kraftwerk 

'90's Metallic Brown Trek 990 , building into a 700c utility / gravel / commuter wonder, it's nice.
Just needs a b/b ! The shell is 74mm and all the b/b's I have are for road-bikes or track-bikes and all those axels are too short..
What size -brand fits > ?

Iride01 

Take the current BB out and you'll probably see the spindle length written on the cartridge. Along with stuff to what the threading is. Which is likely BSA 1.37" x 24tpi.

Then you just need to eliminate the choices that don't match the spindle interface to your crank arms. IE: Square taper, octalink v1, v2, ISSIS and etc.

Probably a deore crank isn't it? What is the part number of them located on the inboard side of the crank arm around the hole for the pedals?

bboy314

The shell width is 73mm. Match the bb to whatever crankset you’re using. If keeping the current crankset, you can just pull out the bb and measure it.

kraftwerk 

There is no BB in the frame...it
was built-up from a bare frame.
A Shimano was there 20 yrs ago...i saved only the cups for some reason..
Thread size: BC 1.37 68

Iride01 

I still bet it's a BSA threaded BB. Your choice of BSA threaded BB's are huge. And the correct one will depend highly on what crankset you intend to put in it.

If you can measure the internal diameter of the threads in the shell, then they'll probably be 1.32" - 1.33" give or take 60 thousands of a inch or so. That will tend to confirm them as BSA 1.37" x 24 tpi. Since it's from the 90's, I doubt you have to worry about Trek putting a French threaded BB shell on it as that would have similar dimensions.

kraftwerk 

Just need to know the proper the axel-length, is it 127 ?
Do axels come in 127 ?
Will probably use a square-taper crank, Shimano 600 or so..
or even a SRAM-splined crank if i can locate a proper BB

Trav1s

Vintage Trek shows this info for the 990
89, black with magenta decals, also white with mint decals
90, black with white splash with white decals
91, black and conch with black decals on white and silver
92, red with black decals
95, ice root beer with sage decals
96 dry ice titanium with black outline decals

This link will take you to service info for the model year: https://www.vintage-trek.com/trek-fi...ein-lemond.htm

1995 Technical manual: https://www.vintage-trek.com/Trek-Fi...manualTrek.pdf

Pages 22-27 in the manual cover the 900 series steel frame bikes and desired info is on page 27:

BB-UN71 73mm X 113mm

I've attached screen snips of page 27 with all relevant service information for your 990. As Adrian Monk would say, "You'll thank me later."

bboy314

The axle length is determined by the crankset, not the frame. So pick a crank first, then find the appropriate BB, made for a 73mm English threaded shell.

bboy314

This info is useful only if you intend to use the bike’s original crankset.

Camilo

Like others have said, simply choose a crank and get the BB specified for that crank.

Trav1s

As someone relatively new to the bike repairs, I read the original poster's request for information as an attempt to understand a baseline using original components as a reference. With that as a starting point, I would use it to explore options that are close to original without straying too far from the baseline. I'm also still trying to figure out the difference between the mtb 73mm and road/hybrid 68mm wonder if the Kraftwerks is in a similar place.

bboy314

The difference is simple - it’s the width at the frame where the bottom bracket threads in. So with square taper BBs, you might have a 68/122 or a 73/122, with both having the same length spindle but the center sleeve being made for one or the other width at the frame.

With external bearing BBs, there is usually a non-structural sleeve between the cups that is sized to fit either width, but can often be removed or cut down as needed, and the spindle (usually affixed to one crank arm) just needs to be long enough for the frame’s shell. Many of these will work with either shell size with the proper spacers.

As for the OP, from their description it sounds like they’ll be changing the crankset to something pretty different from original spec so the new BB will likely be a different size, with only the shell width being the pertinent measurement.

wheelreason

No need, that is 100% a BSA BB shell. Like others have mentioned, the axle length will depend on the cranks you'll be using, be it single, double or triple.

Iride01 

Spindle length will set chain line of the cranks. However chain line is controlled by the rear dropout spacing and the number of sprockets on the rear.

So taking that stuff about the rear will help the OP make a better choice about what cranks and what BB.. Otherwise the chain line of the front could be off and not give good shifting on one side of the rear cassette.

I'd actually lean toward the two piece cranks. They take a lot of the what if's out of the equation. Though I haven't looked to see what's offered for mountain bikes with their wider BB shell and the likelihood that this old frame from BITD is probably the same rear spacing as most road bikes were till disc brakes became popular. 130 mm.

kraftwerk 

Thanks to all who responded, good read! Also, thanks Trav1s for posting the original spec sheet (!) love that kind of archival information.
I can tell you, OX-3 triple-butted heat-treated chrome-molly is very light, strong and springy.
990 has a bit of soul, that is why I have held onto it for so long finally building it up.
The paint hue does make me want to drink a root beer.
The issue with the first crank I tried was just an huge gaff on my part: the BB was for a 68 & not a 72 shell,
so one crank arm struck the chain stay.. have a 72 BB on the way, it will be an appropriate one, hopefully.
If I saw this a bit sooner I would've gotten a BB UN -71. they are around still... again, that spec-sheet has great information.

I will post a photo when the bike is finished, some of you may get a kick out of it.