BB Facing - how necessary?

markshaw77

I have recently picked up a second hand Specialized Allez Comp frame (2003 I think) for a build project I have got going and I want to fit external Hollowtech II BB cups.

The BB threads are in good condition and the shell is standard 68mm width, so the cups screw in with no problems, but the faces still have the original covering of paint on them.

I am working on the basis that, as the paint on one side in particular is very uneven, the cups will not sit parallel and I am going to get some issues in a pretty short time.

So, is it worth getting the BB shell properly faced by the LBS (and what will it cost) or if I just take a razor blade to the offending paint, is the underlying shell itself likely to be good enough?

Thought on a postcard please.....

rake

i understand why the surfaces should be parallel. but i dont understand why they wouldnt do this as a matter of course when a frame is built because at some stage its likely to have a bb fitted. maybe they do? i took the less hassle route and its been fine for a couple thousand miles. thats on my winter bike though.

salsarider79

You could take a razar blade to the paint but facing is about getting the metal to line up. Yes the paint may be too thick on one side but proper facing is best. I charged about £20 when I worked in the last bike shop, and that was facing both sides of the BB, and also chasing the threads to ensure that everything was perfect, 'Down There'.

It's worth doing if you are planning a tasty build.

toontra

Personally I would get it faced if using external BB. Twice I've had new bikes squeak and creak badly from the bottom bracket with external BB's fitted. Both times this was solved by getting the shell refaced by the LBS. One of these was a ti frame, so it's not just paint that's the problem. The metal itself has to be level, otherwise you may end up getting those really annoying noises, and it's far easier to do before the bike is built up.

APIII

I'd get it faced. It'll save you any potential problems/expense later on.

Ben6899

Get it faced. I would suggest that salsarider79's £20 is a little pricey compared to what I have paid for old stubborn BB out-shell faced and chased-new cups/crank fitted.

But then again, I live in the impoverished North.

andy_wrx

A 2003 frame might well have previously been fitted with a cartridge BB.

Being a single sealed unit, that wouldn't be too bothered if the faces were parallel, but if you're fitting external it's a different matter.

will3

I don't understand why the bearings on HTII are not self aligning to allow for a small degree of misalignment. Any engineer worth his salt would design it like this.

Smokin Joe

will3 wrote:

I don't understand why the bearings on HTII are not self aligning to allow for a small degree of misalignment. Any engineer worth his salt would design it like this.

+2

You can get aftermarket self-aligning bearings but it is very poor service from the manufacturers that this isn't a standard feature. It's hardly rocket science and adds next to nothing to production costs.

John.T

will3 wrote:

I don't understand why the bearings on HTII are not self aligning to allow for a small degree of misalignment. Any engineer worth his salt would design it like this.

Cost. You can get them at a price.
It is only since the introduction of outboard bearings that this has mattered so BB shells were not faced in the past.

hopper1

I'm not against getting this done, but... I don't really follow...
If you screw a BB cup into the threaded shell, it'll come to a halt when it meets resistance, caused by steel, carbon, Ti, or paint.
But, regardless of what material stops it from going further, it'll still be true, with regard to the BB shell threads.
The threads are so fine that the cup couldn't 'cock' in the thread.
So what does facing actually achieve?

Ben6899

Yes, the cup will be true to the edge of the shell, but will each cup be totally paralell to the other cup? If the shell isn't perfectly square then the cups won't be. No matter how smart each one is installed.

hopper1

If the shells are threaded 'square', then surely the cups 'should' be square, too! Regardless of what stops them.

salsarider79

Ben6899: The £20 did include chasing the threads, so the BB was as good as it was going to be.

It was pricy, but have you seen the price of the tools you need to do the job???!!!!

Ben6899

hopper1 wrote:

If the shells are threaded 'square', then surely the cups 'should' be square, too! Regardless of what stops them.

Forget the threads for a minute... we're concerned whether the faces of the shell are parallel. If they're anlge toward each other, then no matter how square they are threaded the cups will not be parallel to each other.

salsarider79 wrote:

Ben6899: The £20 did include chasing the threads, so the BB was as good as it was going to be.

It was pricy, but have you seen the price of the tools you need to do the job???!!!!

I have indeed seen the tools. £20 is only pricey compared to the great deal I landed!

will3

Ben6899 wrote:

hopper1 wrote:

If the shells are threaded 'square', then surely the cups 'should' be square, too! Regardless of what stops them.

Forget the threads for a minute... we're concerned whether the faces of the shell are parallel. If they're anlge toward each other, then no matter how square they are threaded the cups will not be parallel to each other.

Actually that's not the whole story. The bearings have to be not only parralel, but also concentric. Quite a task given the shell is machined from either end and the concentricity of the two bearings is governed by a thread.

Ben6899

True.

Anyway, we can be sure that it needs doing for most external BBs.

Avoneer

I'm with hopper1 on this.

Even if a small fraction of the inner face of the BB where it meets the frame doesn't quiet touch, you're telling me that's it's gonna flex the right angle metal shell of the BB at that small point?

Or if you tighten it up onto an un-flat frame, that the BB metal cup will distort itself slightly?

Wouldn't both BB cups be parallel however tight they are done up or even left loose as long as they weren't cross threaded.

Just my thoughts,

Pat...

toontra

Avoneer wrote:

Even if a small fraction of the inner face of the BB where it meets the frame doesn't quiet touch, you're telling me that's it's gonna flex the right angle metal shell of the BB at that small point?

Speaking from bitter experience, the answer to that is "yes". We're only talking about thousandths of an inch here, but under load it's enough for a bottom bracket cup to creak against the shell. I've "proved" this twice so far - both times the facing sorted the problem for good.

It's nothing to do with the angles of the cups as such - that is almost entirely determined by the thread as has been said, so it's not a major mis-allignement issue. It's the fact that the whole surface of the cup isn't in contact with the shell. Grit and debris gets into the gaps and causes the creaking and ticking that is so bloody annoying on an otherwise perfectly tuned bike.

Avoneer

Ahhh, so they do/can flex slightly.

I get it now.

Pat...

ScaldedCat

My lbs are facing my bb and fitting a chainset and headset to a new frame for me for approx £40 so just the facing on its own would be less I suspect.

bill57

Surely this just raises the question of what kind of progress outboard bearings actually represent?

Avoneer

Can of worms I'm led to believe - but they look damn good.

Pat...

John.T

bill57 wrote:

Surely this just raises the question of what kind of progress outboard bearings actually represent?

Lighter and stiffer. Just as reliable if set up properly. IE :- Shell faced. It only needs doing once.

simpo1961

amazing the amount of flex you can get. Saw a demo, guy put a micrometer in a cylinder bore then twisted the block by hand, the micrometer fell out. Strong guy!

nicklouse

Every frame i put together gets a chase and face even if it is to confirm that it was not needed.

if a bike comes in with a short life BB the BB tools come out and normally you can see that the shell had not been faced.

re self aligning bearings sure as an aftermarket "upgrade".

anyway for those that dont know what chasing and facing involves some pics.

taken from the MTB FAQs and tips

Untouched and ready to start.



then chase the threads.



remove the handle to allow the facing tool to be fitted.



Fit the facing cutter.



then face the BB faces.



Not fully done



Done



Other side



remove tool and clean any swarf out.



you are now ready to fit your external BB cups.



Job done.

dbb

nice tutorial.

every home mechanic should have these tools in their kit. ;-)

mididoctors

hopper1 wrote:

If the shells are threaded 'square', then surely the cups 'should' be square, too! Regardless of what stops them.

this is correct

you could still get creaking thou

John.T

mididoctors wrote:

hopper1 wrote:

If the shells are threaded 'square', then surely the cups 'should' be square, too! Regardless of what stops them.

this is correct
you could still get creaking thou

Not quite correct. If the faces are not also square then the cups will distort when tightened up taking the bearings and seals with them. It needs very little to do this.
Nicklouse's 5th picture shows just how far a frame can be out. In spite of removing metal from 3/4 of the shell it still has paint on the last bit. How bad was it to start with?.