Topic: CB750K8 Rear Sprocket Drawing

[Sheik Yerbouti]

Hey guys. I'm working on a project over in project shop that involves converting a 1978 with a junked motor into a battery electric vehicle. This requires me to significantly adjust the gear ratio, meaning I need to get a new, larger (72 tooth!) rear sprocket made. Before I went through the hassle of taking a thousand measurements from my sprocket and CADing up a new one, I was hoping to find a dimensioned drawing that I can base it off. Also, are all of the rear sprockets the same or were they modified at all during the SOHC CB750 run?

Thanks guys!

[steam-powered man]

sure you need 72T?  i've got a cb750 rear sprocket w/51T, brand new in the box and never used.  came with a bike and parts bike i bought last year.  just sitting on the shelf, i'll never use it.

bobp


oh, yeah.  it's aluminum (less rotating mass, yadda, yadda).          

[Sheik Yerbouti]

sure you need 72T?  i've got a cb750 rear sprocket w/51T, brand new in the box and never used.  came with a bike and parts bike i bought last year.  just sitting on the shelf, i'll never use it.

bobp


oh, yeah.  it's aluminum (less rotating mass, yadda, yadda).          

I might be interested in something like that depending on what kind of drive sprockets I can get my grubby mitts on. 530 chain?

[steam-powered man]

tried the sprocket against my F3 with #630 chain (nope) and my 900ss with #520 chain (nope).  width measures
.320inches.  if interested, i can swing by my local bikeshop and see what chain it matches.  be going past them tomorrow.
bobp

[Sheik Yerbouti]

tried the sprocket against my F3 with #630 chain (nope) and my 900ss with #520 chain (nope).  width measures
.320inches.  if interested, i can swing by my local bikeshop and see what chain it matches.  be going past them tomorrow.
bobp

I just calculated some gearing and realized that I need to be atleast 60 in the back to get anything close to the top end I'm hoping to get with my electric motor. I appreciate the offer though!

[754]

 What do you want for the sprocket?
can you ship USPS to Canada?

[scottly]

Sheik, you might want to take a look at dirt-bike sprockets; they tend to be bigger than street bikes. I'll try to take a look at my CR480 rear tomorrow. You would have to make an adapter to fit one to your K8 wheel. As far as the front sprocket, what is the shaft dia. on your motor? Does the shaft have a key-way?

[steam-powered man]

sprocket measures 0.320 thickness with pitch of 5/8inch, appears to be a 525 series, but i'll check at my bike shop.  from another site:

a 500-series chain has a pitch of 15.875mm. A 600-series chain has a pitch of 19.05mm. The second and third digits indicate the chain's width, measured between the inboard surfaces of the inner sideplates. Originally these numbers specified width in eighths of an inch (i.e. a chain ending in '20' had a width of 2/8, or 1/4 inch, a chain ending in '30' had a width of 3/8 inch), but now metric dimensions are used. A chain ending in '20' measures 6.35mm between the plates. If the number ends in '25' or '28', the dimension is 7.94mm.

have shipped quite a few parts to canada, can check shipping cost tomorrow.  a 12in x 12inch padded envelope would work well.  would like $24 for the sprocket, plus ship.
bobp

[754]

 PMing my Zip.. 530 or 520 works for me.. looks like a PBI sprocket...754

[Sheik Yerbouti]

Sheik, you might want to take a look at dirt-bike sprockets; they tend to be bigger than street bikes. I'll try to take a look at my CR480 rear tomorrow. You would have to make an adapter to fit one to your K8 wheel. As far as the front sprocket, what is the shaft dia. on your motor? Does the shaft have a key-way?

I can get one custom machined from electricmotorsport but they're a hundred bucks apiece. The motor has a 7/8 diameter I believe (I can't double check right now) and has a keyway. The drive sprockets are like 15 bucks a piece, so you can switch gear ratios if you're so inclined. If I had a 60 tooth rear, I could have a 10 tooth (6:1) 12 tooth (5:1) and 15 tooth (4:1) to play around with.

[bollingball]

Are you sure that is a 750K8? It has a spline shaft no key way one center bolt.

[Sheik Yerbouti]

Are you sure that is a 750K8? It has a spline shaft no key way one center bolt.

Sure is a 78K8, but the motor in it is an electric motor. 

[scottly]

Sheik, you might want to take a look at dirt-bike sprockets; they tend to be bigger than street bikes. I'll try to take a look at my CR480 rear tomorrow. You would have to make an adapter to fit one to your K8 wheel. As far as the front sprocket, what is the shaft dia. on your motor? Does the shaft have a key-way?

I can get one custom machined from electricmotorsport but they're a hundred bucks apiece. The motor has a 7/8 diameter I believe (I can't double check right now) and has a keyway. The drive sprockets are like 15 bucks a piece, so you can switch gear ratios if you're so inclined. If I had a 60 tooth rear, I could have a 10 tooth (6:1) 12 tooth (5:1) and 15 tooth (4:1) to play around with.

You can get a flat #50 60 tooth sprocket from McMaster-Carr for $46.60. You would have to bore the center hole to fit the K8 hub, as well as drill the four mounting holes. MC part #2299K59

They also list a 72 tooth sprocket with a hub, which would require more metal to be removed, for $83.24. MC part #2741T4.

Disclaimer: prices are quoted from a catalog that is a year or two old...