the rear of the OEM muffler is just an old style FL Baffle, and the end of the tube has been pinched shut.

the 2014 pipes I had, only had the Clutch side like that. I have those on my bike now, and they are ALMOST perfect. (they are the "slash up" so I really like them)

however, the pipes off a 2015 FLHTKL had both pipes with the pinched baffle tube. They are "flat ends" so I have not tried them.

you can just slide a hole saw in there, and drill out the end of the tube.

it may only be 1 1/4 hole, but you should consider the newer than 2009 OEM slip on's are louder (flow more) than the old EVO and TwinCam Slip ons, because the newer bikes had a Catalytic Converter, which absorbs ~25% of the sound, so the newer pipes only reduce 75% of the sound.

(the newer bike slip ons are a great cheap update if you have a buddy who's bike you cannot even hear, just perfect so you can tell what gear he is in & what he plans to do next)

so when you consider that amount of sound increase, and then drill the end of the baffle, and then use a Head Pipe without a Cat, then you have a little bit more sound.

the hole saw is $10 at Lowes.

Also, the OEM mufflers I looked at had a Fiberglass blanket inside, which physically slides back & forth when you shake the pipe. this means the fiberglass will blow out & make the pipe a bit louder.

pic 829 & 841 show the fiberglass mat down in the body

so overall, several marginal gains that add up to a good sounding pipe for the cost of the hole saw on an Evo, and on a newer bike, buy a non-cat head pipe, drill the OEM slips, and you have a little bit of sound.

YMMV/hope this helps.

 

here's what I mean by an old style baffle,

 

This what I used. I used a piece of pvc to keep the hole saw centered.

 

drilled out the gas side of a 2015 pipe on a 2007 Ultra today (96 inch with no other mods)

it sounds pretty dang good, like something aftermarket (it has a big mat/mesh pad inside the muffler body still)

the hard part was getting the baffle out, think about it before you do it.
maybe use a smaller bit to separate the two pieces, then a larger bit to make a bigger hole, so you can pull the parts out

I had to waggle the bit around to wallow out the hole so I could pull the piece out

 

Couldn't you just take a small drill and drill the center of the pinch, and then work your way up with larger drills to about 1/2" or 5/8" and get the same results?

 

I used a 42mm hole saw to remove the center baffle. Its neat , fast and the hole saw is self centering .

 

Mod's should merge these two threads
https://www.hdforums.com/forum/milwa...d-sound-3.html