Originally Posted by
plonz
Wow that is some great information. I especially like your comments on
#6 . If I read this correct, as long as the chainring clears the stay by 5mm and doesn't extend past the derailleur movement, there is a bit of wiggle room in chainring and Q-factor allowances.
My current conundrum is with a late(er) model SR Suntour Superbe crankset that I picked up from forum member @
guygadois. He pointed me in the direction of a Shimano UN55 but does not know the spindle length. I have also emailed SR Suntour and will try calling them as well. In the meantime I was asking to see if there was a way for me to just figure this out on my own. I do have a couple of spare BBs so I can assemble one and start measuring from there.
Here is the sellers pic of the item. They look a lot like vintage Superbe cranks but I'm not assuming same dimensions. Frame shell width is 68mm. Thanks for the help!
HISTORY LESSON... 
From an article
"Sundown for Suntour" by Frank Berto:
http://pages.citebite.com/o2n1u6u4w3qui
"In late 1989, Mori Industries Inc., a Japanese steel tubing company, bought Sakae Ringyo Ltd. ...In mid-1990, Mori bought Maeda SunTour. ...Shortly afterwards, Mori combined the two acquisitions. The new company was named SR SunTour."
Around 1991 SunTour changed from a Japanese company based in Osaka to a Taiwan company.
"...Mori decided to shut down their bicycle component business in March, 1995. Daisuke Kobayashi and Hideo Hashizume, the former owners of SR Sakae Ringyo, arranged a management buyout. ...They bought the SunTour name and the SR factory in Taiwan."
The current SR Suntour is related to the 2 Japanese companies in name only.
I did some searching for SR Suntour BBs. Apparently the company is primarily producing suspension forks and modern cranks that use a different style BB than the square taper standard. Their website says that the square taper cranks that they do make fit "square taper spindles" with no other information.
Good luck getting help from
them....
They look like beefy cranks. It looks like a lot of those SR Suntour Superbe cranks have been hitting the market for the past 5-6 years with no reference to matching BBs!
BTW, the early Suntour cranks were made by Sugino...
Back to basics...
I'd start off finding a BB spindle or cartridge that properly fits in the crankarm taper:
Next, measure the size of the small end of the taper and the length of the taper. That's a good starting point.
Then, work out the spindle length.
A bike co-op or similar operation might be a good place to look for spindles.
The Shimano UN55 cartridge BBs are very good (the old UN72 BBs were better, the UN26 BBs have plastic left side cups). It looks like Shimano started making the UN55 BBs again at their Singapore operation. The last time I checked they were available with British threads in a wide range of widths. I bought a bunch of them for ~$25.00 last year.
Another suggestion would be the Campagnolo Veloce cartridge BBs. They are/were available with British or Italian threads in 111mm and 116mm spindle lengths. I bought a bunch of them too, for ~$15 to $25 USD.
They were made for Campy Veloce, Mirage, Xenon and Centaur Cranks. I've used them with Campy Victory and Triomphe cranks plus I have them on several bikes with late 1980's Suntour Superbe Pro cranks.
The Veloce BBs were made in 3 styles:
AC-H - Aluminum Cups with hollow steel spindle
AC-S - Aluminum Cups with solid steel spindle
SC-S - Steel Cups with solid steel spindle
The ultimate solution is a Phil Wood BB. You have about about 5mm of wiggle room on the drive side.
Good luck...
verktyg
Chas.