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Sure the 0.5mm on Scienceofficer's frame is a big chunk. The plane of the face is now aligned 0.85 degrees different to before. Now screw in the cup, does the bearing now sit 0.85deg different too, and if it does, how?
screw in the cup, does the bearing now sit 0.85deg different too, and if it does, how?
Aluminium is pliable and you underestimate the force you can get from a ~1.37"x24tpi thread that and quite fine. Deforming the 'cup' part will be no problem.
Remember you can snap an M6 bolt with an Allen key!
My understanding is like @walleater's, threads (V-profile threads anyhow) do not lock the two parts in perfect alignment, they are designed for the two parts to be able to wiggle about relative to each other and "find" comfortable relative positions that spread the load out more evenly than would be the case if the threads were a perfect fit. Which is good where a strong join is what is needed, rather than perfect alignment of the two parts.
So with non-parallel facings, tightening the cups up will result in some displacement from the ideal alignment (assuming the two ends of the bb tube are threaded co-axially in the first place), as well as causing some distortion of the cup (this could be a greater effect than the displacement of the threads). What the amount of misalignment and distortion would be, and whether it would be significant, is a matter of observation, but @Scienceofficer's experience suggests that the amount of mis-facing in their frame is sufficient to have noticeable effects. Though we await to hear their further experiences of wear with the newly-faced BB shell.
It may also make a difference whether you have Shimano style bearings with the plastic top-hats inside, or what everyone else does, which is have the crank axle sitting directly in the inner bearing race.
Aluminium is pliable and you underestimate the force you can get from a ~1.37″x24tpi thread that and quite fine. Deforming the ‘cup’ part will be no problem.
If you can get past his internet caricature, Hambini makes specific reference to this.
The gist is that with a 30mm (or DUB) in a BSA BB shell, theres insufficient space for a decent sized bearing and/or the bearing cup itself. The end result is that without plumb BB shell faces, the BB cup deforms and over-pressures the bearing which can run tight, feel rough and wear quickly. Either that, or the bearing cup flexes under cyclic pedalling load and causes fretting on the crank axles (especially soft alu axles).
This is kind of all solved by having a smaller axle like shimano, which leaves more space for bigger bearings and thicker bearing cups or a bigger BB shell like T47. To give more space for the whole BB30 arrangement.
The jury is rather obviously out on BB wear right now. Observations are that it all slid together rather more nicely without force and it does seen to spin better. This could be confirmation bias though. The truth will be measured by BB lifespan. Since I keep a maintenance schedule, it should be easy to measure any differences in durability.
Hope it works out, Scienceofficer. Looking back, I think we peaked with Octalink!
In the majority of fabricated frames, threads are cut before welding.
Machines to cut the threads after fabrication do exist - when Chas Roberts shut down there was a list of tooling for sale that included an ancient and rather scary looking threading machine. You would really struggle to cut a bb thread from scratch into a plain shell with the standard hand taps. And if you manage it, it is unlikely to be properly aligned.
There is usually enough slack in the bb threads that an unfaced / misaligned end face will skew the cup when tightened up (maybe not completely but enough to eat bearings).
Any post weld thread tapping is to remove any local distortion / gunk in the threads. The threads are not being recut to be perpendicular to the newly faced end. But it does add a bit more slack to let the faces control the final alignment.
If you are clumsy with heat, bb shells can be distorted (slightly bent like a banana - they pull to the heat and one side doesn't get heated). This leaves both ends out of parallel and both threads out of line. If not too severe, facing the ends and tapping out the threads allows enough slack for everything to align (ish).
Regardless, lots of bbs die in the UK due to water killing the bearings - either from inside the frame or tracking between the bearing and axle by capillary action.
I still prefer Shimano UNxx Square taper cartridge bbs as they solve all these alignment and sealing issues in a £20 unit. In my 32 years of mtbing they have been the single biggest advance in reliability 🙂
I still prefer Shimano UNxx Square taper cartridge bbs as they solve all these alignment and sealing issues in a £20 unit. In my 32 years of mtbing they have been the single biggest advance in reliability 🙂
The BBs were fine. It was the stretched taper in the lh crank arms that was the issue 😁