this was taken from : Sheldon Brown: Bicycle Frame/Hub Spacing
Spreading the Frame
There are a number of ways to do spread a frame. Probably the easiest way is to use a lever. A piece of 2 x 3 or 2 x 4 lumber, roughly 5-6 feet long works well for this:
1.Remove the wheels, fenders and any seat-tube mounted bottle cage.
2.Lay the bicycle on its side with the handlebars turned to face upward.
3.Insert the lumber through the rear triangle, so that it goes underneath the upper rear fork end, and above the seat tube. The lumber should extend out past the rear end of the frame.
4.Place the far end of the lumber onto a chair, crate or other raised structure, so that only the head-tube/fork area of the bike is in contact with the floor.
5.Press down gently on the lumber where it crosses on top of the seat tube.
6.Measure the spacing to see if it has changed.
7.If the spacing hasn't changed, try again, pressing a little bit harder. Repeat until you get a result, applying a bit more force each time, until the spacing has increased by about half the total amount you are seeking.
8.Turn the bike over, and repeat for the other side.
In the illustrations below, the stays will be bent to the left (up in the photos) by pressing down on the end of the 2 x 4.
The other end of the 2 x 4 is on top of a stool. The longer the 2 x 4, the less force is needed.
The 2 x 4 is under the left fork end,
widening the spacing by pulling the left side upward The 2 x 4 is under the right fork end,
narrowing the spacing by pulling the right side upward
The bike does not actually need to be stripped down this far, most parts can remain in place.