Just thought I'd post an interesting tolerance issue that I (er, my clever mechanic friend) discovered.
I recently converted my '83 R100 into an R100RT with a fairing I bought on ibmwr.org. It was a sizable job and now all together running like a top, but not without some challenges.
I had cleaned the steeringhead bearings in a heated ultrasonic bath (without removing the lower one from the stem/triple tree. Clean and shiny. Before installing the triple clamp assembly, I had installed the fairing spider frame mount, which bolts into the steeringhead tube on the frame with two specialized bolts (not 1.25 threads, finer 1.0) that I bought from the local BMW dealer. I also used a wavy washer on each as I saw in the diagrams. I then installed the triple clamp assembly, forks, etc. and onto the wiring harness mods needed for the fairing conversion.
Well, as it turns out the lower steeringhead bearing wasn't fully seated as my friend discovered. So, he checked seated it and checked the steeringhead bearing play...it was super stiff, even with the wheel off the ground. Even with the steeringhead nut backed way out.
He pulled it all apart again and discovered two notches in the steeringhead stem. they were there when I was initially assembling it, because I hadn't put the spider frame on at that point. It turns out that the fairing spider mount frame bolts need a pair of 2mm washers, not wavy washers. Without them, the bolt ends were interfering with the steeringhead stem. A problem he had never seen in all his years working at BMW and on his own.
Rides, steering and feels like a new bike now. Just thought I'd share this for anyone else considering a fairing project. The same would/will apply to an RS fairing conversion (my spring project).