When I first mounted my engine it would start to lean left while I was riding. I didn't think much of it, and when I was done riding I would put it back and tighten the bolts... This was until without warning it popped of a bolt and fell to the left, luckily onto my pedal and not the ground. I did a little unintentional acrobatics over the handlebars and had to carry the bike nearly a mile home(It wasn't even roll-able). I decided then to drill my frame. It holds rigid in the middle now and as a bonus I have noticed less vibration. Maybe drilling the frame and bolting through it could be the answer? I know it sounds as appealing as drilling a hole in your own head but if it is leaning dangerously it could be worth it. Hope this helps.
