[Mwood95]

Hey there, new member here.
Ive been migging stuff for projects for awhile, just bought an AHP 201xd

My excuse to dive into TIG was I have a 02 mercedes that needs a rear catalytic converter. I bought a new magnaflow cat and Im going to weld it in place of the previous one. I have some experience with welding exhausts as I MIG'd a stainless exhaust for a project car a couple years ago. It didnt go great as my harbor frieght mig doesnt like thin metal.
I have plenty of stainless tubing lying around in the garage to practice on before I weld the cat on my merc.

The mercedes exhaust is probably 400 series. the magnaflow cat is stainless.
I dont really want to pay for the gas to purge these two welds, and it seems like it would be complicated to set up.
I would have to blow argon from the outlet of the secondary exhaust pipe Im welding on all the way through the exhaust. like 2-3 feet of tubing. I dont know if thats difficult or if argon traveling that distance has a hard time pushing other gases out of the chamber. the argon would also have to travel through the cat to reach the primary weld.

the previous owner of this car had a cat welded in on the other side of the exhaust by a crappy shop, and its disgusting. mig wire sticking out, nasty gobs of weld everywhere. If I can get the other side sorted out, Id like to order another cat and replace that one too.

I have seen written elsewhere that the coating left behind by solar flux can chip off and destroy catalytic converters downstream. Is this a common issue?

Any advice on backpurging a catalytic converter thats deep in a secondary exhaust section like this? Id rather avoid it all together honestly.

Mitch

[Coldman]

I assume it's a butt joint. You can put a short sleeve inside the joint then do the weld without purge.

[cj737]

If you want to do it right, weld a V-band clamp on both ends of the joint. This allows you to reach in and clean the Solar flux before assembly. Plus, I think you'll have a LOT easier time aligning the CAT and having the ability to remove and replace it later will pay huge dividends.

I did the opposite recently on my eldest's motorcycle- CAT delete with a new mid pipe. That small of a piece I purge welded autogenous(ly).

[BugHunter]

That's not exactly what I would call a beginner project but if it's what you've got I guess onward and upward.

I have a dual flow regulator for the specific purpose of back purging something like that. Even sitting here thinking about it, I really don't have any good ideas on how to do this and make it last. If you don't back Purge the inside of that exhaust it's going to operate at extreme temperatures and stainless is no longer stainless when it has a surface passivation removed. You are going to be removing it's big time.

[Arno]

If the new cat is a slide-fit over the existing exhaust pipes then no back purge is usually not too bad as you don't tend to penetrate fully to the inside in that case and the sugaring is limited.

On a butt-weld it's going to sugar on the back without some sort of backing. It will tend to rust there and the ridge of sugaring is not great for the exhaust flow, but many exhaust shops just do this. You could too and just ignore it.. (suppose that's not gonna happen )

Solar-flux is a good option. Yes, it will leave a kind of 'glassy' layer, but you only need a pretty thin coat/cover and I would be surprised if any that came off would damage the cat substrate. It's too light/small IMHO to really make a difference. Used that myself in several cases and never had a problem (but then my statistical sample size is waaaaay to small to make any claims..)

Just don't slather a bucketload on/in there.. If you have some test pieces of sheet or tube you can experiment with how little/much you need to have 'enough' coverage to stop the back side sugaring up.

Bye, Arno.

[BillE.Dee]

WOW...right there is several great ideas, Mitch. I would tend to believe that any of them would work.
Coldman's sleeve could be made out of a piece of the tubing with a small slit in it to get fitting inside the pipe.
Arno isn't using a ton of flux.
cj is using a band clamp.
buggy has a twin regulator, but we don't know how much gas will fall thru.

[Poland308]

3inch pipe has a internal space of .05 cubic ft of volume per ft of pipe. Purging may seem like a lot of gas and time but when you do the math it’s much quicker and efficient than most think.

Edit: unless your doing high performance and trying to squeeze out fractions of a hp. Then you probably don’t need to purge at all. Even though an unpurged weld will be susceptible to corrosion it will probably last as long as the cat does.