Well, I went to several auto parts stores. They never heard of a 3.5mm tap. I went to the Sears Hardware store, no luck. I searched on internet, out of hundreds of listings for 3.5mm tap, I found one place with 3.5mm taps, Great Bay Model Yachts in Las Vegas. I called Sig about this problem, Scott said that the 3.5mm meant it was a 3 mm bolt with 5 pitch, or words to that effect. and to use a 3mm tap to clean the blind nuts. I went looking for a 3mm tap here in Brookville. No one had them. I called GBMY about the 3.5mm tap. They were busy. Then I screwed on of my 3.5mm screw in one of the blind nuts to see if my soldering iron would heat it up enough to melt whatever was there. It didn't help. So I unplugged the iron, and tried to remove the screw - I burnt my fingers slightly, but that unlocked my brain. I went to my goody box and got a 3mm screw, and it went right in with no problem - It took me one and a half hours to remember what Scott from Sig said about the blind nut. I've tried all four blind nut, and the 3mm screws all work. PROBLEM SOLVED. But the manual from the kit says that these mounting bolts are 3.5mm Philips head bolts. Oh well, you just never know. Anyway, thanks for all you suggestions. Now I will try to see what else I can screw up.