My current baffles are straight thru perforated, meaning that they have hundreds of little metal protruding dimples on them that protrude slightly into the exhaust stream.
I found the actual measurements of the 258 inserts on the company website, and they are 2 5/8 inches in diameter (hence the name.)
So the clearance around the insert will be at more than 3/4 inch, which seems like it should be perfect my application.
I have ordered a set of 258s and I'll report back after I get a chance to install them in the pipes. Unfortunately, due the devastation from Helene it looks like there are mail delay warning for much of the southeastern US, so they may take longer than usual to get here.

 

So I ordered a pair of the DK TTI 258s which I received a few days ago. My first impression is that they are well made and are heavier than expected.
After a LOT of tugging and twisting I extracted the baffles from my Full Boar 3.5 inch slip-ons. I was surprised to find that most of the packing was still intact around the baffles, as I've heard that most of it eventually disintegrates and gets blown out after many miles of vigorous riding.
Fortunately, I didn't have to enlarge the existing screw holes in the slip-ons to accommodate the stud on the TTIs.
The clearance between the TTIs and the inside of the muffler looked like about 3/4 inch, and the stud on the TTIs was marginally long enough to mount them as the instructions suggested. I installed them with the concave part towards the exhaust pipe exit and as close to perpendicular to the inside of the pipe as possible.
My biggest concern when I was installing these was that they would make the exhaust way too loud, especially when I saw how much packing was left around the original baffles. That would have been a deal breaker for me because I love to listen to music when I ride.
After the install was complete I fired up the bike and was immediately impressed with the much more distinctive exhaust note. I blipped the throttle a bit and the volume wasn't nearly as bad as I feared. It was louder, but not ear splitting ridiculous.
Today I took the bike out for a road test. I felt the the low end grunt that I had been missing as soon as I turned the throttle. It was a VERY noticeable improvement and reminiscent of my 82 Sportster from years ago.
I did a combo of riding on local roads and several miles of interstate, and I was very pleased with the torque, power, and sound. Throttle roll-ons were much more responsive. It felt like the bike had woken up and come to life.
Despite the good reviews, I was a skeptic that some tailpipe insert could really boost performance. Now I'm convinced that a lot of riders are probably spending more money than necessary on expensive aftermarket slip-ons and still missing out on the low end grunt that they're looking for.
My biggest concern is that I'm worried about the mounting hardware loosening up over time and that I'll have one fall out. I'm planning on a have a welder friend of mine put a small spot weld on the interior and exterior nuts once I've done a bit more riding to make sure they can't loosen up or change position.
If I had a seen a pair before ordering I would have ordered the longer stud ( as one commenter suggested), because it's easier to trim off some off the stud than to stretch it
Thanks to all the members who offered comments and suggestions when I posted my question!

 

So the thunder torque inserts are used in lieu of the baffles that are already in the pipes?

 

I installed mine without the baffles, which is how the company says you'll get the best performance increase.
The company has numerous videos on YouTube where they explain that they can be used with or without baffles, depending on which size insert you order and the diameter of the pipe or baffle.

 

Thanks!

 

NOT TO HIJACK THE THREAD !! i have reinhart DBX 4.5 inch on my 2022 SGS and i bought the Tuneable baffles from Fuel Moto - love them , they where so easy to install and just helped with that bit of back pressure those M8 need . S and S 475 cam , Dyno Jet tune, stage 1 , factory headpipe still . the bike works so good

 

I think the header collectors on my SBC were 3" or 4" and the header muffler cores were 2-1/2". Funny how riders are installing nearly the same thing on tiny Lo-Po HD engines.

I don't know how or where this trend started but exhaust gas velocity is important to promote VE in certain operating ranges and bigger is not always better.

I wonder if @Steve Cole has anything to say about this?

 

What is Lo-Po?

 

Yep, the FM inserts are just what the Rineharts need.

At first, I was disappointed with how much quieter they seemed with the inserts, but after a good long trip (and a proper dyno tune) the Rineharts are again loud as hell.

 

Davekp - LoPo - Low Performance, Harley V-Twins are not racing engines, they are low rpm tractor engines. Now, before you get upset, look at them from an engineering standpoint, cam low in the engine, requiring long pushrods, not the best rocker arm geometry, intake and exhaust ports not configured for best (good, but not best) airflow, still basically an air-cooled engine, requiring sloppy tolerances for thermodynamics, and a low rpm limit.