In my opinion and most tire repair centers will agree, that you should never plug a radial tire. They should be patched from the inside.
As for how close you can get to the sidewall?? As long as the puncture went up through the tread and did not enter past the edges of the tread, you are OK. The other problem you may have is how big the hold is. Some larger punctures do too much damage to repair even if they are in the center of the tread.
I've never had any problems plugging a radial tire in the tread. At one point, I kept plugs in the truck and would plug on the side of the road and air the tire with a cigarette lighter pump. Still got lots of good miles after that, too.
Dad has a kit to plug his own. My brother used to sell them. He drove around with over 100 plugs in each tire to demonstrate how well the plugs worked. He sold a bunch of them. Never had a problem.
Thanks for all the help, I think I should be able to plug it , it is in the tread and it is just a nail stuck in there so the hole shouldn't be too big. I was just concerned about it being on the outside edge. Thanks again.
I don't care how many plugs you have put in tires. The fact that you may have done it doesn't make it right. People do a lot of stupid things and get away with it...at least for a while.
I am just telling you that any good tire repair shops will not plug a radial, and It also voids the tire warranty. I would not let anyone put a plug in any radial tire of mine.