Hans and I are talking about pretty much the same thing here:
The complete distributor drive assembly mechanism originates down in the base of the block in a gear mesh with the crankshaft. [cutaway drawing in Tech Manual] When considered as a complete assembly (all the way up through the distributor) it must, like all shafts, have some provision for end play. The attractive solution is to contain it at each end with suitable surfaces to bear the force and friction, then add The Spring somewhere along the assembly's length where convenient. Someone decides how much tension -- compression, actually -- is appropriate to keep the shaft constituents from banging back and forth along their length like a train in the switchyard, and without over-stressing the thrust washers and wafers that are in all the Bosch distributors I've ever had apart. An absent or non-functional spring should allow a little variation ("wander") in the spark timing, but probably very hard for most of us mortals to detect. This would be the result of the gear mesh at the crank end being cut an an angle (for good reasons not germaine to this topic) so distributor drive shaft end play, if undamped by the spring, would advance and retard the spark with each lengthwise movement of the shaft. This rotation effect shows up clearly when removing or installing the distributor drive shaft: it turns maybe 15-20 degrees as it meshes with or withdraws from the drive gear. The much smaller endplay here would give a much smaller degree of spark variation with the distributor in place but no functioning spring to contain endplay oscillations. Someone like The Engineer could tell you more accurately how much to expect. But even then it would move only as the engine changed back and forth from acceleration to deceleration and the reverse, in which case any such effect would be lost in the greater advance and retard of the timing mechanisms. But there would be thousands upon thousands of small, sharp impacts all along that shaft assembly as the miles added up, stealing energy and eventually leading to enough component damage for that assembly to fail. In fairness (read: charity) to your 'know-nothing' sources, this could take awhile, and in that sense they're right. And, on the other hand, you'd hate the results when it broke.
So it seems to come down to just good engineering practice to design any given shaft with proper provision for endplay, because good engineering yields longevity...and the converse definitely applies.
Or, as a practical matter it's less hard on the distributor if the forces on it from the distributor drive shaft are mediated by that spring. Simple, eh?
Did I mention that I sometimes miss The Engineer's postings? Bring him back to us, Michael, or at least flip him a salute and say that he is missed and wished very well here. Denny
