Ok, about 2 months ago I did my first rebuild on a 300EX. I have to say I was proud of myself. It went smooth, it started and ran great. Now it has about 25 hours on it and it started smoking bad. I mean real bad. I tore it back down and seen that the rings look worn on one side. It looks to me like maybe the cylinder is out round. I didn't check the bore roundness. I guess that was one rookie mistake. Now, I just put the piston in it. The rings are shot. If I get the cylinder bored to get it back into round. Can I use the stock bore piston as long as I dont get it bored to big? Wiseco only sells .20 and .40 overbore pistons. If I get it bored will I have to use one of those sizes? Or can I use a stock bore if I only go .10 over to true it up. I know its a lot of questions but I need help on this.

You must get it bored. If the rings are wore, the piston is usually worse. Get a .020 piston and have the cylinder bored to it. If you bore it .010 to "true" it, you will have too much clearance. The 300ex should run at .003" piston to wall. .010 + .003 for clearance will be almost 4 times as much as is needed.

I will never pull a engine apart that has blown up and not bore it. This goes for auto and atv. The only time I won't bore is for preventative maintainance (rebuild every year on the banshees).