This on going argument over 4130 is apples and oranges. Rob you need to re-read the statements you made at the beginning about quenching 4130 in water...........I build stuff for the DLA so I know a little bit about heat treating lots of different materials (everything we do requires,certs for materials and certs on all testing and certs on all processing). Building parts for aircraft and other mission specific items, have all been thoroughly tested and engineered. (they know what they want).
Welding on race cars, dune buggies and other things made of 4130 or other similar alloys is another matter entirely. ZT is an excellent welder and has a good knowlege of what he does. As far as his style of welding with a mig, only mag-particle testing, xray, or pull testing would verify whether the strength of his welds for a given application is appropriate. I can't imagine any testing body not passing his tig welding on any level....it is perfection. I have only seen one other example of tig welding that would come close, and it came from the Nissan GTP program out of Calif. I knew a guy who worked there (Pat Foster) the former and now deceased funny car driver. He said the guy was a freak........took him 8 hrs to set up and weld something, but it was perfect. Anyway.........most production motorcycles look to be mig welded together. If they are 4130 and they don't get stress relieved after welding there's a good chance on ocassion they would be subject to cracking. This case involves an exhaust pipe mount that has broken off.......imagine that, the engine that turns up in the trees has caused a bracket to break off. The fix is simple, you could mig it, tig it, braze it or gas weld it, how long will that last, who really knows. To suggest that you have to "field" heat treat 4130 after welding it would be a wild assed guess and could certainly cause more problems.
Also, you can zap the ECU if you are not careful on ground location and disconnect the battery. I also have unplugged the computer just to be safe. 
