Bicycle Mechanics - Converting 1 piece to 3 piece

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.

I may well be wrong on this, but I thought that 1 piece cranks used a larger diameter BB (and thus, BB shell)... might have trouble converting to three piece in that case.

Harris Cyclery sells conversion kits to let you use a modern bottom bracket/crank in a frame made for one-piece (Ashtabula) cranks. Look here and scroll down a bit to "Conversion Kits" find them:

http://www.sheldonbrown.com/harris/opc.html

Heh. I should have known Harris Cyclery would have something like that ;)

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.

I have a similar adaptor rings that are DMR brand.

Also have a ladies Raleigh MTB that has the special axle that fits in to the american BB to fit the 3 peice cranks.

So if I use the conversion peice that is here http://www.sheldonbrown.com/harris/opc.html, along with this http://www.ebikestop.com/eastern_euro_bb_set_for_20mm_spindles-CR7199.php, I can use the 3 piece crank from eastern that my buddy donated to the cause, or am I over simplifing this?

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:

http://www.jensonusa.com/store/imgWrapper.aspx?img=/bb/BB407A02ENG__68___122.jpg&type=3

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.

So if I use the conversion peice that is here http://www.sheldonbrown.com/harris/opc.html, along with this http://www.ebikestop.com/eastern_euro_bb_set_for_20mm_spindles-CR7199.php, I can use the 3 piece crank from eastern that my buddy donated to the cause, or am I over simplifing this?
No, you are over-complicating things.
You just need an American BB for 20mm spindles.
http://www.ebikestop.com/eastern_american_bb_set_for_20mm_spindles-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.
-Bill

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.