MIG welding 409 stainless exhaust pipes
I've mentioned previous that the DD has a TME cat back exhaust. I knew it was stainless, but not much else about the material. The pipes have always had a slight patina that looks like rust, but more like "stain" than rust (no thickness to the color). More research shows that the slightly magnetic pipes are made from 409 stainless.
So, the issue I've lived with since I bought the used TME setup, is when "bolted up" to the transition pipe behind the CAT, always resulted in the muffler stainless tip pointing down rather than ~ horizontal. For the last hurrah on the DD, I decided to fix this and modify the pipes so the muffler tip is properly horizontal.
A little research on MIG machines shows that they are capable of welding stainless steel. Not as pretty as TIG welds, but quite functional. On the sage advice of my machinist buddy, I bought a small roll of 309L .030" wire for my Millermatic 175, and he loaned me his TRI-Mix gas (which includes Helium). All the welding forums agree..... this combo of MIG wire and proper shield gas will yield quality stainless steel welds.
I took a bunch of field dims, and purchased an eBay 409 stainless pipe which was flared for 2.5" ID slip fit onto the back of my CAT transition pipe (3" --> 2.5").
Once the materials and pipe arrived, I again elevated the car so I could get it level on jack stands, and then carefully measure what I had. After I had all the dims, I removed the TME pipe which runs over axle and began the surgery:
Using a cardboard cereal box, I wrapped the pipe and made a square cutting line. Free handing with a 4" Metabo wheel, I had a nice square pipe to weld to:
I needed a compound offset to get the TME over axle pipe to approach the CAT exit pipe. Fortunately, I determined the discarded piece I removed from the same TME pipe once shortened and "clocked" would bring my over axle pipe concentric and
mostly square with the CAT exit. After prodigious measuring, I cut the dropped piece, clocked it and tacked it:
Prior to this exhaust surgery, I took a couple scrap pieces of the 409 stainless pipe and practiced MIG welding them butt fashion. You end up using more shield gas and LESS voltage than the Miller guide suggests, but I ended up with good penetration that didn't blow thru the .065" wall exhaust pipe. Having achieved competency with the butt joint weld, I took a breath and welded the real butt joint together. Ugly a$$ black welds are what you should expect when MIG welding stainless.... but they are quite functional:
With the over axle pipe revised in stainless, I used a pair of Volvo scissor jacks to get the final "measure twice, cut once" dimensions of the last piece. From memory, the lengths needed to create last butt joint were 113, 112, 111, and 112mm. Here the mockup to determine these critical dims:
With the last 409 stainless pipe laid out perfectly, I made the METABO surgical cuts, cleaned up the butt joint & confirmed clocking.... and made the 2nd butt weld:
Now with the most difficult pair of butt joints welded, I chose to use the METABO blade and make 8 equidistant cuts at my slip joint, so that I could pull it down with an exhaust clamp and not weld that joint behind the CAT (retain flexibility for removal/repair). I am pleased with my cuts on the slip joint (used a square & sharpie, then free handed with the METABO):
This last slip joint was easy to assemble, since I had a V-band ahead of the 3" high flow CAT. With the full exhaust assembled and the car sitting at ride height, I've got more than 90mm between the road and the lowest point of my exhaust..... would have been ideal at 100mm,
but I'll take it. In fact, the bottom of the supposed TLAO 3" DP is lower than my rear piping.
The shameful angled tip TME is gone, and I didn't have to settle for a crush bent exhaust transition from the local redneck exhaust shop. Another box checked off........ and ever closer to driving Cheap Thrills to the windshield shop