Glad I was able to help. I have been driving my truck with my re-tapered ptiman arm for a while now. I tapered it on the truck with a 1/2" drill and ALOT of cutting fluid (I used some cheep "dark" cutting fluid from Home Depot - nothing special). If you don't get the speed and pressure just right it will chatter, but when you get it right it takes material off very quick and smooth. You will have to give it more pressure to start it but once it starts to cut you can let off and let the tool do the work for you.
I am sure the chattering is partally from the straight flutes and that using the $$$ stock car products one might work better and/or be easier to use...but for enlarging a taper that already exists (rather then tappering a plain hole) I think the much cheeper straight flute tool works well.