I bought a nice quality old road frame from approximately 1980, so it likely originally had an old-style non-cartridge BB with a quality installation. I am installing an external-bearing BB/crank (SRAM S300/GXP BB, to be specific), and would like to verify whether the BB has been faced or not.

According to the Park Tool website, during the process of facing, "When cut metal appears in a complete circle, facing is finished." I do see bare metal all around the circumference of the BB shell presently-- does that mean the shell is already faced? Should I verify by measuring distance between the sides of the shell with a calipers in different spots?

You can't tell just by the lack of paint and calipers aren't really going to tell you enough as it's not just shell width but parallelism that counts. One way to check is to install the external bearing cups moderately tight and try to slide the crank spindle through them. If it lines up with both cups with no wiggling or forcing, you are good. If it requires twisting or force to get the spindle through the second cup, facing is needed.

If he frame has been built up already, odds are it's been faced. You can also tell by the character of the face. Concentric circular tool marks (tree rings) mean it's been faced.

Once a shell isfaced, there's never a need to face again except to possibly shave raised burrs and nicks. (recesses don't matter).

BTW- contrary to the info on Park's site. There's no need to face down to the full circle. Once it's faced 2/3rds or so around, or to where there are three faced areas forming a tripod, the cup or lock ring will be supported square to the axis, and can span the recesses. This issue rarely comes up, but over the years I've seen the ill effects of the I want to get this perfect mentality where BB are faced to where they're too narrow (short?). The record that I know of is 63mm. Believe me, an 68mm BB is far preferable to a perfect 63mm one.

Using a razor scrape paint if it is thick , install cups.Ride

At what stage in the process of removing 5mm of BB does one start to think "maybe I've gone too far?"

+1 This is a great explanation the issue isn't rather the BB has been faced so much as rather will the BB and spindle align correctly. On a high quality frame it will the majority of time. In the rare case it doesn't the BB will need to have some work done at better shop that does work on vintage frames.

It's an example of tunnel vision. The person who did this was having problems with chatter and the scalloped face it causes. He was so frustrated dealing with it he stopped thinking about how much metal he was removing. Noticed the problem only when installing the BB assembly. BTW- this is not the only time I've seen overly faced BBs (under 68mm or so), it's just the record (that I know of).

Folks hear so much about facing a BB shell, that they often lose sight of the need to stay at the 68mm width.

Often I'm presented a BB that looks like it might be faced and is already at 68mm. If I have a reason to worry about whether the face is square, I test it with a disc covered with fine emery, that replaces the cuter on my facing tool. In 2 turns I can have good confirmation of the face without removing any metal or paint to speak of.

While this and using a file on each face can insure a uniform face, it won't account for parallelism with the opposite face. For an internal cartridge BB this can all that's required.

Brad

I believe the razor blade suggestion was based on a BB that was faced before painting (common in production bikes) where one may (but shouldn't be) concerned over paint thickness.
Otherwise, very few people are good enough with a file to get a flat face by hand, and as pointed out there's no assurance of parallelism or axiality.

I can relate to that.

The short answer to the OPs question is to simply take a caliper and measure it at multiple places.

Consider the difference between a rectangle and parallelogram.

you can get a pretty good impression of the face's square by using marker or, ideally, partially dried marking blue and the actual cups, by coloring the BB shell faces and quickly hand-tightening the cups into the frame.

If the cup only rubs off a small part of the dye when you screw it in, there's an obvious high spot and potential problem. Whereas a properly faced, square BB shell will contact the cup quite evenly all around and the dye/marker will rub off the frame and transfer onto the cup along a large part of the circumference.

The best answer that the home mechanic can achieve with common tools is a multy step one.

Remove any paint on the shell's face. i like to first cut the corner between the face and the shell's outside surface to keep any paint chipping from coming off the outer surface. Then take a flat file, having fine single cut teeth, and gently file across the BB face working the file around. This is not to really remove much of the face but to bring out any high or low points of the face. Assuming the face is evenly scuffed with the file, use a square (I like a small 6' blade one) and place the body against the BB shell's face and check the blade along the shell's surface. Do this at a few rotational points. This will indicate to you whether that face is perpendicular to the shell. Repeat on the other face. Then measure at a few rotational points around the shell's faces for the width of the shell.

If the square's blade is not parallel to the shell's outer surface evenly at the few points of measurement then you need to file down the high sectors of the face so that after re scuffing with that big flat file the square's blade is parallel to the shell. Repeat for the other side. All the while keeping in mind the overall width.

BITD and with cartridge tapered square BB units the shell width has some tolerance in width. The LH lock ring or retaining sleeve (bearing cup) just threads in or out a touch more or less. But the more current external bearinged units have less range of acceptable shell width (another loss that's not mentioned amongst the hype). So more care will be needed here.

Lastly do not confuse the shell's faces with the threaded internals. The threading can be not quite coaxial and yet the faces can be square to the shell's outsides and also parallel to each other. With BB units that have flanges or lock rings this is not a big difference. But for a BB unit like the classic Phil Woods which use retaining rings that don't have any BB face interfacing elements it's the threading that you want right. This is why a proper thread chaser and facing tool is really the best choice. Andy.

IME even expensive frames may not have the BB faced. My last bike at £1500 had not had the BB faced and it had a thick layer of paint. While I was lucky enough to be able to do this myself, I would have been outraged if I had to pay for this to be done.

At the danger of being seen as an old Luddite, I have to say that it is now my practice, after experiencing the lack of durability with external BBs, to exchange these as soon as practically possible to a chain-set with a traditional cartridge, despite the small amount of weight this adds.

I recall $20 was what I paid the Trek shop a year or so ago as my LBS did not have the tool. They shaved very little, the crank slid in like on rails and I liked the peace of mind.

I don't think many DIYs worry about facing or chasing the BB shell until there is a problem with one. I think with external BBs there maybe more interest in this procedure over time.

Brad

What I have read elsewhere, which makes sense to me, is that facing was not prioritized in the cartridge BB era, from 1990-2005 or so, but prior to that it was important as BB cup position was important for bearing positioning with traditional loose-ball BBs. And my frame is from ~1980.

Alan -

Since the frame already has a quality installed old-style cup and bearing BB; first examine the cups all around to look for gaps between the fixed cup and the frame and between the lock nut and the frame. Then pop the chain off the rings and turn the cranks slowly by hand with no background noise. If all is well and quiet and smooth running, then the chance that you have a poorly prepped BB is almost zero. I wouldn't even worry about it.

/K

If one uses a large-jaw caliper then that'll show if it's parallel or not.

Again do not confuse faces that are parallel with faces that also are square with the shell. Just like drop outs that are parallel but drop out tools won't be concentric. Andy.