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I've been welding 4130 race care chassis for nearly 25 years now and you should tack and weld it with TIG method. You can tack with ER70S6 if you must but it just causes you to have that tack in the middle of the weld that looks bad. For TIG use either ER70S2 or S6... Most people still swear by S2 but the mill spec will allow S6 to be used anywhere S2 is called for and if it's good enough for mill spec I think the race car world can allow it.. The original wire was Oxweld 65 if my memory serves me correct.. Anyway I like S6 a lot better because the higher silicon makes everything flow out a little nicer. Be sure and use a foot control and don't over heat the tubing because it makes it brittle. Also 1/16 wire is the choice size for this. ..
I agree with Gunner12R on choice of filler......
also...there are quite a few old threads to be searched... it is worth your while to read them... after all it is your neck in the balance......
I would NOT use 309 filler for his application... this is an obsolete practice that was followed in the mistaken notion that the SS filler would make the 4130 welds more impact resistant and durable... I used to follow that practice myself until I was shown the error of my ways... It was common practice in race cars during the 1950's and 1960's... and also btw by the people that taught me (in 1968).... but testing and experience taught me better..............
I would also seek out the tech inspector for your local sanctioning body for guidance..... after all he/she will grade your work and either pass or reject it...... so better not to stumble on issues that they consider important....
BTW ... Here is a link to an AWS article regarding 4130 fillers....
Lincoln ac/dc 225
MM200 black face
Whitney 30 ton hydraulic punch
Lown 1/8x 36" power roller
Arco roto-phase model M
Vectrax 7x12 band saw
Miller spectrum 875
30a spoolgun w/wc-24
Have you checked the rules from the association yet?
Drag racing is as I read in the US governed by the Hot Rod Association.
I know that they are the only association that state CrMo frames must be TIG welded but am unsure if the rules go further stating post welding heat treating and so forth.
Check fabrication rules first.
After all someones life may depend on your welding practise and your understanding of the rules.