Facing BBs for HTII

Never understood why it makes a difference. The BB has a lot of thread to go in before the cup touches the frame so by that point it is not going to go anything but straight is it as it is not done up that tightly.

Or, I have never done on any of the 10+ frames I have used with HTII and never had any issues...

Once upon a time it seemed to be a ‘must do’ to prolong HTII life, but now I’m not so sure.

I was never that convinced it ever needed doing beyond taking stray paint of the machined surface. It always seemed to me to be done by OCD shop mechanics because - the tool to do it exists. 

When it is torqued up the threaded section stretches to create the preload. This is when the flatness of the face comes into play as it will determine the angle the cap sits at and how even the stretch is.

Personally i would get it done as you never know where in the tolerance band a frame will lie or if it is ever checked during the manufacturing process. If you have future BB problems at least it rules out as a potential problem.

In general, threads make poor locating features - there's a small tolerance between a nut and a bolt to allow the nut to able threaded onto the bolt.

Another feature is usually used e.g. shoulder of a shaft, dowel pins. Using a facing tool ensures that both faces are parallel, as the BB is tightened down it should align against the face. Having said that, I can't imagine it making a big difference as long as the paint's removed.

The BB has a lot of thread to go in before the cup touches the frame so by that point it is not going to go anything but straight is it as it is not done up that tightly.

The two things aren't linked. Once the bearing cup touches a highpoint on the shell and you continue tightening (to around 40-50Nm for most types) the cup will deform and you'll end up with cockeyed bearings. TBH, even if the threads aren't parallel or concentric, parallel and concentric shell ends will pull everything into line.

Though, you'd have to be buying a really shoddy frame to have *enough* misalignment to cause an issue.

My thoughts are:

Remove any paint from the BB shell end faces, install the cups and then push the spindle through. If the spindle slides through without much resistance, then the bearings are nicely lined up. If it's tight going through the second bearing, then maybe investigate further.

The threads might not be in straight, the shell might have distorted during welding, there may be excess paint on parts of the face. To ensure the mating face of the shell is parallel to the mating face of the bearing retainer I would suggest facing is essential? It’s a one off expense and could extend bearing life. I used to carry it out for free when replacing bottom bracket bearings and then keep a frame number record to avoid duplication!

The two things aren’t linked. Once the bearing cup touches a highpoint on the shell and you continue tightening (to around 40-50Nm for most types) the cup will deform and you’ll end up with cockeyed bearings.

After over 10 years of using them in many different frames (none of which were faced) with no issue I am in the whatever camp on this one.

I have always wanted to know, and I may struggle to explain this....

The BB shell is faced to itself by clamping forces.
It is not faced in line with the driveline / chain.
Therefore facing would make the face flat, but not necessarily straighten a wonkily welded BB.
I recognise a flat face is good for bearing life, if of course the BB cups flex/stretch leading to highpoints.
The bearing though still could be under odd loading if the BB is not aligned to the chain line - which of course moves....

So does facing make *that* much difference?

No, facing won’t straighten a shell that is welded/bonded wonky. Facing, however, will make both ends of the “tube” parallel with each other. Some shells are distorted slightly- a bit like a squashed toilet roll inner - I believe facing helps bearing longevity, some don’t. What is certain is that it doesn’t hurt and may help!

The drive line alignment is all over the place depending where you are up / down a rear cassette anyway so a slightly skewed BB shell isn't going to add much to those existing bearing forces. Facing is just to get the bearing clamp faces parallel to each other which can help with bearing durability. I've got a facing tool so do generally use it with separate cups (not so important with one piece cartridges). But we ride in a swamp so always find ht2 BBs die from water and dirt ingress way before the bearings wear out from pedalling.....

BBs and headset cups will almost always follow the alignment of the face not bore. 

Brazing head tubes gives me lots of fun. Brazing too near the ends can ovalise or flare (some people leave the tube long and cut down later to reduce flaring). And all the heat concentrated on the back face of the head tube at TT and DT joints can pull the ht into a banana shape.

BB shells are thicker and shorter so usually less of a problem.

The nub of it is that external BBs are a crock, not really designed for the general tolerances of high volume frame manufacture, but they had to come up for a reason to blame ham-fisted shop mechanics for a bearing life of weeks when we were happy with UN52s and the like with a life in decades.  You can at least these days get external bearing BBs that cope with some degree of axial misalignment and provide better sealing than the original ones.

As a shop mechanic I never once encountered a problem due to BB thread alignment - if there was a lump of paint or resin on the shell face, you’d just pass a file over it.

I hate external BBs, but in fairness old school adjustable cup and cone BBs had the same problem with face alignment.

UNxx was the biggest bearing durability revelation in bike history. Unfortunately now dumped by Shimano for the junk UN300 design 🙁

Screw in the BB until it touches down then use a feeler gauge to check for gaps, some frames need it some don't, sometimes it's obviously needed if you can get your fingernail in in one area but not the other or you can see a gap on one side by shining a torch from the underside.
I bought a Cyclus facing tool years ago when they were going cheap, I've used it on several frames most of which definitely got material shaved off on one section before touching down on the entire face to make it perpendicular. My last Mega frame didn't need it though once I checked with a feeler gauge.

If it was a high end frame, I wouldn't, but given how cheap a Ragley frame is, clearly costs are cut in the manufacturing process and tolerances and quality checks are probably not as high, therefore to avoid potential issues, I would.

Ragley Trig frames have an RRP of £650, that's not cheap! They've just been on sale.

Given current market you could probably buy a fair few BBs for not much and less than cost of getting faced. I’ve never thought of facing and BBs often last longer than frames  given my frequent changing or N+ing. 

Dovebiker, when you've driven 2 hours to a DH area, case a big double on the first run and snap your UN52 BB in half, you want a more sturdy solution 😉

The plastic spacers that Shimano use are purposely plastic so they deform slightly to minimize the need for facing. 

Hmm, it's a 'maybe' then.

If the BB screws in easily and the threads are nice and clean perhaps I'll see how I get on without facing, but on occasions I have found frames with grubby threads that need chasing, so we'll see.

The plastic spacers that Shimano use are purposely plastic so they deform slightly to minimize the need for facing.

Which was my next question!

Which BB are you fitting? Shimano road BBs like the BBR60 don't use plastic spacers.

FWIW I've never had to face any frame I've personally owned. I've only done a couple while building bikes professionally (certified bike mechanic blah, blah, blah). The worst alignment I've ever seen was on a Lynskey-built Ti frame that we just sent back. 

I face mine. I have the tool to do it and it’s a quick job.

Which BB are you fitting? Shimano road BBs like the BBR60 don’t use plastic spacers.

It's a Tiagra (part of a GRX400 groupset) and I had noticed that the online instructions don't mention spacers (although the BB is stated as suitable for 68mm & 70mm shells). I assume that it just goes in as it is regardless of shell size (the Trig's 68mm).

I suppose that this might make it more susceptible to an unfaced shell.

How much does a BB cost and how much does facing cost? If the BB is ££££ and the facing is £ then it makes sense to face so you don't screw the BB or bearings. If its the other way round then don't bother.

I've never done it on any of the frames I've owned and never found BB bearing wear to be excesive.