Our Sportster project is sitting here all pretty with the stock rear section still intact. Since we’re doing a couple hardtail sections on some Evo Sportsters, we decided to fab up a motor dummy fixture that will maintain the stock motor mount locations while we cut off the rear section to make room for the new Led Sled hardtail going in its place. On most frame jobs of this nature, the motor itself would be used as a fixture, but our motor’s being prepped for the 1200 kit while we do the hardtail.
Before bolting in the motor dummy fixture, we took some measurements from a piece of drill rod fastened to the motor mount of the Led Sled hardtail so that it protrudes past the top plane of the backbone. The drill rod gives us a nice fixed point to measure the backbone length where the slug begins on the hardtail so we can transfer that measurement to the stock frame for cutting. The lower frame rails on the hardtail are measured from the lower rear motor mount to the slug beginnings and that length is also transferred to the stock frame. We leave 1/8-inch more tubing on the stock frame cut lines so we can dial in the fit precisely. Better to cut too long than to cut too short. Bolt in the motor dummy nice and snug, fire up the Porta-Band, Sawzall, or cutoff wheel and have at it. Have someone pull on the frame when you get close to cutting through the tubing to keep your tool from binding.
Once the stock rear section is cut off, we test fit the new hardtail and see what fine tuning is required to achieve the desired fit. Once we’re happy with the fit, we remove the hardtail, drill at least three staggered plug weld holes on each section of tubing on the front frame section where the hardtail slugs slide in, then de-burr and chamfer all tube ends before welding. Re-install the hardtail, bolt to the motor dummy securely, and fire up the welder to plug weld in all drilled plug holes. After those have cooled we TIG up the chamfered seams where the new hardtail meets the stock front section. I usually weld one-quarter of the circumference of the tubing at a time, moving symmetrically so the welds pull equally while cooling. That could be complete overkill, but at this point it’s a habit. Let it cool, pull the motor dummy out of the newly welded frame, and start bolting on your chopper goodies for mock-up.
You can see part 1 of this feature at Barnstorm Cycles/SPCL'79 Sporty Project Part 1
SPCL’79 and Barnstorm Cycles