Swap questions ~GPZ to CB500~

[olds-cool]

I asked the same questions over @ SOHC4 but there are a wide range of bike here and maybe a better chance of answers.
I was looking for tail options on Ebay today when I came across a GPZ tail. I decided to see what other GPZ items they had available and found that the early 80's wheels are the same size as ours. That got me curious since I really like the wheels. I then started looking at the front ends and swingarms. The front tubes are 35mm, same as the CB500 (and 750). So now I'm curious as to whether the whole front end, lowers, tubes, and trees, would fit on my 500? A quick trip to ohiocaferacers confirmed the tube size as 35mm, perfect. Now when I look at bearings I see that the GPZ uses Upper: 25 x 47 x 15 and Lower: 28 x 52 x 16.5 while the CB500 uses Upper: 26 x 48.5 x 15.2 and Lower: 30 x 50 x 14.4. Those numbers are not that much different. Would this front end swap work? I don't know what the stem measures, which I guess would be a determining factor. I'd like to keep the GPZ tree as it is much cleaner looking. Next thing is the swingarm. I couldn't find any measurements listed in the auctions. I did see some nice pictures of boxed swingarms with a monoshock that look pretty close to our own. I've seen some of you simply weld tabs on to mount our factory shocks and at least 1 person that has welded a mounting plate for the monoshock (freakin sweet!). I'm not so much worried about shock mounts at this time. What I want to know are measurements and if it will fit? The rear wheel uses a disc brake and since the K1 used a drum, some type of conversion is going to be needed to run a disc. It would be much easier to use the GPZ swingarm since all of the mounts are there for the brake system. Also could I straight swap the rear sprocket for the CB sprocket or is this going to require some redrilling?

So to sum up my ramblings, I have these questions...

• Will the complete front end swap or would I be better off using just the lowers on my stock tubes so that I can get the wheels and brakes?
• What are the dimensions of the GPZ swingarm and will it fit in a CB frame?
• Will the sprockets swap or require redrilling?

Fronts

Rears

Swingarms

Front end

 

[Druro]

hmm... where to start.

Ok, for the triple tree, it wouldn't be a straight swap. You'd have to go with either custom bearings or custom stem (stem would be easier).

It also might be possible to fit the forks from the GPZ into the CB, but you'd probably need to sort the axle out, and it might not fit (you will need to check the spacing of the fork tubes).

For the swing arm, worry about the width and bolt size. Usually you can grind off some space, and make some "Top-Hat" spacers to fit other swingarms (assuming it uses a larger bolt then the CB).

For the sprocket, that won't be a problem. Easiest thing, as you will just order a new GPZ in the same chain size as the CB. The issue there will be spacing the sprocket out.

I think for you, you will need to get some measurements from people. One thing to consider is that if you are using a modern swingarm is that it will be stiffer then the old one... which could cause you some handling problems.

Wish I could offer some more detailed help... but i'm a Suzuki guy (mostly...)

 

[olds-cool]

Thanks for the info, it gives me a few new things to consider. Hopefully someone will be able to provide some measurements of the GPZ parts so I can get a better idea of what I'm looking at. Someone at SOHC4 suggested finding bearings that matched the outer of the CB but the inner of the GPZ. I have very little knowledge of bearing sizing. I'm sure someone at a bearing supplier can help with that though. Now the question is stem length.

 

[fugue]

For the forks, I would suggest having the GPZ stem pressed out of the lower tree and the CB stem pressed in. The CB stem/GPZ tree may need to be modified to make the stem swap work, but if you could accomplish that, you would use the CB bearings and the front end would bolt right on.

 

[olds-cool]

I guess the stem would be an easy route. Still hoping someone will pop in with some measurements. Closest scrap yard is almost 2 hours from here and it's snowing.

 

[Swagger]

I gotta second pressing the stems. By using your stem in the GPZ triples you get what yo uwant in a fairly easy manner. You may need to mode the stem or clamps a bit but that's far easier than having the stem turned to fit your bearings or god forbit trying to spec oddball bearings.

I say go for it!