C110 close ratio 4th gear

On paper the standard C110 gearbox is pretty close, nicer then any standard OHC gearbox, but while riding I felt the step from 3rd to 4th was a bit tall, so for the racebike I was wondering if anything could be done about that easily.

I have a lot of ratio's on paper of different gearboxes, so looking at OHC models, which have the same distance shaft to shaft and a similar 4speed system I found another gearbox with a shorter 4th gear that might just work in a C110.
I had a look through my leftovers of various gearboxes and came up with an SL70 4th gear what would be my best bet.
An SS50 4th gear could also be used, but then I would have to remove more material from the 3rd gear for it to work, and I didn't want to take off too much.

Seen here the original C110 gearbox with the parts to modify taken off, and on the bottem the new 4th gear, which is too wide to fit in a C110.


To make the SL70 primary 4th gear work, I had to take off a little bit from the 3rd gear cogs so they would slot into 4th. So it went on the lathe with the anglegrinder attachment.


After that, it's time to narrow the SL70 primairy 4th gear, again, grinding it on the lathe.

Same was done the the secundairy SL70 gear, and after that a lot of cleanup to get the lathe free of all the grinding dust, including taking the chuck apart and cleaning the insides.

C110 gearbox, with close ratio 4th gear upgrade kit (in the middle) :


And a quick pic mounted in the engine.


Now I really want to finish the engine and get the rolling chassis done so far that I can feel what a difference this 4th gear makes. Theoretically it should make the step from 3rd to 4th about 40% smaller.