I have a 1987 Panasonic dx1000 that I am converting to a single speed. I was planning on replacing the bottom bracket. To get the measurements I measured the tube to be 70mm (2 3/4") and the spindle to be 125mm (4 7/8"). When looking for replacements I cannot find this size. Would it be better to go with a 68x122.5 mm or a 68x127.5 mm? It was a double cog and removed one so that it's only a single. Any suggestions?
I have just rebuilt a 1990 Raleigh bike (exact model unknown) and tried several bottom brackets to find the best. This turned out to be the one that brings the smallest chainring (in your case only chainring) closest to the chainstay without touching it. The one I've ended up with is 117.5mm whereas the original was 122.5mm. The front derailleur works perfectly (not applicable for your setup) and the chainline looks great, albeit aimed at the larger cassette ratios rather than the smaller ones. My logic is that when on the larger sprockets I am putting a lot of pressure on the pedals, so a straight chainline is best for chain strength and wear. When in the smaller sprockets I am usually on the flat or downhill, so a bit of chain angle is not such an issue. Thus, I would go for the shortest BB you can get away with to keep the chainline close to the chainstay, and minimise flexing forces on the BB stubs (are they called that?)
The goal here is to make the chainline of the new SS setup correct while avoiding frame clearance issues with the crank/ring. Depending on what you're doing in back, you may also have some ability to configure Q-factor to your liking.
Using your existing bottom bracket and the crank with whatever ring you're going to use, measure what chainline it gives you and where the tight spots with the frame might be and how much room you have to work with there. Also ascertain that that BB is putting the cranks symmetrical in the frame. Then measure the existing spindle's length and its offset if applicable. You now have all the numbers you need to do the math for what spindle length will yield a given chainline. (As well as what spacer configuration is needed if you're going to be compensating for a crank that wants an offset BB.) Then just figure out what your rear chainline is going to be. If your rear hub setup is one where you have choices here, you're deciding it based on what Q-factor you want within the constraints of frame clearance.