Topic: Which CB750's had an offset sprocket of 10mm on the rear wheel?

[Yamahawk]

I started thinking about my 1971 CB750K1, and the PO has installed a different set of forks, and ComStar rear wheel/front wheel on it. My question is, were the offset sprocket carriers only on disc brake models, or was there an offset sprocket on the ComStar drum brake models with the 17" rear wheel? I am curious, as to whether my bike has a mismatch on the front and rear sprockets, as I have this ComStar rear rim on it. Here is a side pic, of the bike.
Charlie

[robvangulik]

I know the F2 and 3 (dual disks in front and rear disk, with comstars) and the K7/8 with spoke wheels, (drum rear and single disk front) had the wide chainline with the 630 chain.
I have no idea what bike your wheels came from, but don't think they came from a CB750 sohc.

[Yamahawk]

Thanks Rob,
   Do you think these came off of a DOHC CB750? I have been trying to determine where they were from since I have owned this bike, which hasn't been but a few months. I figured they may have been off of a CB750A model. Is the 10mm offset only due to the bikes being set up with a 630 chain? I know back in the late 70's and early 80's the 630 chain was popular on the larger displacement bikes from the factory. My 1981 Yamaha XV920RH has a 630 chain, for example. I am just thinking as to whether the rear on this bike had the offset. It is a 530 chain on there now.
Charlie

[BobR]

Thanks Rob,
   Do you think these came off of a DOHC CB750? I have been trying to determine where they were from since I have owned this bike, which hasn't been but a few months. I figured they may have been off of a CB750A model. Is the 10mm offset only due to the bikes being set up with a 630 chain? I know back in the late 70's and early 80's the 630 chain was popular on the larger displacement bikes from the factory. My 1981 Yamaha XV920RH has a 630 chain, for example. I am just thinking as to whether the rear on this bike had the offset. It is a 530 chain on there now.
Charlie

  I believe in 77-78 the front sprocket was moved out 10mm along with the rear to accommodate larger tires. They also went to the ridiculous 630 chain. Recently I discovered my 1974K4 has a later F2 engine in it. My front sprocket is 10mm farther out than the rear. It worked as I put about 4K on the bike that way. Trying to correct it now though. Bob

[Jerry Rxman Griffin aka MuthaF'er]

10mm offset was to accommodate the new double row output shaft bearing on the 77/78 F and K engines and that allowed the "stronger" at the time 630 chain to be used. 

[BobR]

10mm offset was to accommodate the new double row output shaft bearing on the 77/78 F and K engines and that allowed the "stronger" at the time 630 chain to be used.

 Ah ha, thanks. Guess I shouldnt believe everything I read. Anyhow there is some good info here about the rear sprocket carrier change to "offset" the new offset, lol. Bob

http://www.cyclexchange.net/Chain%20Sprocket%20Page.htm

[Yamahawk]

10mm offset was to accommodate the new double row output shaft bearing on the 77/78 F and K engines and that allowed the "stronger" at the time 630 chain to be used.

  I believe in 77-78 the front sprocket was moved out 10mm along with the rear to accommodate larger tires. They also went to the ridiculous 630 chain. Recently I discovered my 1974K4 has a later F2 engine in it. My front sprocket is 10mm farther out than the rear. It worked as I put about 4K on the bike that way. Trying to correct it now though. Bob

So, do you think the 17" ComStar rear drum brake wheel I have is from a '77-'78 F or K model? Did they have drum brake models of these two bikes? I am really curious as to what bike these wheels and forks came from. Is there a measurement I can take from the hub of the rear wheel to the center of sprocket to determine if this has a 10mm offset, and if so, what would it be? Thanks guys for your help!
Charlie

[jgger]

My '79 650 has drum rear and comstars from the factory. They don't look like your wheels though. Maybe they are European wheels??

[BobR]

10mm offset was to accommodate the new double row output shaft bearing on the 77/78 F and K engines and that allowed the "stronger" at the time 630 chain to be used.

  I believe in 77-78 the front sprocket was moved out 10mm along with the rear to accommodate larger tires. They also went to the ridiculous 630 chain. Recently I discovered my 1974K4 has a later F2 engine in it. My front sprocket is 10mm farther out than the rear. It worked as I put about 4K on the bike that way. Trying to correct it now though. Bob

So, do you think the 17" ComStar rear drum brake wheel I have is from a '77-'78 F or K model? Did they have drum brake models of these two bikes? I am really curious as to what bike these wheels and forks came from. Is there a measurement I can take from the hub of the rear wheel to the center of sprocket to determine if this has a 10mm offset, and if so, what would it be? Thanks guys for your help!
Charlie

 Im sure someone will be along who actually know but I believe Comstars came on 77 and later which would mean the sprocket would be 10mm farther out than your K1. Of course the PO could have compensated for it already. BTW have you checked the number on your engine yet? After 20 years I discovered my K4 has an F2(77) engine in it.  Bob

[kmb69]

10mm offset was to accommodate the new double row output shaft bearing on the 77/78 F and K engines and that allowed the "stronger" at the time 630 chain to be used. 

The double row 5206 bearing is called out in the parts manuals as early as 1972. It was the output shaft that was changed in 1977 to accommodate the 10mm offset for the 630 chain and possibly larger tires. 

[Yamahawk]

Bob, I have checked the serial # of the engine, and it is a 1971 K1 motor. The wheels are off of, I thought, a 1977 CB750A, but not sure of this. They are nice looking ComStars, and they seem to line up ok, but I have no reference to check them for offset. The chain looks straight as an arrow, it tracks well, I haven't checked the front sprocket for wear as 754 mentioned in on of his replies to your quest, so I am not sure if I need to worry about offset. That's why I am curious about these Costars and where they were originally from.
Charlie

[kmb69]

They are very nice looking Comstars.
If the rear is a 17", it probably came from a 78 Automatic.
Pretty sure the 77 Automatics still had spoked wheels.

[BobR]

Bob, I have checked the serial # of the engine, and it is a 1971 K1 motor. The wheels are off of, I thought, a 1977 CB750A, but not sure of this. They are nice looking ComStars, and they seem to line up ok, but I have no reference to check them for offset. The chain looks straight as an arrow, it tracks well, I haven't checked the front sprocket for wear as 754 mentioned in on of his replies to your quest, so I am not sure if I need to worry about offset. That's why I am curious about these Costars and where they were originally from.
Charlie

 Wished I never discovered my offset issue. Blissfully road the bike for many years without a problem.. Now that I know it bothers me... I needed to remove the chain guard and use a straight edge to confirm my offset issue. Bob

 

[Jerry Rxman Griffin aka MuthaF'er]

10mm offset was to accommodate the new double row output shaft bearing on the 77/78 F and K engines and that allowed the "stronger" at the time 630 chain to be used. 

The double row 5206 bearing is called out in the parts manuals as early as 1972. It was the output shaft that was changed in 1977 to accommodate the 10mm offset for the 630 chain and possibly larger tires. 

There you go! Thanks for the lesson Leroy. 

[Yamahawk]

They are very nice looking Comstars.
If the rear is a 17", it probably came from a 78 Automatic.
Pretty sure the 77 Automatics still had spoked wheels.

Yeah brother, I think that may be where they came from, now...  does the 1978 CB750A model have a 10mm offset? hehe...
Charlie

[kmb69]

They are very nice looking Comstars.
If the rear is a 17", it probably came from a 78 Automatic.
Pretty sure the 77 Automatics still had spoked wheels.

Yeah brother, I think that may be where they came from, now...  does the 1978 CB750A model have a 10mm offset? hehe...
Charlie

Yes, they did. But an earlier sprocket carrier would fit on that wheel. Can't tell for sure, but it looks like you have a 530 chain, correct? The PO may have used the original carrier.
 

[Yamahawk]

Ok, cool! It is a 530 chain, so perhaps the PO did use the original carrier. That would take care of the sprocket offset then, and I'm good with that. It is interesting to see the different parts that he used on the bike. The instruments were like from a CB400 or something, and I replaced them with a good used 1972 set from my buddy at the cycle salvage. They did bolt up to the 750A instrument bracket, and look good!
Charlie

[BobR]

They are very nice looking Comstars.
If the rear is a 17", it probably came from a 78 Automatic.
Pretty sure the 77 Automatics still had spoked wheels.

Yeah brother, I think that may be where they came from, now...  does the 1978 CB750A model have a 10mm offset? hehe...
Charlie

Yes, they did. But an earlier sprocket carrier would fit on that wheel. Can't tell for sure, but it looks like you have a 530 chain, correct? The PO may have used the original carrier.

 He may have or he may have just swapped the sprockets. The carriers are easy to tell apart. Bob

  Edit: Couldnt just use the sprocket. The swingarm is wider on 77-78 models. Thinking the easiest way would have been to use the original(early)carrier and inner spacer and deal with the outer spacers after that but who knows. Bob
.

[754]

 The double row sprocket shaft, did come out around 73.
 I don't consider the 77 and 78 as having off set sprockets, they simply made z longer shaft and all the sprockets remained flat. Offset sprockets are not flat.
 The easy way to tell look for sprocket only wearing on one side. .
 It's notvthe end ofvthe world if it's out, but it's not ideal.,

[KeithB]

Just to add a bit more (useless?) info, the 77-78 bikes had a wider swing arm.

[Yamahawk]

Just to add a bit more (useless?) info, the 77-78 bikes had a wider swing arm.

...10mm wider?