Joined: Feb 2012
Posts: 19,381
Likes: 5,528
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
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.