Topic: Early CB750 tach? (WAS: Re-gearing a tach?)

[Scott S]

 I have a CB500 with an early '71 build date. It has the smaller tach and speedo. I'll soon be swapping in a complete CB650 engine and I'd like to keep the original gauges if possible. I know the 650 uses a different tach ratio than the 500.

  Is there such a thing as a "gearbox" or something to change the ratio? Or does anyone know of a place that can re-gear my tach to the 650 ratio?

Am I correct in thinking that the '69-72 750 gauges were smaller as well? Would the '69-72 CB750 tach fit my '71 CB500 gauge bracket?

[Scott S]

 Bumping with new info/questions.

[oldbob]

No, the early 750 gauges were large, even the K0s.

I don't know about re-gearing the tach, but to do so would require opening up the case. If the case gets opened, I wonder if a different face might accomplish the same thing. Admittedly, you would have to work up your own face, but with digital technology it shouldn't be too bad. The hardest part would be determining where to position the marks on your new face. I have an old dwell meter that also functions as a tach. I would use that to determine the actual angular velocity of the crank, and note where the Honda tach's needle is pointing. I would repeat for various crank speeds from 1000 RPM to something just south of redline.

Bob

[Stev-o]

A lot of guys use the 650 cam in the 550. And they use the 650 cam gear so thectach will work...

http://forums.sohc4.net/index.php?topic=53444.0

[Scott S]

 No...that's not quite correct.

 I have a 650 cam/tach gear in my other 550. I had to get a 750 tach. The 650 and 750 tachometer use a different ratio than the 350/400/500/550.

 That was fairly easy to do on my '78 550K because it had the "larger" gauges that came on the later models. However, my '71 CB500 has smaller gauges. I was hoping to either keep the early style gauges or find a matching tach with the correct ratio.

 I think what I CAN do is run the later, larger 750 gauges on my early gauge bracket.

 Another Q: What about the later, DOHC guages? Do the tach/speedo ratios work with our SOHC bikes?

[754]

 If you want guages to fit in your cups, try looking the 350and 450 twin ratios, they may fit on your bike.

[Scott S]

 I'm pretty sure most all speedos are the same ratio. How can I find out what ratio the 350/450 twin tachs are?