In general, there are two schools of thought on modifying ANY motorcycle's spinal region: a resounding "NO...Good God, Don't" from worrisome riders, and chop-aholics that posit a "My tank WILL rest on top of the frame, so let me die in a crumpled mess from a broken weld or stress fracture". This post is meant to "school" the second school, so the inevitable isn't so.
Step 1: Buy or Turn Bungs and Set Up the Welder
Mild steel bungs 3/4" OD, 1" length, threaded 3/8-16
They are cheap from Lowbrow Customs. http://www.lowbrowcustoms.com/index.php?l=product_detail&p=1368
Buy 3 in case you lose one or, God forbid, burn through the threads.
Welder Set Up:
Set to 200 amps. You wont be burning nearly this hot but it helps to have the pedal half way depressed for moderating your puddle burn. So in this case we weld at about 100 amps or less. Post flow set at 10. No pulse. Half Penetrate.
Note: Weld using TIG for mild steel in DC-. In this case we are Welding Cast Steel with Nickel rod so we needed to "ball up" the end of the electrode to facilitate the even burn of the Nickel filler rod. So the AC setting was only used at the very beginning of the process.
Step 2: Heat and Weld (a Tack)
Its a good idea to preheat the cast steel. Otherwise, you could burn through the mild steel without burning the cast (this is why you purchase more bungs than you need).
Simply heat the surface of the steel to around 100-150 degrees, using a propane torch.
After testing on a 10 ga. steel plate, get ready to jump into! You must rely on your welding skills or the power pf positvie thinking, both of which will see you through til the end (though a combination of both is advised).
Don't worry...at this stage you are merely tacking. Get a good puddle and fill with your 3/16" Nickel fill rod.
Now, if your tack looks like mine... take a celebratory swig of something and continue AFTER it has cooled for about 3 minutes. This is important. Let welds cool enough to avoid burning the cast out and prevent the formation of martensite in the heat-affected zone.
Step 3: Alternate Welding
In this step, you recognize that the steel bung and cast neck into which the bung is buried have adeqautely welded together and you continue tacking. This is tacking, not welding, techincally, I guess. By tacking in series, alternating sides, you create a friendly environment for the two new buddies to co-habitate. In other words, you are the cool chaperone at the junior prom.
I tacked both bungs at their respective holes (haha) simultaneously. By this, I mean I tacked one, then the other, then let that one cool by tacking the other, and so on. Alternating between the two regions of the spine (cast neck and lower backbone, near seat mount) provided enough time in between tacks to create the "happy heat zone". Cue the awkward slow dance jam, please.
Step 4: Finish and Polish
I forgot to mention an important step in the dance; the brush off. Don't forget to brush your welds soon after the tack, leaving them nice and shiny. Keeps the current flowing nice, steady and clean.
And that's it, basically. Remember to go slow and don'r rush the tacks, but dont let them cool entirely, either (as in, you finished your adult beverage mid way through and went to the firdge for another: resist, my friend).
Celebrate the fact that you have a strong will and even stronger motorcycle spine! Now Frisco mount that tank with stainless hardware and LEATHER OR RUBBER WASHERS IN BETWEEN THE BUNG AND THE TANK-MOUNTED TABS. If you hard-mount, the tabs on tank with vibrate and break off, leaving you balancing your fuel bucket between your legs for the next 45 miles of South Dakota badland.