Topic: Speedo and Tach Gauge size 550 vs 750

[youngerthanmybike]

I'm sure this is posted on here somewhere, but I couldn't bring myself to sort through all of the search results for the answer to one quick question. My bike is a 77 550k, by the way.

Are the actual speedo and tach units the same size between the 550 and 750? (and 650 for that matter?)
As in, the outside diameter, height, mounting, etc.? I've read the early 500/550 units are smaller, but mine is later, so there.

Or, another slightly more difficult question- anybody know if the guts from a 750 tach could be fit into the shell of my 550 tach? If they're not the same size, I might try this out.

I'm finishing up the 650 cam swap into my 550 and I want to know if it's gonna look funny if I have a 750 tach next to my 550 speedo.

[youngerthanmybike]

Anybody? 

Can anybody at least give me the dimensions of a 750 tach?

[Clyde]

Yes, the later 550 gauges are the same size/mounting as the 1973 and later CB750 gauges.The early 750 gauges use a different mounting method.

I have used a set of 550 gauges on my 750.

The speedo is ranged 0-120mph (0-200km/hr) for the 550 and 0-140 (220km/hr) for the 750. They will both be accurate on either bike as they use the same revs per mph. (2240 = 60mph).

The tacho however use different ranges and ratios. The 750 is 4:1 and the 550 is 20:3 (sometimes referred to as 7:1). Apart from the F2 most 750 gauges have a redline at 8000 or 8500rpm.

I ended up selling the 550 speedo once I got the correct 750 speedo and my instrument guy reranged my 550 tach to work accurately of the 750. (This is done by changing the magnetism inside the unit)

[fastbroshi]

  Hmm, so I guess if you wanna switch to a 650 cam you just...

get a cam
 
650 tach cable

a 750 tach

applique to apply to the 750 tach face to match the color and redline of the 550

  Does that sound about right?  I've never done this, but sounds like a good idea.

[youngerthanmybike]

Close, I think. Here's what I'd change:

Get a 650 cam...

Use any tach cable...

Get a 650 tach drive gear...

Use a 650 or 750 tach....

Fix with whatever matching gauge faces you want!

I just wanted to confirm that the tach's themselves were the same size. Cool! Thanks guys!!!

[Clyde]

I think the 650 tach ratio us the same as the 500/550. I have never owned a CB650, so I am not positive.
If so a 750 tach will not read anywhere near correct! At 6660rpm on a 500/55/650 engine, a 750 tach will read 4000rpm

[youngerthanmybike]

Clyde- I'm pretty sure it can't be the same ratio. The tach drive gear tooth count is different and I think the cam gear count that it mates to might be different as well. That's why I need a different tach, otherwise I'd reuse my 550 tach.

[Clyde]

I am not sure at all. As I said I have never owned nor worked on a 650
There is a simple way to check.
Disconnect your tacho cable at the gauge and wrap a piece of insulation tape around the square end in the form of a pointer.
Turn the engine a few times in the normal direction to take up all the backlash and then count the number of times you turn the motor over to make the tacho cable go one revolution. This the ratio!
It is probably more accurate to count the times you turn the motor over to do do two turns of the cable just to be sure.