I have a 1981 Schwinn Super Sport. Last year I respoked both rims and while I was at it replaced the hubs (dynamo{front**, 8 speed cassette{rear**). Now I am in need of replacing the bottom bracket (It is pitted and worn). Right now I have on the bike the original SR Apex triple crankset (170mm with 34/46/52 chainrings) and the spindle measures are A=34mm B=51.5mm C=42mm. What would be suitable modern replacement?

Anything really. There are lots of external bottom bracket cranks out there that would do the trick and be much easier to install and work on then the old square taper crank with adjustable bearings you have now. Shimano...damn them...has the best system I've seen for installation. Dead simple and highly effective. This Tiagra cost $100 with the bottom bracket. A new square taper with a new bottom bracket would cost about the same.

Do you want to replace the bottom bracket or the entire crankset? Your title says one thing, your text another. Bottom brackets are cheap and easy to replace and there is no need to replace your cranks and chain rings if they are not worn or damaged.

The OP should have explained what those were - https://sheldonbrown.com/bbsize.html



In addition to that, given that the sum of his measurements is 127.5 I suspect this is a spindle that has a stud on the end and a nut that fit on that, and that he measured that portion as well, which is incorrect.

Not necessarily. There are nutted-type spindles with 127.5mm overall length, excluding the threaded portion:



Source: Sutherland's 4th Edition

True, and I thought that was possible, but thought it best to get correct measurement confirmed. I somehow missed the 127.5 spec when I checked Sheldon's page.

The measurements do not include the threaded portion.

There is a UN 26 in 127.5 [my Koga WTR uses that , R'off chainline, M730 Crank,
or original Stronglight Mygal
chainring on inside of crank spider .. disc chainguard on the Outside.:]

so perhaps that will be a replacement, if retaining crankarms.

as I said Shimano current 'fit and forget' cartridge BB is offered in 127.5

There is also a UN 54 in 127.5. I had to use one on my Trek 7000 MTB to get the non-drive side crank arm clear of the chainstay.

According to Sheldon wouldn't I need to take into account the offset that exists in my original spindle?

No. It wouldn't. Lat time I checked bikes look about the same now as they did then.

The 2mm or so offset on older SR and Sugino stuff is just fine you just need to add a couple of mm when ordering BB if you intend to keep the same crankset.

This where you have an advantage by bringing in the BB you have, Fixed cup and all, down to the shop.
and 1st hand, hold it and the new one both in your hands.. then you will Know.

You can get a nice FSA Exo triple with external bearing bottom bracket from Nashbar for $89 vice the $229 MSRP. It is a beauty and the BB looks very strong.

It's the latest stuff, but the bearings are out there in the gritty wheel spray too.

OP could get a new 50t, chainring, already have the 34t, then its the same Gears
as the compact double, so popular now, then might not need so long a BB axle..

I would expect the 127mm UN26/54/55 to work -- it's basically symmetrical, but would only move the drive-side crank inward by about 2.5mm. (A bottom bracket spacer, available in many widths, could be used to scootch the whole thing to the right if you don't have that much extra clearance.)

If you like the gearing of the SR triple, and it's in good enough shape, I'd keep rocking it, personally.

Many thanks for the info provided as well as the advice. I am going try a Shimano UN 55 with a spacer and keep my crankset.