First we have your run-of-the-mill stock CB750 frame.
Now slice those down tubes. My weapon of choice is the sawsaw, you could also use a cut off wheel. In my opinion the sawsaw is safer, cleaner, and doesn't take of as much metal. Note how I cut at a 45 degree angle downward. More about this later.
Now hack that backbone. Don't be sceered!!!
OK, heres whats going on here. This is the tubing I'm going to use to extend the two front down tubes. You will need to get as close as you can to the OD of the down tubes. The tubing I used was 10 or 20 thousands undersized so I had to enlarge it. I used a grinding stone on my dremel tool. The ideal way to do it would be on a lathe. Also note this tubing is way thicker than you would normally use, but its what I had on hand.
What I did here is drill holes on both sides of each tube...180 degrees from each other. These holes will be the plug welds.
Here are the sleeves sliding over the top of the down tubes. The were still a little undersized so I had to do quite a bike of whacking with a hammer. The high side of each of the cuts in the frame should marry up the hole drilled for the plug weld.
The old broom stick in the neck is a good way to check for straightness/alighnment with the rear wheel.
Mocking up the backbone.
Now lets cut out all the seat area. Note I have not welded anything yet. All peices should be cut and mocked up before the first bead is laid.
And here we are. Not that none of this rear area is sleeved, all the tubing is simply butt welded. I plan on cutting some 1/4" plate to gusset this entire area.
Heres the backbone welded up. This was slide up on the backbone about 3 inches. Also note the plug welds on either side of the tube.
Down tubes welded. Seam is welded as well as a plug weld at the top and bottom of each sleeve.
Finished product-one true ole' skool chopper with stretched down tubes and drop seat.
