MBZ Driveshaft Characteristics
 

The first time I ever removed an MBZ driveshaft, the 46mm nut on the end of the front shaft was loose but the rubber boot had kept it from ever coming off completely. Since then whenever I have had occasion to repeat this job, I always install the nut as per MBZ, with very little torque. After all, the two shafts need to be able to slide freely on the spline inside the nut. Lately I have been wondering why is this nut even there? Any ideas?

The nut is there to prevent them from sliding since there is obviously no need or it.

The nut is there to keep the slots in the drive shaft from "opening" up and then allowing slippage between the 2 splines.

I have considered this, but then I wonder why have these slots to begin with? Why not just make the female spline without the slots? (Good sliding, no opening of slots, and no jumping of the spline teeth).

The purpose of the slots in the female splines, the taper on the OD of the female portion, and the nut (which has an internal matching taper), is to LOCK the two splined sections together. It should be obvious that the shaft is splined to provide, 1. an initial length adjustment to allow for manufacturing variances, and, 2. a means of reducing the length of the shaft for service work.

The flex discs incorporate enough play to absorb longitudinal movement of the engine/transmission, and the final drive center section. This method is in contrast to the deliberate slip joint method employed with live axles.
The MB splined joint is NOT a slip joint.

I had thought the same yesterday but didn't get a chance to post.

And, locking the two splines together eliminates torsional slop. Given the rear axle is independent, plunging of the spline does not occur.

BMW also uses Guibo / Giubo joints but I don't know how they are setup.

These upgraded discs look interesting, but unless you are making big power the stock discs will make for a smoother ride. ( they are for BMW but I bet they would make some for MB if we asked )

revshift ( dot ) com/shop/bmw/e30/flex-disc-couplers.php

I´m not sure this is the case. Keep in mind that the flex discs are attached to either yoke by 6 bolts connecting through steel spacers. The rubber part only surrounds the bolts, so there wouldn´t appear to be any longitudinal movement possible (not the amount a driveshaft spine might encounter) - only minute side to side movement. These are strange beasts that we love.

Each opposing flange uses 3 bolts from one direction and 3 from the other. This does allow for longitudinal movement of the joint.

Have a look at a " Lovejoy " coupling commonly used in industrial apps. While it does not have any bolts, the principle is similar and power flow the same.