Welding and metal questions on modified front subframe

[moparguy]

[FONT=&quot]Not at all related to our general diesel truck world but since we have a ton of experienced fab folks here I'm looking for seasoned realworld advise.[/FONT]

[FONT=&quot]The project; [/FONT]

[FONT=&quot]Widen a Mazda Miata na subframe to the max to facilitate installation of new headers on my 302 Ford powered Miata.[/FONT]

[FONT=&quot]The questions; [/FONT]

[FONT=&quot]Anyone know Mazda's and know for sure that the factory front subframe is non-heat treated mild steel?[/FONT]

[FONT=&quot]Anyone have comments on the best type steel for welding to the stock subframe?[/FONT]

[FONT=&quot]I've built a jig with 1.5" sq tubing with 8 points of contact to hold the subframe in place while it's modified. Bolted a factory front subframe to it and widened the inside clearance to the max....". The rear control arm mounts are basically just hanging on a whisper since I removed substantially all the subframe they were attached to. [/FONT]

[FONT=&quot]My plan for reinforcing or rebuilding the subframe is to use A513 1" X 4" .125 rectangular tubing or possibly 1" X 3" if I can't find 1x4. Reason for the A513 rather than the A36 mild steel I suspect most use in building home brewed V8 Miatas is A513 has a higher carbon and yield strength (72,000psi vs 36,300psi for A36 mild steel). This gives it a greater resistance to bending and it's used in the automotive frame world. [/FONT]

[FONT=&quot]Anyone have comments on the type steel that's best in this application if I'm off in left field?[/FONT]

[FONT=&quot]I'll be welding with a Lincoln 175 amp 220v machine using .035 wire and 75/25 gas. I don't think I need to pre/post heat anything.[/FONT]

[FONT=&quot]My thinking with the rectangular tubing is I'll have a better looking, stronger/safer subframe when compared to what I could fabricate using flat metal only.[/FONT]

[FONT=&quot]Any comments or suggestions are appreciated.[/FONT]

[FONT=&quot]Thanks, Ron[/FONT]

 

[Deleted member 995608]

I'm not a metallurgist, but I've fabricated since my teens and I'm almost 41 now. I've got experience in structural steel, stainless, alum, and other steels such as copper and brass.

A36 is actually enough for your application but use whatever you feel is adequate. The jig is an excellent idea. Just remember to stitch weld and allow cooling between welds. The jig will aid in twisting but it will not prevent it once you pull your project from the jig; even if you wait until cool before removal.



Gusseting is crucial when mating the sub with the existing frame. Build your own fish eye gussets and drill plug welds for plate reinforcement, not straight cut plates. Google fish eye gussets for more info .

Good luck Ron and post pics when ya get a chance.

 

[Thump]

You're talking cold/hot rolled electric welded vs DOM steel tubing. Both are fine for what you're doing and won't need normalization like chromoly or other type metals.
Keep in mind that the higher carbon content, and resulting harder metal, will be more prone to work-hardening and stress cracking over time. This is especially true in the heat-affected zone from welding.
A question I do have is how dissimilar are the metals you're joining? I asking due to possible galvanic reaction or possibly some concerns over filler metal while welding. Both of these are VERY mild concerns, but you're getting into ASTM specs, so I figure it's good to consider.
What was said about stitching and moving around to reduce pulling is good. Test your heat and speed on some scrap to get the puddle and bead where you want it, too.

 

[moparguy]

Thanks for the reply's. This project has been sidelined while I worked on re-decking a 8'x40' flatbed trailer that's been re-purposed as a bridge to our cabin. Also had to do a lot of backhoe work building approaches to the the bridge since it's about 4' above grade. But that's about finished so it's back in the shop this week.

I've picked up some 1x3 a39 .120 rectangular tubing along with 5' of 6"x.120 plate to rebuild the subframe. Images below show an uncut subframe (not mine) and mine bolted up to the jig with a piece of plywood clamped to the bottom, allowing me to cut the subframe in one pass with a 9" bimetal blade in the sawsall.

 

[Deleted member 995608]

Cool pics. Actually nice to see what you're working with. Keep us posted on the progress.

 

[moparguy]

Few pics of the almost finished subframe. Still have to exactly locate the motor mounts, clean and paint. One thing I had reinforced is you can't weld if you can't see. Even with my reading cheaters, it was near impossible to see the weld puddle. Had plenty of heat so not worried about unsafe, just not at all happy with the lack of pretty. But, had a welding x-ray tech on a project once say that there was no bonus points for pretty when it came to welds.