Well folks, I was surprised to find my supercharger pulley has disintegrated. I didn't know it was non-standard, I bought the car with no known history. Apparently it was well tuned - pulls very hard and gets reasonable mileage. The pulley was a 65mm aluminum one which split in two along the root of a belt groove.
After looking at options, I bought a used stock pulley with a new bearing already installed, from NeedsWings.
The picture shows the first step of turning it down. Workholding was a challenge, this is a third attempt, which worked well. Next I flipped it around and held the nose in the 4-jaw. Dialed it in and made the rest of it bald. I cut the new grooves with two custom ground tools. I only took it down to 67mm. If my measurements and calculations are right, there is a little more than 3mm of wall thickness remaining at the root of each groove. A 65mm pulley would have only a bit more than 2mm, which seems risky to me. Hopefully 67mm will work okay with the tune.
This is by far my most ambitious machining project. I was worried that the stock material would be a stainless that work-hardens when cut, but happily it is pretty mild and very free-machining. I'll install in the morning and add a picture of the result.

 

It looks great! Best of luck to you!

 

End result (almost). I did stick it back on the lathe to refine the shape of a couple of those ribs, and then gave it a quick polish. Installed this morning, see post #6 below.

 

If memory serves me well then the largest inside diameter is the U/Cut at the shoulder the bearing sits against.
I have all the sizes somewhere, pulley diameter was a huge topic here once. The aluminum ones were a waste of money as where the early steel ones.
If any pulley can be trusted it is the OEM ones.
I’ll see if I can find the figures, the use of a smaller bearing is preferable to the OEM diameter.

 

Your memory is correct about the minimum internal diameter. In fact, the aluminum aftermarket one split right at that shoulder, which coincides with the root of one of the grooves. By the way, I cut the new grooves in this one with a rounded bottom to reduce the stresses in that area.

 

One additional twist, the auto tensioner doesn't seem to be the correct one. There is no 17mm boss or any other attachment point for levering the tensioner in a counter-clockwise direction. I managed a workaround, but it was a giant pain. The slightly larger pulley size wasn't an issue, probably because of some belt stretch. The belt is not in as good a condition as I had hoped, so I'll order a Gates replacement.
I took it out for a quick test drive and gradually worked up to some strong pulls. Seems like I'm up and running again. I did note that at wide open throttle, the tune was clearly going to let me get into redline territory. Maybe that's a bug of that particular tune. This car is mainly for commuting, so it shouldn't be an issue if stay alert.

 

There is no 17mm hex, you use the e bolt in the center to reduce belt tension.

 

Thanks for the info. My searches turned up instructions and pictures with the typical hex stud for gaining leverage. I guess I was looking at the wrong how-tos.
Just to be clear, by "the e bolt in the center," do you mean the one in the center of the tensioner pulley?

 

The SRT6 tensioner works differently than the NA. The "bolt" for lack of better words you see in the middle of your tensioner requires an E10 torx to loosen (some have used an E11 but I'd be worried about stripping the tensioner bolt). Good luck!

 

Thanks for your input!

 

Never use the 11 size as it will round the corners on the bolt head.

 

Roger. I have a full torx set.
I ordered the NeedsWings improved idler kit, so in a week or two I'll have a chance to figure out the proper operation of the tensioner.