I have planned to attach the oil cooler to the firewall and acquired a nice aluminum duct adapter to fit over it. It requires a 4" SCAT tube from the aft engine baffle to feed it. So that means a 4" duct flange will need to be riveted to the aft baffles to secure the 4" SCAT tube to. AS&S sells duct flanges economically enough (I think they're even in stock) but appears the size ranges only go up to 3". That's about as far as my search efforts went trying to locate one. So lets see if I can stick one together.
I've been looking for some aluminum welding projects to practice on since my aluminum welding skills in general, suck. This seemed like a good opportunity. Had a scrap sheet of 0.032" 6061-T4 big enough to make several attempts at (good thing too). First off, It's a weldable alloy that's important, cuz not all of them are so. Plus, it's reasonably thick enough even I can stick it together without blowing it full of holes.... mostly.
Initial steps were to use the fly cutter to cut a 6" ring which will form the riveting flange. This should form a 1" flange around the finished part.
Next, I sheared a 1" wide strip to form into a circle which will be welded into the center of the ring, forming the duct stub the SCAT will clamp to.
Simple enough, cut a four-inch hole in the ring, weld the one-inch strip into said hole and yur done! How tough could this be?
First attempt worked out.. mostly. Finished part was kinda wobbly. Heat distorted the riveting flange. Kinda looked like a really small aluminum fedora with no top. Was thinking I could anneal it with the torch and smush the flange flat and prettier when I attempted to fit the 4" SCAT to it. Turns out 4" SCAT tube is exactly 4" and will not fit over a 4" tube... at least I couldn't do it. Next, tried fluting around the perimeter of the duct stub in hopes of making the SCAT fit nice. It did sorta. But not well and it's getting uglier. Better off cutting my losses on this attempt and starting over.
The good thing about starting over is the drill press and shear processes is a simple repeat setup, only hopefully I have learned something from the earlier attempt. First up, duct stub will be 3-7/8" to allow the 4" SCAT to fit, and the formed strip will meet with an overlap instead of a butt joint. (I'm finding that easier to weld)
Fast forward, I wasn't paying enough attention to detail and during the tack-up phase things got away from me. Didn't notice the problem until I finished up. Flange was even wobblier this time. No go. At least the SCAT fit well.
Third time was the charm (I hope). Made the rivet flange narrower to reduce the distortion and spent a lot more time spreading the tacks around before running any beads. Longest run between tacks was about 3/4". Everything stayed put, and the distortion was minimized. It's probably ok.