Topic: Cb750F front sprocket problem

[7not11]

Hi folks,

I am rebuilding a cb750f engine and have enjoyed the process so far but now that the engine is back on new issue- the front sprocket from the 78 cb750k bike I tore down for the build doesn’t fit the shorter sprocket drive of the new engine. Any advice on how to move forward?
I’ll include some photos below. Any advice is greatly appreciated!

Best,

Tony

[7not11]

Pics for detail.

[7not11]

Additional pic of the donor engine sprocket came off of- note the generous usage of jbweld by previous owner

[754]

Does not fit your chain.
 Or does not even go onto the shaft.
 which one Is it one of the above?

[ekpent]

Looks like a 630 chain sprocket maybe in the picture. Will they fit on a shaft designed for a 530 ? Never tried here on my end. What year or serial number are your 'new' engine cases.

[754]

They should interchange, not clear what problem is .

[7not11]

Hi folks,

The sprocket shaft seems to be a lot shorter and I can’t figure out how to mount this sprocket onto it. The first photo shows the motor I’m putting it onto, it’s off a 77 super sport. I’ll note the serial numbers when I’m back in the garage.

Thanks again!

[754]

Quite simple, you push the sprocket on until you can see the groove. Then you slip on the lockplate you did not shown, turn it to line up with the threaded holes, then put the two small bolts in.
 However. ..
...the 75 f looks like short shaft, later are longer.  Tell us what motor is going in what frame, and we can tell you what to do to line up the sprockets

[Jerry Rxman Griffin aka MuthaF'er]

+1

It is NOT a direct swap.

[madmtnmotors]

...the 75 f looks like short shaft, later are longer.

Nope, just the opposite. The early F models (75/76) used the 530 chain with a raised shoulder on both the inside and outside of the front sprocket. The later (77/78) used a narrower sprocket that only had the raised shoulder on the inside (facing the engine) with the 630 chain. This shorter stem on the output shaft was a result of shifting the front sprocket out 10mm on the later models (77/78) with the wider bearing on the output shaft and the retainer was changed to a simple center bolt and washer.

So you went to all the trouble to rebuild the engine and you are going to use the old wore out front sprocket of unknown mileage? 

[754]

Ok your sprocket retainer is in the pic of the 15 t sprocket .. is it for 5 30 chain or 630.
 There is a lot of wear on the side of the sprocket shown..if the other side is not worn, that is a CLEAR INDICATOR the sprocket are running out of  line.. so this is caused by short front shaft and wider carrier on the back... OR long shaft on front narrower carrier on the back.
 So we need to know year of frame,  year of motor , wg0hich swingarm you have,  then we can advise what to use to hold rear sprocket.

[bryanj]

Early sprocket on early motor held on by plate and 2 bolts, later sprocket on later engine held on by large bolt in middle

the methods of fitting are not interchangeable

[7not11]

Hi folks,

So it looks like my new build engine is a 77’ F2 motor and the donor engine was a 72’ K2. Ok after clearing that up and previous comments I probably can’t just bolt on. Which sprocket or kit should I get given the new info?

Thanks all,

Tony

[7not11]

Frame is a 78 cb750k, frankenbike!

[754]

 I think you guys are missing how this works , the earlier 530 sprockets  will go on late shaft  , using washer and bolt.
 So again, which swingarm ?
 And do you want to run 530 chain? Or 630 ?

[7not11]

The swing arm is the stock off the 1978 Cb750K bike- not sure what details are needed. I will choose whatever chain size will make this easiest, couple of months wrenching now!

Thanks,

Tony

[754]

Tony, if you do not have the correct swingarm there are a few mods need doing.
 You have the right one, I hope you have the later rear wheel with wider sprocket carrier, then you are set.
 You can run both sizes of chain ...you choose. .

[madmtnmotors]

The 77 F motor output shaft will align with with the 78 K swingarm and rear wheel/rear sprocket carrier. The simplest would be to source the 630 chain and sprockets for any 77/78 F or K model assuming your evaluation of the chassis being correct for the 78 K.

Converting the later models (77/78 F and K) to 530 chain and sprockets is discussed here:

http://forums.sohc4.net/index.php/topic,180710.msg2094423.html#msg2094423

[7not11]

Roger on all replies, I'll be ordering a 630 sprocket for my model (77 cb750f super). Any ideas about mounting it once I receive it? Should it just be a simple bolt on the end like stated earlier? I'll pour over my manuals to see.

Thanks,

Tony

[HondaMan]

The sprockets for the F2/3 engines require a bolt and large washer that bolts the sprocket onto the end of the countershaft, using those reduced-size splines you can see. The old sprocket you have used the floating-mount countershaft-and-sprocket setup. On the F2/3 engine, the sprocket is rigidly mounted to that shaft and doesn't use the trick little pieces that let it float to its own alignment. The onus is on you to make sure the chain alignment to the rear sprocket is true as the result.

[754]

530 is probably cheaper, lighter and revs out quicker  fwiw

[7not11]

Does the fastener for the newer sprockets look like the attached?

[754]

I think so, I lost mine, I did not realize that the proper 77 and 78 sprocket  cover has a piece cast in to prevent the bolt coming off. Make sure you use that. ..they can be tricky to install on a stock frame.

[7not11]

An update- the bolt I got was too large. I’ll try sourcing the right one- any know a part number or good directory for it? I’ve had trouble finding it. Attached is the bolt screws in.

[madmtnmotors]

Something looks wrong with that picture. It like the seal is not fully seated and protrudes out from the case. Can you clean the grease off the seal and clean part of the case around the seal for a better look? The seal should sit flush with the case.