I have picked up a Trek 600 frame and fork and am building up a bike from parts I have lying around. I am modernizing the bike and putting on 8-Speed STI shifters and the associated cogs, wheels, derailleurs, etc. I have a Shimano 2300 square taper crank that I am going to use but I am having trouble figuring out what length spindle I need to go with.

The bottom bracket is a 68mm English thread so I have that nailed. I believe the original spindle was either a 113mm or a 117mm in length. I have a feeling since the crank is a Double I should go with the 113mm spindle.

Any comments are appreciated. Thanks.

Spindle length selection is only slightly easier than a Voodoo ritual. The place to start is to find (if possible) the maker's recommended spindle length for that crank, but even then, some bikes have more or less chain stay clearance so you often wish you'd gone a few mm longer shorter.

Unless your stays re wider than average where the chainring passes, try the 113mm first and cross your fingers, and don't complain if it works out that you could have gone with a 110mm. Also keep in mind that there's more than just chain stay clearance involved. You also want to end up with a chainline close to the centerline of your cassette (or slightly outboard).