Helisplining

The kickstart shaft is hardened so its best the lever splines are softer so they suffer as its easier to change. Have you opened up the gap on the lever that the bolt reduces, if its large enough it will allow more torque to be applied to the bolt and the lever hole reduce enough so whats left of the splines grip.

Another thing that can cause problems is: There are a lot of clamping bolts around which have too short threads. When tightened, to their maximum, the kickstart lever is still not tightly clamped to the shaft. A thick washer, under the bolt head, will often solve the problem ( was this a "cunning plan', by Norton, to sell more replacements?!!).

Getting a spline to contract using a pinch bolt is a flawed design. If you could find a Triumph kickstart that would clear the silencer, then machining the spindle to fit with a cotter pin would be a better solution. The folding T160 kickstart might be a good start point. Your stock of knackered kickstarts could be welded up and machined to do this. The minor problem is that the lever will only fit in one position, dictated by where you put the flat on the spindle.

Its a common problem, filing away a few splines either side of the slot may help, finding a longer HT bolt and a nut and washer to get it too tight to shift is the best plan, also file away any bolt threads that not in use to avoid damage to the shaft spline.

Mine seems to work loose so a good blob of Loctite on the bolt thread and the splines, extends the tight zone, clean it all with carb cleaner first to let the Loctite stick.

I would be interested to know how they used to press the splines in the lever - probably a fancy broach tool with a lot of cutting teeth.

There is no need for the splines in the lever to be hard for the fixing to work.

In a home machine shop you could slowly cut the splines, one at a time with a simple vee tool, into a sleeve using a rotary table on a press. Then loctite that sleeve into the bored out lever.