I don't know...i have a the 49 c.c. slant head, mounted to my frame with rubber.
I have 0 vibrations at at idle or at any speed. it runs very smooth and does not vibrate the frame, handlebars or the seat.
My friends 66 c.c. non slat head, is mounted solid to his frame, and it does not have any vibrations either.
we both upgraded to higher quality nuts, bolts and washers to replace the ones that come with the kit, and we used blue loc-tite on every nut & bolt.
so far 200 or so miles on each of our bikes, nothing has vibrated loose, nothing has fallen off, and both of our engines / bikes run very smooth with no vibrations.