I found that a dab of silicone gasket goo on the exhaust pipe threads cures any rotational issues or loosening. Sounds like you got the balance in well without added weight, is that right? So far I only have 3 planes that needed lead added, The SIG Somethin Extra, only because it has a 72 and not the recommended engine, the SIG 4*120 because I moved the elevator servo to the tail, but its going to get a DLE 30 on it, so that lead can go away. The only other plane I had no choice on was the ACE 4-60 I picked up for the Saito. I put my OS 70 on it and still had to add a few ounces to the nose for it to balance. That bugger was built tail heavy. All the rest of my plane have no dead weight, I was able to compensate CG by moving things around. My LT-40 started out with about 1 1/2 ounces on the nose, which later fell off somewhere, sometime, but hasn't affected it at least that I notice. My SIG 1/5th Cub balanced out perfectly, not a drop of lead was needed.
What do you use to balance them? I use the Vanessa Rig and it works perfectly, PITA to get stable, but once the plane is hanging true, it is so easy to get the balance done. I can probably find a different use for the Goldberg balancer I have, AKA the Plane DUmper.