DannytheLuddite wrote:Hi busaste
Thanks for once more coming up with a detailed and helpful answer. After reading it, I went back and measured the BB spindle and, sure enough, the RH side projects 28.5mm but the LH side projects only 25.5mm. Maybe that is why I also have an iffy right knee...
So I am now thinking that if possible I ought to leave the existing BB alone - there is after all nothing wrong with it. My quest was for an 7/8 speed double chainset that (a) fits on a square taper spindle and (b) has a small chainring with no more than 40 teeth and (c) doesn't look completely out of place on a 1970s bike. The one I bought, a Shimano Sora 3300, met those criteria but is hopelessly out of line on a 122mm spindle. It has to be a 7 or 8 speed chainset because I'm now running a 7-speed freewheel and matching chain, and on two or three occasions the chain has dropped between the two chainrings and jammed there - not good. Plus the chainrings are worn, and generally don't go very well with a new narrow chain. Perhaps I'll just keep watching on eBay and see if anything better comes up.
If you want to know how the story ends, I eventually found a 108mm Klein chro-moly spindle on eBay. (I wanted 110mm, but I reckoned 1mm less each side was not going to make much difference.) I used a big hammer to knock out the old spindle and bearings, and a small hammer to tap in the replacements. The new bearings are lovely and smooth and, with the new chainset fitted, the chainline looks just about right.
When I removed the old spindle, I was surprised to see an F W Evans price sticker on it. So they must have replaced the spindle when I had them replace the bearings back in the early 1980s - I can't think why. It had circlip grooves but no circlips.
Now I just need to get my knees sorted so I can ride it again