When I had to repair sand and grit damage in a NR headset I found it very hard to find lower bearing replacement parts that were not brinelled already. I finally did, but there were quite a few made that are now not usable.
I can't agree about too loose being ok at all. The reason to have a bunch of balls is for them to share the load pressure. They will only do this if for both bearings the upper and lower races (or cups and cones) are aligned in plane well-nigh perfectly. If the bearing is loose, then when weight is added, and certainly with transient loads like bumps, the steer tube will move slightly in the head tube. This small misalignment tends to focus pressure on a few balls and their contact points. It also creates impact contact between ball and race. This can be worse than constant contact with varying pressure.
Not only should they be aligned right to eliminate motion, but even with an SP frame there is some miniscule elasticity, which allows the steer tube to flex with bumps. This flex also momentarily takes the bearing pairs out of plane. With just the right amount of preload, contact can be maintained all around the circle of bearing balls much more consistently. I don't have any specs about a preload torque, but I tend to tighten NR headsets until the play disappears, then a touch more, barring any feel of roughness or drag.
I don't think impact is the cause of brinelling. I think, and believe I read in one of Zinn's books, that it's caused by long-term use of a bearing that contains dirt and has gone dry. Dry bearings don't always feel rough. Hardened grease that has lost its suspended oil might have been pushed away from the ball track, resulting in the balls running essentially metal to metal. I think this constant grinding is what damages race surfaces.
On Mon, Nov 23, 2009 at 2:37 AM, Kurt Sperry <haxixe@gmail.com> wrote:
> I have a 1970 bike that I bought used in the early eighties with
> obviously quite a few miles with what I am reasonably certain is its
> original C in a diamond NR headset. For around ten years that bike
> was my only transport and I rode it everywhere piling up many, many
> thousands of miles including a fair bit of off road duty. It's still
> my daily rider and I've had the headset apart I think twice in those
> twenty odd years. All the parts including the bearings are the same
> ones I bought it with and it still feels fine.
>
> I'm left wondering what the hell you have to do to damage or wear out
> a properly adjusted NR headset because it seems damn near impossible
> to me in one lifetime.
>
> Kurt Sperry
> Bellingham, Washington
> USA
> _______________________________________________
>
--
Ken Freeman
Ann Arbor, MI USA