Topic: Cb750 driven sprocket swap - change late model to NON offset sprocket?

[fxef79]

Ok. So I know that later CB750 motors had an offset sprocket setup (10mm?). And that you could use a late model sprocket on an early motor if you needed extra clearance for a wider tire, as long as you matched the 10mm offset on the rear (swingarm spacers, offset sprocket carrier, etc). 

But is the opposite swap also possible? Meaning, can I take a non-offset early drive sprocket and put it on a late model motor without major work?

Here’s the specific case:
I’m replacing a 73 motor in a trike frame with a motor out of a 78 K8. I’d prefer not to have to mess with the rear end alignment.  Since it’s a trike frame I can’t do the usual changes to rear swing arm spacers or sprocket carriers to match.

Can the non-offset driven sprocket on my 73 motor be taken off and straight swap onto the 78 motor shaft? Any other case or shaft or other mods I’ll need to do to accomplish this? 


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[754]

You would have to run an offset sprocket reversed up front.
 Cant you just put spacers on rear sprocket ? Show us a pic of how it moùnts.

[fxef79]

I’ll partially answer my own post, having started down this road, but it really means I have more questions. 

So I pulled the front sprocket on both motors. Neither sprocket is “offset“. They are both perfectly flat. The one on the trike is a 16 tooth, the one on the K8 is 17 tooth. They swap no problem.

 But here’s the problem. The shaft is different. And the attachment for the sprocket is different. On the 73 I have the “plate” and two bolts, with the chain oiler bolt in the center. On the K8 what I have is something I’ve not seen described before and doesn’t match parts breakdowns. The sprocket has the two holes for the two small bolts, but no bolts and no plate. And you couldn’t use them anyway - the end of the shaft doesn’t have the groove that the plate would rotate into. Instead what I have is a center bolt with a small washer that goes through a large fender washer which then goes through an odd large spacer and all of this holds the sprocket on. The spacer has nothing that would keep it centered other than it appears to be the same inside diameter as the outside diameter of the shaft. Show this assembly holds the sprocket on.

But here’s my real problem, using a straight edge and measuring off the edge of the case where the lower rear engine mounting bolt goes through, with either sprocket on the K8 motor it is offset to the left, or outside, by about a quarter inch in comparison with the relative position of the sprocket and that same point on the case on the 73 motor. This is a very rough measurement, I’m using tiny wooden ruler and a tape measure. I’m guessing the actual measurement here is right out about 10 mm, which is what I suppose I should be expecting.

So now, here’s the question, what do I need to get in order to have a driven sprocket on a K8 engine that has the same offset from the engine/frame mounting points as a 73 engine? Do I need a specific special sprocket for this? Has anyone used one and maybe has a link for where I can get them?


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[fxef79]

I’ll partially answer my own post, having started down this road, but it really means I have more questions. 

So I pulled the front sprocket on both motors. Neither sprocket is “offset“. They are both perfectly flat. The one on the trike is a 16 tooth, the one on the K8 is 17 tooth. They swap no problem.

 But here’s the problem. The shaft is different. And the attachment for the sprocket is different. On the 73 I have the “plate” and two bolts, with the chain oiler bolt in the center. On the K8 what I have is something I’ve not seen described before and doesn’t match parts breakdowns. The sprocket has the two holes for the two small bolts, but no bolts and no plate. And you couldn’t use them anyway - the end of the shaft doesn’t have the groove that the plate would rotate into. Instead what I have is a center bolt with a small washer that goes through a large fender washer which then goes through an odd large spacer and all of this holds the sprocket on. The spacer has nothing that would keep it centered other than it appears to be the same inside diameter as the outside diameter of the shaft. Show this assembly holds the sprocket on.

But here’s my real problem, using a straight edge and measuring off the edge of the case where the lower rear engine mounting bolt goes through, with either sprocket on the K8 motor it is offset to the left, or outside, by about a quarter inch in comparison with the relative position of the sprocket and that same point on the case on the 73 motor. This is a very rough measurement, I’m using tiny wooden ruler and a tape measure. I’m guessing the actual measurement here is right out about 10 mm, which is what I suppose I should be expecting.

So now, here’s the question, what do I need to get in order to have a driven sprocket on a K8 engine that has the same offset from the engine/frame mounting points as a 73 engine? Do I need a specific special sprocket for this? Has anyone used one and maybe has a link for where I can get them?


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The K8 sprocket and bolt/washers. To me it looks like the inner spacer/washer had been loose and flying around free, marking that larger circular area on the sprocket.



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[fxef79]

The 73 setup.



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[754]

Shaft is 10 mm longer on K7 and K8.
 And part of the chaincover will hold bolt from backing out .. for a while.,
 I had a bolt back out or fall out, on a K8 when i used wrong cover.
 So again what dous tge back look like..

 The 2 little holes on your sprocket means it fits any year shaft.

[fxef79]

73 case and shaft.




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[fxef79]

K8 case and shaft.




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[fxef79]

You would have to run an offset sprocket reversed up front.
 Cant you just put spacers on rear sprocket ? Show us a pic of how it moùnts.

“Offset sprocket REVERSED”

Omg. Duh.

I seriously didn’t even consider that was an option. Until I started pulling it apart and saw what it looks like I assumed the sprockets would only go on one way. I have a bunch of “book smarts” on this stuff, but only now getting my hands this deep into them.

I see now... no reason a sprocket couldn’t be flipped around.

You, sir, are a genius.


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[fxef79]

You would have to run an offset sprocket reversed up front.
 Cant you just put spacers on rear sprocket ? Show us a pic of how it moùnts.

As far a spacers on rear sprocket, nope, not really.  This is what I’m working with on the rear.



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[fxef79]

You would have to run an offset sprocket reversed up front.
 Cant you just put spacers on rear sprocket ? Show us a pic of how it moùnts.

“Offset sprocket REVERSED”

Omg. Duh.

I seriously didn’t even consider that was an option. Until I started pulling it apart and saw what it looks like I assumed the sprockets would only go on one way. I have a bunch of “book smarts” on this stuff, but only now getting my hands this deep into them.

I see now... no reason a sprocket couldn’t be flipped around.

You, sir, are a genius.


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Oh wait.

That’s going to put the chain on the case below the shaft. With the flat sprocket it already is leaving some mark on the case, no way a reverse offset is going to work.

I may end up having to just run it slightly off like this, until I can get the 73 engine rebuild done.


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[754]

Mount your sprocket on the 78 shaft and check clearance.... use drill bits if you have an index.... they are like 16 thou apart from next size... offsets are like 10mm or just over 3/8.. not sure if you can get a 16 T.
 . if you have to come out a 32nd, you can run shim between shoulder on shaft , and the  sorocket.
 16 is small circle, you may  need different size to ckear seal boss,

Akso i can tell you how to make an offset , out if 2 sorockets..

[fxef79]

Mount your sprocket on the 78 shaft and check clearance.... use drill bits if you have an index.... they are like 16 thou apart from next size... offsets are like 10mm or just over 3/8.. not sure if you can get a 16 T.
 . if you have to come out a 32nd, you can run shim between shoulder on shaft , and the  sorocket.
 16 is small circle, you may  need different size to ckear seal boss,

Akso i can tell you how to make an offset , out if 2 sorockets..

16t sprocket goes on the 78 shaft just fine. Clearance looks good. I’ll just have to live with it being offset a bit. Chain on this trike is longer than stock so there’s a bit extra leeway I think.

What I’m not clear on is how it should attach. I’m not sure I have the right bolt and washer assembly. I can tell this K8 has had sprocket changes, since it’s got a 530 chain - 78K would have had a 630 chain, right?

In fact, something doesn’t add up on this K8 - not that I plan on putting it on the road, but something is odd. It has a 17t front and 40t rear. Yea- 40. For a 530 chain, wouldn’t 18t/48t be “original”?  And when they went to 630 chain wasn’t it something like 15/41?  So I could see someone going to a 40t rear with a 630 chain if they wanted to turn less rpm at highway speed - but a 530 chain with a 17/40 is REALLY tall. And I’ve triple checked. It’s a 530 sprocket not 630.

Although, not even as tall as I’m dealing with on the trike. It’s got a 37 rear. And 28” diameter tires. I’m afraid I’m going to have to find some new wheels and tires to pull that down.

And a 15t front, too. Although that’s even tougher on the chain isn’t it.


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[bryanj]

The K7/8 were new when i last worked on one but i dont remember a spacer behind the centre bolt and washer, perhaps the 530 sprocket is thinner and needs it ireally dont know.
No matter how long the chain run if they are not in line both the chain and sprockets will wear quickly

[Medyo Bastos]

Cant you just run a jack shaft?


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[fxef79]

Cant you just run a jack shaft?


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If I wanted to put lots more money into this. All I wanted to do is swap the motor and put it on the road already.


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[fxef79]

Cant you just run a jack shaft?


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If I wanted to put lots more money into this. All I wanted to do is swap the motor and put it on the road already.


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Also, it’s only 10mm of offset.


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