Originally Posted by
DannoXYZ
mconlonx's statement may incorporate a large margin of error. If you sample actual bikes in the field, it's probably closer to 99.9% of them are within +/-0.05mm parallel and don't need facing.
The benefits of going from 99.9% to 99.99% of perfect is not going to be noticeable for the majority of bike-parts consumers.
Well, not so much engineers as the corporate attourneys and their CYA policies. Very few shops have the jigs and tools to accurately measure the squareness and parallelism on the faces of BB-shells and head-tubes (unless they have a resident framebuilder working in the back room). Not to mention the demographics and middle of the bell-curve of bike-shop employees. Very doubtful there's many MEs in the mix. So the safe thing to write in your installation-manuals is to say, "Always face BB-shells and headtubes prior to installing our product.".
It might be more accurate to say 99.9% of cyclists wouldn't be able to tell the difference. Which if the cstoners that cone through out doors are any example - isn't exactly a complement. Most of them apparently also can't tell when a pedal arm is loose, teeth are missing on a cassette or when brake pads are worn down to the metal.
None of which changes that's its good practice. The assumption that bicycles are carefully assembled in some factory by conscientious employees is pretty much a fantacy unless you're talking about bikes worth many thousands of dollars. Just another reason I like to build my own stuff up from scratch.