I've learned the lesson more than once: Loctite everything on the 390. I learned it again today when I was truly surprised that the upper rear engine mounting bolt had lost its nut and backed out. It looks like it must have happened quite a while ago, judging by the reshaping done to it by the drive chain.
Fortunately, I had a spare engine bolt that was just a bit longer than I needed, so I cut and re-threaded it... and installed a new nut with blue Loctite.
This bolt is not noticeable unless you have the sprocket cover off, but you can inspect it by looking from an angle.
Fortunately, I had a spare engine bolt that was just a bit longer than I needed, so I cut and re-threaded it... and installed a new nut with blue Loctite.
This bolt is not noticeable unless you have the sprocket cover off, but you can inspect it by looking from an angle.