stainless steel under-chassis welding

[oldskoolkid56]

Hey, few questions for you guys. I have a 89 240 and im looking to sell it this coming summer, its got less than 32K on the sr20de under the hood.. so im looking to make this car mint to get a good price for it.

Anyone who ha sa 240sx in the northeast knows that the chassis rusts out from under them. Its actually a flaw under the hood in the 89-90 models (dont know about the 91+) but theres a small slit between the firewall and the chassis on each side, when it rains water gets down in the chassis and sits,,, til it rusts. This car is from NY, soo, the drivers side is rusted pretty bad. Probably wont even pass inspection in Md where i live now.

So im looking to weld it over (which is a common practice to fix this) Now i am very profficent at welding, but am no professional. Im assuming i cut out all the rusted area, grind the rest down to bare metal, and weld a boxed stainless steel support there. I would just like some details from anyone who has done this or had it done. Like:

what gauge stainless steel?
is ia 200 amp arc welder suitable for the job?
what should i spray it with when finished to prevent anymore rust?
any thing else i should weld over while im at it?
should i weld over the slits in the engine bay? or will it not rust again (or at least for another 15 years)

any tips or advise would be great,, thanks alot

i posted this message in the 240sx forum, but they have no real answers, and told me to try fabrication...
-andrew

[duplox]

Don't use stainless. It'll cost you lots more money and offer no advantage if painted.. I'll get to that. I'd use whatever thickness the rest of the chassis is. Most likely ~3/16ths steel plate.
Not sure about the welder, but mine is a 180amp mig and it has no trouble with 1/4". Just make a nice groove between the two pieces of metal and fill it up with weld.. making sure you get adequate penetration.
Paint it with POR-15 paint. www.por15.com. This stuff is the best, but rather expensive. If you don't want to spend this much, I'm sure there are other cheaper paints out there that will do a decent job. If you use POR-15, it should never rust. It does cover very well, my brother and I painted a '78 bronco frame with probably 2 pints, a couple of layers.
As for welding over the slits, I think it'd be a good idea but as I've never seen an 89-90 240's engine bay, I really couldn't tell you. I can't see how it could hurt. With a POR painted frame, it shouldn't make a difference, but better safe than sorry, right?

You've got the right idea with repairing rust... anywhere it gets really thin I'd consider cutting out the thin part and patching it. Also, I'd grind/cut a small section, patch it, then move on to the next. I'd hate to see you cut a bunch of holes in the chassis, weld it all up, and end up with a crooked car.

[mike@af]

I'd say use 1/8" 4130 Chromoly, it has a higher carbon content then mild steels so its doesnt rust as fast, and also is very strong.

If you are dead set on Stainless, use some 1/8" as well. However, I'd use a TIG welder and .035" Steel Filler Rod. The biggest probably you would encounter is welding stainless to steel, because of the melting points and different nickel contents.

[Cbass]

My recommendation would be to user 10/11 guage sheet mild steel, it's about 1/8th". There is really no point to using stainless, and it just makes the job harder, and more expensive. If the body of the car was stainless steel, that would be a consideration, but it isn't, it's just mild steel. It's a good rule of thumb to keep the metals you're welding as similar as possible, it just makes it easier to weld and you'll always get a stronger joint.

MIG is the ticket for this kind of welding, it's much faster and easier than TIG, especially in hard to reach spots. A SMAW (arc) welder is not very suitable for this task, unless you're really handy with a 3/32 7018 rod. I'm a fabricator/welder by trade, and I wouldn't want to do it with stick unless I had to. That's not to be said that it couldn't be done, but you might end up blowing through here and there, having to grind out welds and touch them up here and there, etc.

I can't recommend Por-15 enough. Think of it as Tremclad rust paint on steroids, for cars.

[TheSilentChamber]

Cut out the area thats rusted and prep it with a wire wheel, weld in the patch panel that is the same thickness as the surrounding area, no point in going thicker, dont use stainless use mild steel with a high carbon content, as thats what the car is made of. Your not going to be able to use an arc to well stuff that thin, your going to need a MIG or TIG, tack weld little spots about 4" appart and cool it with compressed air and work your way around till its one solid bead. If you run one continus bead it will warp it. grind the welds flush, paint. You can use rust preventive if you want but if you prep it and paint it properly you wont need it.

[mike@af]

Originally Posted by Cbass

MIG is the ticket for this kind of welding, it's much faster and easier than TIG, especially in hard to reach spots.

True. However, I personally am much better at TIG than MIG. I can run down beads with MIG, but I still like TIG much better.

But this is up to the owner to deicde. And Make sure you do what TSC said, tacks about 4 inches apart to keep from warping.

[Cbass]

Sheet metal distorts very easily from heat, although 11 guage is not so bad for it. I agree with the Silent Chamber, give it 1" long welds about 4" apart to start with, and continue welding these tacks in a cross pattern until you have welded the whole way around the piece. Every second you're welding on that metal, you're pouring heat into it, and it can only take so much before it disorts.

[oldskoolkid56]

hey guys, thanks alot for the advice. ill try and find where i can use a mig welder, but as it stands right now i own an arc welder. i mean, im decently skilled with it and use it for any welding job, bought it because it was more versitile for what i do and its cheaper to upkeep. but definately thanks a whole lot, ill let you know how it turns out.