Topic: CB750K0 17T/ 18T front sprocket

[71 V12]

Hello all K0 experts

I am restoring a 22xxx 2/70 CB750K0. The bike arrived to me 16” rear wheel and all kinds of funky 70’s mods to it.

The plan is to get back to original spec. I am stuck on the sprocket choice though. I see early SC where 16 front 45 rear. But we’re quickly changed to 17/48 and then 18/48 became standard spec as far as I understand.

I think my build would have been 17/48. Question is how much difference does a 17T or 18T actually make to the performance? And is there any other dimensional differences between front sprockets or are they all 100% interchangeable from year to year?

Same question for the rear, is a 1970 48T the same as say a 72 48T rear sprocket?

Many thanks,

Kevin

[newday777]

17 and 18 fronts are interchangeable. 18 gives better gas mileage for long distance riding, but less off the line quickness.
48 tooth rear should be the same.

[PeWe]

17:48 and stock wheels match gearbox very well and make bike more fun to ride.
The rpm difference compared  to 18:48 is only 250rpm at 100kmh (60mph).
A little bitt less need to gear down during accelerations.
Order both 17, 18 and test. Easy to change front sprocket.

[Progas]

17 and 48 always is the best choice for all around performance.

[69cb750]

The plan is to get back to original spec. I am stuck on the sprocket choice though. I see early SC where 16 front 45 rear. But we’re quickly changed to 17/48 and then 18/48 became standard spec as far as I understand.

#3528 to CB750-1044649 came from factory with 17/45.
K1 came with 18/48

[71 V12]

The plan is to get back to original spec. I am stuck on the sprocket choice though. I see early SC where 16 front 45 rear. But we’re quickly changed to 17/48 and then 18/48 became standard spec as far as I understand.

#3528 to CB750-1044649 came from factory with 17/45.
K1 came with 18/48

Interesting! So presumably 17/45 spins the rear wheel slightly faster? Is the chain still 100 links with a 17/45 set up?

Are gearbox ratios the same throughout K0-6 production or were the differences for the K0?

Many thanks for the replies,

Kevin

[69cb750]

So presumably 17/45 spins the rear wheel slightly faster?

17/45 and 18/48 ratios are very close, tach reads the same.

Is the chain still 100 links with a 17/45 set up?

Yes

Are gearbox ratios the same throughout K0-6 production or were the differences for the K0?

69 to 72 gearbox ratios are the same, may be the same to 76.

[ekpent]

 I actually think that the early bikes started out with a 16/45 tooth count and to try to combat the chain breakage moved up to larger front sprockets and appropriate rears to match. If doing a totally bone stock resto you may want that depending on serial numbers and for a rider maybe newer updated size.  Personally I like for my riding style the 18/48 and have even tossed some 18's on a couple of 1975 and 76 F Supersport 750's that came with 17's.  I'm more of a cruiser then a stop light racer.

[69cb750]

I actually think that the early bikes started out with a 16/45 tooth count and to try to combat the chain breakage moved up to larger front sprockets and appropriate rears to match. If doing a totally bone stock resto you may want that depending on serial numbers and for a rider maybe newer updated size.

CB750-1000001 to CB750-1003527 came from factory with 16/45
CB750-1003528 to CB750-1044649 came from factory with 17/45
CB750-1044650 to CB750-1121806 came from factory with 18/48

[Mark K]

I've got the 18/48 setup, was told changing to a 18/51 would be good for slightly quicker acceleration off the line. Will this present any issues?

[MauiK3]

I've never seen 51 on the back. You may need a longer chain, check clearances to everything, that's a large sprocket.
All that matters is the ratio you are after, if you are a stoplight racer, small in front, large in back, if you want comfortable cruising, increase the front.

[Mark K]

Thanks. I was told 51 by a drag racer, should've figured that one out myself! 😉

[Don R]

 18/51 would be easy on the chain but it might want 102 links, I've never done it. It will be closer to the chain case and center stand and might slap them when it gets loose. I'm sure guys have done it and can say for sure. All just conjecture on my part.
 I like the 18/48 myself, it's less buzzy at 75 mph. For in town my sandcast still has it's 17/48 is a bit more fun and I don't rack up long miles on it.

[PeWe]

I rode 4 long tourings in Europe in the 80's. Around 6000 km each time. 17:48 worked fine. Speed thru Germany was planned to be around 120-140kmh but that was seldom possible. Either around 90kmh in the slow going lane (long trucks reduced speed in uphills to half), 160-190kmh in the faster lane. Flashing headlight in mirrors when riding too slow which could happen in 180-190kmh too.
836cc, ported head and cam (~80whp) helped to be a part of the traffic flow.
I forgot I replaced 18 with 17 to help the bike to rev out before that.

I thought that 17:48  was stock gearing in earlier bikes.
Bike need to have possibilty to accelerate out from bad situations that can occur.

[bryanj]

The 16 was subject to a Honda recall so its a moot point as to wether 16 or 17 can be called "standard"

[71 V12]

Many thanks for all of the reply’s, very helpful and informative.

I am not a long distance tourer, more interested in the off the line performance.

I will go 17T front. Am I correct that 45 rear will give best off the line?

[ekpent]

Many thanks for all of the reply’s, very helpful and informative.

I am not a long distance tourer, more interested in the off the line performance.

I will go 17T front. Am I correct that 45 rear will give best off the line?

That would be geared pretty low for the open road but would be snappy through the first few gears I guess.

[robvangulik]

45 teeth will actually make for longer gears/lower revs, 48 will pull harder, as will a 16 in front!

[MauiK3]

Smaller front, larger rear equals more off line performance, larger front smaller rear equals better cruising.

[71 V12]

Thanks again for all the feedback. I think I will try 17/48. As Per said easy change to 18T front if needed.

May not be factory for my year but it sounds like the way to go for what I want from the bike.

Regards to all,

Kevin

[seanbarney41]

simple to figure final drive ratio given the same rear wheel diameter...makes it easier to compare sprocket sizes.  Just divide the rear sprocket tooth count by the front sprocket tooth count...
45 divided by 16 = 2.81  so  2.81 to 1 ratio means front sprocket has to turn 2.81 times for every revolution of the rear
45 divided by 17 = 2.65  so  2.65 to 1 ratio means front sprocket has to turn 2.65 times for every revolution of the rear
48 divided by 17 = 2.82  so ....
48 divided by 18 = 2.67

Resto guys must realize that it's pretty visually easy to see the difference between a 45 tooth rear and a 48 tooth rear...no need to start counting some old greasy sprocket teeth.

Also chain quality has improved drastically since 1969.  A 16 tooth sprocket should be absolutely no problem for a new good quality chain.

I don't think any sizes other than 16, 17, 18 front and 45, 46, and 48 rear are available anywhere.  But many places will do custom size sprockets for a price.

[ekpent]

45 teeth will actually make for longer gears/lower revs, 48 will pull harder, as will a 16 in front!

WhoopsI thought he was still talking about the 51, my mistake.