Topic: F Gearing on a CB750K4 Engine??

[nickbee]

I picked up an engine from a ’74 CB750K4 that I’m going to tear down and rebuild for a café racer project.  I read that the F models had lower gearing for quicker starts off the line.  Since I do mostly city type driving I would be interested in making that change.  What parts would have to be swapped out if it is possible? 

Thanks! 

[Mark M]

The simples solution is with a change to final drive sprockets, bigger rear or smaller front will give better acceleration but less top speed. The reverse will slow acceleration but raise top speed. Two extra teath on the rear is probably about right without affecting top speed too much and you should get away without needing another cahin link. On the other hand a smaller gearbox sprocket (1tooth) will give about the same result and is cheaper.

[nickbee]

thanks Mark!  Not sure why I didn't think of that.  So do you thing one less tooth on the drive sprocket will put me at about the gearing of an F?

[Einyodeler]

The `72 - `76 K models used 18 tooth front and 48 tooth rear sprockets.
The F1 came with 17 front and 48 rear , so you just need the 17 front for the same ratio.

[TwoTired]

Word on the street is that a smaller front sprocket puts more stress on the chain, wearing it quicker.  A well worn chain may break.  And, a broken chain often puts nasty holes in the engine case.

FWIW

[nickbee]

Thanks E, but I thought the internal gearing on the Fs was lower also?   Someting I read in my handy CB750 Gold Portfoli book. 

Things that make you go mmmmmmm...

[nickbee]

Are the engine cases the same from the K and the F?  I guess I'm curious if the gearing from the F can be used for the same internal ratios...

thanks!

[nickbee]

Here is the section on the trans from an article in cycle guide on the 750F (for your reading pleasure…

As in previous CB750s, the F model’s crankshaft rides in five plain inert bearing.  Power is still delivered to the clutch through two single-row chains that drive a jackshaft, but two teeth have been added to the jackshaft sprockets; they now have 50 teeth each for a ratio of 1.985:1.  The wet clutch has been modified to allow more oil to get at the plates, which is an effort to reduce the friction plate distortion and glazing that make the clutch grabby and noisy.   The seven friction plates have also been altered slightly and the center has been changed. 

The first three ratios of the five speed transmission are the same, but the forth and gear ratio has gone from 1.097:1 on the K5 to 1:133:1 on the F, and the fifth gear ratio has been changed from 0.939:1 to 0.969:1. 

The transmission sprocket has also shortened the overall gear ratio by giving away one tooth for a new total of 17 teeth.  It drives the same 48-tooth rear wheel sprocket through a new R K brand chain designed by Honda R and D.

All those changes mean that the Supper Sport’s first three gears are 23% lower than the K5’s, and the forth and fifth are 27% lower. 

[Clyde]

The F gear box definitely does have the same ratios for 1,2 and 3, but 4 and 5 are lower than the K and closer together as mentioned above. The F also has a different cam and compression ratio and does rev a bit harder than a K. The redline on an F is 500 higher than a K.
On my F(76F1), I find it revs a bit too high for cruising and I have gone to an 18 tooth front sprocket, which I find perfect.