Bottom bracket replacement
 

I have a late 70s/early 80s Miyata 610 with a pitted square taper bottom bracket spindle. Instead of searching all over for a replacement spindle, I figured I would just replace it with a cartridge.

The spindle has the following marks on it: 3S 83-D
Bottom bracket shell is 68mm. The crank is an SR Custom triple.
it is hard to measure accurately but it looks like the spindle from tip to tip is approximately 124mm.
To the best of my abilities, One end is 35mm, the drive side 37.5mm and the middle is 61mm based on the measurement points in Sheldon's site below.

Could anyone kindly let me know which cartridge I would need and what threading it might be? Much appreciated.

Ive found this on Sheldon's site: http://sheldonbrown.com/bbsize.html

suggesting the 3S bb has a symmetrical value of 127mm. That being said, the 121.5 in the table for 3S is off the mark for my current spindle.
I've found this: http://www.ebay.com/itm/Shimano-BB-U...item46142d41a1

127mm x 68 Eng thrd.

But this is only a start. I'll look at the stock set u before i take things apart. How much clearance is there between the inner ring and the chain stay? Can this clearance be less? Is the shell really 68 wide? How well does the ft der shift, is it at a limit in either direction? These factors might suggest a slightly shorter or longer drive side crank placement. Also the older 3S spindles were not symmetrical, the LH side was shorter then the driveside (by about 2.5mm). So as most current cartridge units are symmetrical the LH side crank arm might end up further away from the frame. Not a big deal for most. In the end a test fit is the only way to now for sure the best replacement. This is why many will let their LBS do this, assuming the LBS is sensitive to these details. Andy.

I agree that the bottom bracket shell is almost certainly 68 mm wide and English threaded but, as recommended, confirm this before buying anything. Shimano makes cartridge bottom brackets in 122.5 and 127 mm spindle lengths and one of these should work but, again, a test fit is essential.

Thank you both. Unfortunately I am not able to determine how the crank sits without reassembling the bike as it is currently in 67000 pieces. From what I can see the FD limit screws are fine and I don't see any chainring rub on the stays. I'll call up the lbs tomorrow to see what they've got in terms of 68mm bb.

what with the relatively small cost of a decent quality Shimano BB. this is one instance, where i take the bike to the LBS, play dumb (or am just objective :)) , and let the mechanic tell me what i need. then i buy it from them. if they don't have it i will usually order it from them while there.

in the real world, i have enough spare square taper BBs on the shelf or on bikes that can easily be removed to just try one and make a reasonable guess as to how long it needs to be. then i'll order it online and have it within a week.

Sheldon says a 3-S has a symmetrical equivalent of 127mm.
Sheldon Brown's Bottom Bracket Size Database

I used my Sutherland's manual for my conclusion. Andy.