Tips for making fish-gill slits in a muffler? IE how to make them "flare out"?

[Cerberus]

I don't want to make them on my "important" muffler til I get them down and I'm trying on thinner mufflers w/o much success at anything but hack-job work....I just don't "get it" how to make a proper fish-gill or louver, it's as-if you need to stretch the metal (which of course does not stretch, lol )

Would also be curious to hear people's thoughts on the more "meta" subject of placement of gills (or of muffler-exits in general), for instance I'm placing some gills on the top/front half of a muffler that's got an internal baffle plate, the OEM design has zero exits in the rear-half of the muff and neither will my finished product, at any rate for placement of my slits/gills I'm planning to:
- keep them higher on the muffler, like near or on the top of it (because fuel sinks downwards, so would think that higher=better and lower=worse, generally speaking)
- put them dead-center because my baffle prevents a "direct path" from the exhaust-port, to the front-portion of the muffler. This way I keep the 'factory concept' of that first muffler-half being a sealed "extension of the cylinder" before gas escapes-into the front portion where it can exit the muffler (I did significantly enlarge the baffle's passages in this case!)

Thanks a ton for any insight, have tried on a couple throwaway mufflers and hate my results, will not share them LOL they are horrible, cannot get myself to do it on my actual project-saw!!

 

[ammoaddict]

I've never done it but I saw a video of someone cutting the slot with a Dremel cutting wheel and driving a cold chisel in it and prying it open. Have you tried googling it?

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[USMC615]

I've never done it but I saw a video of someone cutting the slot with a Dremel cutting wheel and driving a cold chisel in it and prying it open. Have you tried googling it?

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This will work.

 

[old guy]

Just use a wide flat blade screwdriver & pry em open, I've done a bunch.

 

[Maintenance Chief]

I cut a slit with a dremel wheel. I then use a small anvil or heavy chunk of iron and take my 10 oz ball peen hammer and stretch the metal from the inside first.
Since the slit is a open end it will buckle up when the metal stretches and flair up.
After I've created a hump in the area behind the slit I shape it with a square piece of steel or anything really , striking it with the hammer.
034 front view
034 side view
Craftsman 3400 side view.
Just do a lot of little forceful hits with the hammer don't try and get it all in a few smacks.

 

[cash192t]

I have a pair of vise grips, think hvac or sheet metal pliers that are made for bending metal. While I havnt done a muffler I have done just about anything but that lol. they are cheap enough you could buy a pair, cut the pliers head to the width you need. may have to taper them with a grinder after. after you cut your slit with dremel, grinder, whatever. insert pliers, lock on and bend. gives straight even bend. on thicker stuff I heat the ends so they bend without tearing.