I'm replacing the crankset and bottom bracket on my old steel frame road bike, which looks like this one, so I'm guessing it's from around 1985. I picked up a 68 mm square taper bottom bracket and found that I couldn't fit the non drive side bottom bracket cup in. The drive side is fine, but the other one can't fit in at all, even with all the other parts removed (to ensure nothing was getting in the way). The old one (from a cottered crank) fits in fine and rotates without problems, and there doesn't seem to be any deformation or damage. Do I simply have the wrong part? Is the possible the part is defective?
Here is a picture of the bottom bracket. I don't have calipers so I couldn't get a decent measurement, but the left cup seems bigger, and the pictures seems to confirm this.