Hi guys and gals, just stopping in to ask a few questions about my new project. I have this old GT frame that was originally set up for 1 piece cranks and I wanted to upgrade to 3 piece for the weight savings and the durability. The problem is I can't find the specs for the frame anywhere and don't know what bottom bracket I need. I am assuming that I need american BBs, but just wanted to make sure the swap is doable. The frame is a GT mach 3 or something like that I believe. Thanks for any help in advance.
Rans Rockst (Retro rocket) Rans Enduro Sport (Retro racket) Hase Kettweisel Tandem (redundent recumbent), Merin Bear Valley (The gopher).
There's actually two different ways to do it.
The adapter, like the Harris one mentioned, presses into your bike's frame and allows the use of what the BMX guys call a "Euro" bottom bracket like the square taper ones commonly found on mountain bikes. It takes ordinary square taper cranks.
BMX 3-piece cranks are their own thing. They consist of two sealed bearings that press into the frame, a spindle that presses into the bearings and, of course, crank arms that clamp onto the spindle ends.
If you use the Truvativ setup from Harris, you would just use a "standard" 68mm x whatever length you need bottom bracket. An example of a "standard" square taper bottom bracket is a Shimano UN-26. It looks like this:
You can see it is a 68mm x 110mm length from the markings. The BC 1.37 x 24 is the threading. This BB is compatible with the Truvativ adapter kit.
...you would just use a "standard" 68mm x whatever length you need bottom bracket.
The specific type of bottom bracket (e.g. square taper like in the picture above, ISIS, etc.) and length of the spindle are primarily determined by the crank you choose. The 68mm designation is the width of the bottom bracket shell--or in your case, the width of the conversion piece.
No, you are over-complicating things.
You just need an American BB for 20mm spindles. http://www.ebikestop.com/eastern_ame...les-CR7798.php.
But first, double check that what you have is a crankset w/ a 20mm spindle. Most Eastern cranks I know of have a 19mm spindle.
Most BMX 3-piece cranks have/use a 19mm or 22mm spindle. If you don't already have the spindle you'll need to get one to match your cranks. Once you have the spindle sorted out you just take it into any LBS that does BMX stuff and say you need an American BB to fit this spindle. If the donated cranks are old, the hardest part will be finding the correct spindle for them.