Front wheel speedo drive unit

[honda-san]

So, anyone know how to tell what model a front wheel drive unit is correct for?

Specifically the drive gearbox for the 350-4, 500-4, 750-4 all look the same and can be fitted to any of these bikes. BUT, there are three different part numbers - 350-4 and 400-4 use the same "317" middle part number item. 500-4 uses a "323" middle number item, and CB750 uses a "300" middle number item. That I can see there are no features / markings on these that enable easy model  identification. Most likely the difference will be the internal gear ratio, which is not much help when trying to identify which bike a particular unit came from.

I have at least half a a dozen of these, not all of which are identified as the bike they originated from. It would be possible to to work out the gear ratios by spinning a front wheel an exact number of turns and measuring the corresponding number of rotations of the output tang (that the cable connects to), but actually not so easy to do with the necessary accuracy given the likely subtle differences between ratios.

Any good ideas appreciated!

[Orcade-Ian]

Hi Chris,
You've got my inquisitive side motivated!  With the wheel still in and the drive unit connected to a cable, it would be very easy to revolve the wheel 1 complete revolution - with a suitable pointer/marker fitted.  With degree disc mounted on the squared cable end, you could then count the number of complete and partial turns for that one revolution.  Yes, you would have to mount all of your unidentified units to do a full check.  I only have 350, 400 and 550 fours to check but if owners of all models are willing to help, you could end up with a definitive set of results, perhaps for Ash's Dropbox.

Ian

[Oddjob]

Is there no physical difference in size? You’d imagine there would be if the internal gears are different sizes. I can’t remember seeing any identifying marks on my 500/550 speedo drives and I took them to pieces.

[Johnwebley]

I seem to remember someone saying the front hub on the 500 and 750 are the same

Do they both run 19" tyres?



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[Bryanj]

Yes and speedo drives are same

[Oddjob]

So why the different part numbers Bry?

[exvalvesetdabbler]

Most honda and Yamaha used the same internal gearing in the Speedos so it would make sense that the gear in the drive accounts for wheel circumference.

2240 rpm on the cable for 60mph

Full list here
https://web.archive.org/web/20250324054306/https://mostynindustries.com.au/speedo-tacho-ratios

Regards
Dave

[Bryanj]

Not sure Ken, there used to be a supercision list and i suspect its on there

[honda-san]

Yes, 500 and 750 both have the same 19" front wheel and drive plate, so hard to imagine why their drive gearboxes have different part numbers. Having checked via both Fowlers and DSS web site, none of the part numbers supersede to any of the others - suggesting there must be a difference.

I too had pondered setting a wheel up, and attaching a degree disc to the square cable tang and making measurements. Just wondered if I had missed something obvious that would enable easy visual identification. 

[Oddjob]

Maybe a vernier gauge would show any difference in the size of the housing. Maybe so slight but it’s difficult to see by the naked eye

[Bryanj]

Only reason i can think of is if the angle of cable outlet is different, cant remember if 750 has wider tyre than 500, know rear is

[K2-K6]

They both being 19" front (750 & 500)  but are they same profile/cross section  ? 

The 750 could utilise a larger volume, along with associated higher weight capacity from a change in that specification.

[honda-san]

Same width tyre on 500 and 750. I did measure using a digital caliper - no difference dimensionally.

[Oddjob]

The only thing I can suggest is to compare the amount of teeth on the gears.