Borbin the 🐱

The 3rd gear gets pressed on first. Note the angular position of the gear as it locks into the later 4th gear.
I built this press just for this. I could have bought myself a 20t press, but it takes two much space, and I rarely need it. It was also a lot of fun building it!

The two round holders at the top of the press are designed to accommodate a round bar for increased leverage.

With an oversize of 3/100mm there is still a lot of force required for this. The gears are secured with Loctite 638.

Note the angular position of the 3rd gear to the position of the key.

Once the 3rd gear is aligned (described in Align 3rd and 4th gear), the 4th gear along with the key is pressed on.

Old and new input shaft side by side.

My CJ750 motorcycle has a BMW R100 engine, and driving with 100km/h (60mph) on the freeway requires almost 5000rpm. This has to be changed.

The gearbox for the CJ750 (and the M72) has the following ratios:

With a tire circumference of 26 inch and a final drive ratio of 37/8, the rpms require to drive 100km/h (60mph) in 4th gear for all available gear ratios 26/20 (1.30), 25/21 (1.19), 24/22 (1.09) and 23/23 (1.00) are:

4050rpm is good for 100km/h with a sidecar, so the 24/22 for the 4th gear is selected.
The change from 1.3 to 1.09 is actually the same as using a high speed final drive with a 35/9 ratio instead (1.09 37 / 8 9 / 35 = 1.3). I'm using an EU quality final drive with a 37/8 ratio to avoid the russian and chinese junk, so thats why I need the adjustment in the gearbox.
To only change the 4th gear to 24/22, the ratio changes would be 1.57 - 1.34 - 1.42

The change ratio decreases with increasing gear number for all gearboxes I checked.
For example, one BMW gearbox had 1.51 - 1.37 - 1.25:
1st: 3.90 - 1.51 - 2nd: 2.58 - 1.37 - 3rd: 1.88 - 1.25 - 4th: 1.5

But 1.57 - 1.34 - 1.42 (with only the 4th gear changed) is not monoton decreasing.
The only possible replacement for the 3rd is a 28/18 from the DNEPR MT804. Fortunately, this gets a ratio change of 1.48, and the sequence is now monoton decreasing. And it almost matches the ratio if a high speed drive would be used (1.71 35 / 9 8 / 37 = 1.437).

So the new 3rd gear is a 28/18.

But the CJ750 input gear shaft is designed have only the 4th gear changed. This requires that the 3rd gear needs to be removed and the input shaft modified so that the new 3rd/4th gear can be pressed on.

The inner diameter of all output shaft gears are 26mm with a 10.5mm tooth width and only need a new spacing. Leszek from oldtimergarage.eu has verified the measurements for this unique project, and now it’s all set to launch! 🚀

First, the gearbox needs to be disassembled.

Exploded CJ750 Gearbox, Type 52:

Gears on the upper output shaft are all free spinning and can be replaced easily, while the lower input shaft has only the 4th gear pressed on. The 1st, 2nd and 3rd are fixed and part of the input shaft structure.

Project plan

For the upper shaft, new spacers will be created to align the new (free spinning) gears, and for the lower input shaft, the 4th gear is pulled, the 3rd gear removed and the shaft machined for the new gear set to be pressed on.

To get started, the 4th gear needs to be pulled off the input shaft. This is done with a standard gear puller. But what was not that standard, was the force required to get this actually off.

I ended up putting the puller’s screw in the vise, and used a long bar to rotate the gear around it for extra leverage. The deformation of the rather solid plate give a glimpse of the applied forces. Once this was finished, I was done for the week with any physical activities.

With the 4th gear off, the next step was to fill the gap between the 3rd and the 4th gear.

I used an older input shaft to try the brazing. Brazing involves applying just enough heat to melt the filler material without affecting the base material, allowing capillary action to bond the parts together.
I tried welding, but it put way too much heat in the part that it would change the metallurgical properties and the hardening of the part.
Using a TIG welder, brazing with Silicon-Bronze rods was actually quite easy.

The input shaft used for the project.
I used water to keep the material temperature low.

The gap is all filled with Silicon-Bronze.

The 3rd gear was removed with a grinder.

The input shaft had a severe runout at the bearing. To fix this, I machined the front part and created a bronze bushing. The bushing for the bearing is pressed on in a machine vise and secured using Loctite 638 (green colored glue).

The shaft is machined to its final dimensions.

The old and the new input shaft.

The new 4th gear is secured with a key, while the 3rd gear locks into the 4th gear.
A standardized key is not wide enough for the slot and is padded with two small metal parts.

As a common misconception the sealing surface is often polished for minimal wear, but for oil seals this has the opposite effect. The sealing surface needs to have a certain roughness to support an oil film. If the surface is too smooth, the oil film cannot be established and the seal runs dry, causing the parts to heat up and finally destroy the seal.

If on the other side the roughness is too large, the seal gets worn out quickly, resulting in the same fate.

And this is what happend to my gearbox seal. For quite some time the coupler on the gearbox side was hot even after a short drive. The diameter of the sealing surface of the coupler is 36.5mm and the oil seal is TC36x48x8. This tight fit also contributed, but the sealing surface apparently was indeed too smooth for the seal when the sealing surface was done with 400 grit sandpaper.

The perfect roughness is Ra 0,2-0,8.
But how to get to this?

A 220 grit sandpaper gets to about a roughness of Ra 0,5 on metal. A few passes with 220 grit sandpaper around the sealing surface resulted in this, which still felt very smooth.
And with a new TC37x48x7, the seal is not too tight anymore (37mm instead of 36mm).
Back to service!

To remove a tire, use a lot of soap, tire spoons and rim protectors.
The trick is to squeeze the tire on one side with clamps to have more room on the other side. This is for removing and installing.

To install a tire, inflate the tube slightly and put it into the tire. Adding the tube later is difficult and can result in a twisted tube.
Add soap.

Use a tire pusher tool to push the tire over the rim of the wheel.

Using a tire pusher tool makes the process very easy. Using tire spoons require some space between tire and rim and can make installation more difficult. Especially with the stiff side car tires.
A tire pusher tool does not require this space.

I built a tire pusher tool using an old axle, several pieces of plastic and a rectangular tube steel for the handle.