At idle, my conversions seem to shake the plane a little more than a glow engine. At any thing above an idle, the vibration, if any thing, seems less than a glow engine. (Possibly due to the lower rpm's and bigger props.) As a kid, I tried running glow engines clamped in a vise, and they always vibrated themselves right out! Scary! Somewhere in these threads, there is a pic of a guy holding a converted weedie engine with a prop and all, in his hand, running!! Not recommended for sure, but who would think of trying that with a glow engine??

In any case, I don't think it is something to worry about. The older engines from the '70's and 80's I have heard vibrated more, but I certainly haven't had any problems myself. I do specially reinforce the firewall area, as it is a heavier and more powerful engine.

Just my opinion,
AV8TOR