One of my bikes is a 2006 Trek and I am looking at replacing the BB and crank (as well as some other stuff).

My question is the size or type. I have looked everywhere, including the manual, and can't find any info on them.

What, where, or how do I find this information so I know what to get for replacement.

thanks

You can measure what you've currently got and/or look for size markings.
If you can figure out what model Trek you have, bikepedia.com normally lists at least some measurements.
You can take the bike into a local shop, and they should be able to not only figure that out, but give you good advice on what to get as a replacement, and order it for you.

have you tried here?:
https://www.trekbikes.com/us/en/bikes/2006/archive/

which model trek do you have?

2006 would mean the bike is most likely to have either ISIS, Octalink or external.

First, what's wrong with your current crank that you want to change? What do you want to achieve by changing it?

Second, what model and type of Trek do you have; road bike, MTB, hybrid, cyclocross, etc.? What is the current crank and gearing?

More details as to what you have and what you want to do are needed.

It is a Trek 1500 (road).

It has a 105 triple Octolink but never use it (third or small chainring) and since I have a couple other bikes I thought I would use this to learn a bit more about bike repair/replacement of parts.

I would like to put a double on it, but I do know that I have to replace some of the other components as well.

Have tried the shimano and trek websites but cant seem to find it. Should I just remove them and measure?

Your bike is English threaded with a 68 mm wide bottom bracket shell. That is a VERY common type and by far the most available configuration so finding replacement parts will be easy. Remember, the drive side cup of the bottom bracket is LEFT-HAND threaded when you go to remove the old one or install a new one.

Your current 105 triple Octalink has a 118.5 mm spindle. A double Octalink crank will require a 109.5 mm spindle. Since road Octalinks only came in those two lengths, the choice is pretty easy. 105 Octalinks (BB-5500) are still readily available in both lengths. You should be able to find compatible NOS 9-speed era 105 (FC-5500) or Ultegra (FC-6500) double cranks if you want to keep the Octalink type.

Your bike will also take any of the newer external bearing bottom brackets and cranks such as the current Hollowtech II 105 (FC5600) or Ultegra (FC-6600 or FC-6700) in either double or triple form. Since the spindle is built into these crank arms, there is no choice of length to make. These are "10-speed" cranks but will work ok with a 9-speed chain and cassette.

Many thanks for the information.

There's no reason to stick with octalink cranks - there are more modern cranks that you may to look into that take external bottom brackets. More variety, more selection. Octalink is shimano cranks, that's it.

Sugino made some Octalink cranks for a few years but they were never really sold in the US in great numbers.

I believe Cannondale sold some house-brand cranks with Octalink bottom brackets some years ago. Maybe these were just private labled by Suguino.

I know ISIS was developed as a patent-beater since Shimano initially wouldn't license the Octalink design. Maybe they relented later.

To the OP: Operator is correct that you aren't limited to Octalink. Your frame will accept any style bottom bracket from cup-and-cone and cartridge square taper to the latest external cup bottom bracket designs with their matching cranks.

I would advise avoiding any ISIS bb/crank setup. Many of these were poorly made and unreliable and good ones are hard to find.

I have a similar question regarding BB/crank size and compatibility with my bike.

Five years ago I replaced the BB/crank on my 2004 Giant Rainier. It currently runs a Nashbar ISIS drive 48/38/28 (which needs replacing) with a 68x113mm BB (which needs replacing as well). I have shopped a Shimano Acera crankset with the same specs and a Shimano BB (square taper).

The confusion is if I get the 68x113mm BB, the crankset specifies a 122.5mm spindle length. Do I need to find a crankset with a 113mm spindle length? I ask because I am assuming the current 113mm BB fits the BB cup exactly, and that a 122.5mm spindle length would be too long.

Thanks in advance.

Shimano never made an ISIS crank.

If you are replacing the crank and BB, go with the recommended BB spindle length the crank calls for. Make sure the BB is the same width (some mtn bikes have 73mm BB shells).

It is a Nashbar crank - sorry if I didn't mention that. I see what you mean. The cranks call for a 122.5mm spindle length, equating to a (68mm)x122.5mm BB. This is my first attempt at replacing the front end of my drivetrain - tools are next.

I am just now getting around to buying some replacement parts and have come across a pretty good deal on a new Ultrega Double Hollowtech II with bottom bracket. So, if I replace current 105 Octolink with a Hollowtech, the 116mm spindle length on the Ultegra wont be a problem. Would I foresee any future issues with using a "10-speed" crank with a 9-speed cassette? Noise, Dropped chain, etc?

thanks

Shouldn't be any problems with that setup.