I received the 4.5" Back-Tap and over about 2 days played with it doing a little bit ever so slightly. With this 4.5" long tool it takes longer since the top of the shaft to the bottom of the plug well is exactly 4.5". Sort of similar to busting your knuckles by trying to access the knurl and the nut. BTW: I ordered a 5-3/8" tool made by Snap-On as a back up. The longer the better for tool control in the case of this bike, I don't know of other Guzzi's.
I did rent a bore scope to look at the thread damage and it was all the way down towards the bottom. But, I just worked at expanding the backtap slightly more and more each time and when I hit the sweet spot it felt just right when pulling it out and I got the very small shavings left on the grease. I put in a "used" plug and it went in easily by hand and removed easily as well just like the other plugs/holes. I took measurements and it seats the same depth as the other center plug on the right side.
I have new plugs on the way and I need to now check the valve clearances; the reason for this event. I now have two tools.