These are hard to get items. I wish that that wasn’t the case because they are very important. Few discuss them.
In this case, these are 50mm x 47mm x 1mm stainless steel shims for T47 bottom brackets. I need to get a few 0.5mm as I needed one today. Broad strokes still work and in the case of the new PVD M2-F2, the shell was shimmed 3mm. With a 0.5mm shim on hand, that would have been 3.5mm.
Think of it this way, imagine that we have a 85.5mm shell and the bearings are approximately 80mm apart. It may take 3mm of shims along the spindle to take up play in the system. Fine. This is what the kids do.
Instead, if we shim the shell rather than the spindle out 3mm to take up space, we’ve increased the distance the bearings are apart by almost 4%. That’s significant in the world of marginal gains.
Here’s an old graphic that I made when comparing PF41 to PF30 that may help you understand. I miss labeled PF92 and PF86 rather than PF41/92 and PF41/86 but this was early in developing the correct nomenclature.
We’ve increased the alignment of the bearings geometrically and given them a longer lever to work against the forces.
This is the correct way to do this work.