Welding moped header

Was not sure if I should ask here or in automotive, so apologies if I got it wrong. Whats happening is that I am fixing up an old moped, and naturally I need to add some go fast parts like a less restrictive exhaust. To accomplish that, I hacked the old muffler off the header and bought a new muffler to weld on. Only problem is, I cant weld. I signed up for the upcoming hot process class so I can start learning, but in the meantime I was hoping that someone hanging around could help me out? I’d be glad to introduce you to my friend Andrew Jackson for your trouble.

I’ll attach a picture, the only thing im not sure of is that the diameter of the header tube is about a tenth of an inch smaller than the inner diameter on the silencer. I have a clamp to go on the silencer but its not going to reduce the diameter enough to make a good fit on its own. Not sure what I’d have to pick up to make it work, Im open to suggestions here.

2 cycle or 4? 2 cycle requires a tuned exhaust. 4 cycle however doesn’t matter as much. Not throwing my hat into the ring, just trying to help you avoid some heartache if possible.

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Its an old 2 stroke! I know any changes to air flow will require some tuning, but Im not sure what you mean by a “tuned exhaust”? I dont have a ton of hands on experience with 2 strokes, but I’ve been poking around the moped forums for a few months and havent heard anything like that, lots of guys have taken the approach I’m attempting with success. I changed out the reed valve and once I get it together I’ve got a range of jets to get the carb dialed in, but I’m not aware of any other tuning that would be required beyond that?

I did a bit more reading about expansion chambers, Im guessing thats what you were talking about! Had no idea that was why different styles of exhaust on a 2 stroke can have such different power bands. I thought it was solely due to the restrictiveness of the pipe. That being said, the pipe I bought is made for a ~50cc 2 stroke and I know at least one other guy is running it on a similar setup to mine, so I think it will be fine.

Step one for a good weld is making sure your parts are clean. No paints, no oils, no rust or mill scale, it needs to be really shiny. Inside and out. Depending on how skilled the welder is, you can clean up less distance from the weld than a less skilled welder will, but the heat affected zone (HAZ) must be completely clean. I’d recommend at bare minimum an inch from the joint both ways, but 2" would be better.

If it’s a stainless steel weld, and you’d like to use TIG, you will want to use the second regulator hose and back-purge the inside of the tube with Argon.

A small gap is OK, but good fitment is better. Maybe flare the pipe a bit to make sure the two mate closely?

If you can’t find someone to help you weld, you could always try a Tiger Patch. I used that to install Sportster mufflers on my Ural along with pipe reducer/enlargers

thanks, I had never heard of the stuff I may try it just so I can get it running this weekend. I unfortunately cant find a reducer off the shelf with the correct size, I believe I’d need 7/8" ID to 1 1/8" OD? weird sizes…

That I can’t help you with. I used some EMGO reducers but looking at their specs they won’t help you. You can wrap the Tiger patch multiple times so maybe that will help get you to the size you need, along with your clamp may be sufficient. Either that or I would go the way @malcolmputer mentioned and try to flare the header side + tiger patch instead of using a coupling.