provide.

Welcome aboard swashy
Just to help you out in terminology on these bikes, you say you have a drum brake on your 76, then it is not a cb750f as you said, but is a cb750 Four K6 model. The cb750F is a different model, which came available in 1975 and 1976 and came with disc rear brake already and spoke wheels(see first 2 pictures). You do need to properly identify your frame model when talking about it and any modifications you want to do(like you mentioned) as parts are different between models. (I was a parts manager in 76 and 77)
No you can not bend your swingarm out wider..... Not a good plan. You will need to get a better plan and parts together to do a brake swap.
You can put a 75 and 76 F model swingarm, spoke wheels w/disc brake if you can find all the parts and the width will be the same width as your K6
As scotly said you can also use a 77/78 swingarm from a F2 model if you are dead set on comstar wheels and want to convert your K model drum brake spoke wheels to comstar wheels w/ disc brakes(comstar wheels are not a good choice in wheels as they had tendencies to crack, so inspect them well!) But, yes you will need to space the top of the shock out further so the shock travels properly without stiction and deal with the sprockets spacing differences the F2 had compared to your 76 K model, if indeed you have a K6.(pictures of your bike will help!!!!)
The 3rd picture is a what a 77 and 78 F2 with comstar wheels came looking stock.

The last picture is a stock 1976 K6.(I'm on my second K6)

Here is a good read to help you in identifying your bike and differences in the sohc models. A long read but well worth the time in helping you learn these great bikes.

http://www.honda750expert.com/

[newday777]

Your picture of your chain looks very out of alignment and is a disaster a the ticking time bomb if you continue in your thinking.... Chain alignment is a very important factor. There is a lot more you'll need to take into consideration.

[newday777]

swahy
What is you reason to want to swap out the rear to disc brake?

Here is a video on swapping to disc he includes the weight differences.

[Bankerdanny]

It is a common mistake for newbies to refer to all the 4 cylinder Hondas as F's where F=4. As others have said, in the context of the 750 (and the 550 also) the 750F was a specific model with some important differences than the other 750 models, the 750K and 750A Hondamatic.

The wheel on your bike is a reverse Comstar off 79 or later DOHC bike (750F or 900F). The original Comstar found on the 77/78 750F and 750A was silver and with smooth spokes as opposed to your black wheel with raised silver edges on the spokes.

The Comstars are not really an upgrade over the stock wires other than being able to run them tubeless. From a weight perspective they aren't an improvement, and you can run twin front disks on your wires, you don't need Comstars. The rear disk is probably somewhat better than the drum in track situations, on the street there isn't a whole lot of benefit.

The rear caliper mount and caliper in the picture didn't go with that wheel, it is from a 75-76 750F. The 77-78 F's and the DOHC 750F's all had the caliper separate from the bracket while the 75-76 had the caliper integrated into the bracket. Someone has messed with it a bit because the allen head screw front and center is where the bleeder screw should be.