The black on the threads , how would these things move ? If they were loose surely it would be noticed whilst riding ? The idrive shell is made of some form of alloy , could the aluminium be have some reaction with the metal of the idrive , the only noise I got from the idrive , which was s click happened when the dogbone worked itself loose , anyway , to be fair , there shouldnt be anything on the threads bar the grease that was applied at the time of fitting , i may try some PTFE tape when I refit them , give em a while and then check them out
They move through two mechanisms:
1) Precession. Because the crank is a very uneven power source it places large unbalanced loads on the BB shell. Imagine for a minute that the BB shell (that's the bicycle part of the BB) is made of rubber. You can then visualize how the shell ovalises in the direction where the crank points as the crank pulls the centre of the circle around 360 degrees. Although the shell is not made from rubber, metal is nevertheless elastic and that actually happens, sans the exaggeration I forced to demonstrate the point. This is the force that causes the one side to spontaneously unscrew if it wasn't a reverse or left hand thread.
2) Dynamic joint. A dynamic joint is something that can move because it is literally not welded. Take a bolt and nut and play with it. Notice how the nut can move against the bolt even though it is screwed halfway in. Now, if you back that nut up against the workpiece, it appears as if the nut cannot wobble on its axis anymore but that's an illusion. If the workpiece moves, the nut wobbles with it. A screw of the type you see on your BB is exactly the same. Although it backs up against a flange, it can never be perfectly stable. With pedaling, it moves. And when it moves under pressure such as when you torque it up, the two parts fret against each other and metal particles break loose. That's the black rouge you see in your grease.
The external BB is poorly designed to counter these two forces described above. Because it comprises two shallow cups on the same axle (crank), they can move independently and don't support each other. An internal BB does. Further, think of a car wheelnut with its tapered flange. That design is perfectly stable and doesn't move. We know that because you don't need reverse threads on the one side. Had he BB cups been designed with tapered shoulders we would not have had the problem. Further, if the bearings inside were like car wheel bearings, angular contact bearings, the bearings would have lasted longer.
Hollowtech is not perfect but it is OK in screw-in versions. With press fit it is a disaster. Screw-ins can be removed and "serviced" (i.e. thread cleaned and regreased). Pressfit can't. Also, Shimano has started to sell single cups. This means you can buy left ones separately and replace only those when they fail. If you buy in bulk for a workshop, you'll know that the fail ratio is about three to one. Our workshop used to buy then separately (many won't because there's more money to be made by denying that they are available separately) and our ratio was three (left) to one (right) without fail. That doesn't mean everyone's personal ration will be three to one, that was the average over a large sample.
PTFE tape silences it for a while but that eventually wears off. The best treatment is regular removal and cleaning and refitting with copper compound instead of grease.
