I tightened things up as the engines move about in the frame under race conditions. Not really for vibration reasons, although with the engine loose, vibration does rise dramatically. I didn't go for a real snug fit, probably 0.1mm, which is better than 1.5mm!
When the engine moves about it wears the back of the cases and on a narrowcase loosing around 0.5mm or more off the width is quite common. The mounts then bend in not making good contact and wear escalates. Cracks in the rear mounts are also then not far away. To repair the wear you can either machine the casing faces flat and shim, or machine back further and fit "top hat" buttons to the cases to return to the original thickness. We went for the original width, as with race bikes you are sometimes swapping engines and keeping to a "standard" helps.
However BE WARNED, with modern race tyres we found that with all the forces when used hard, the cases split at the rear mount! So transfering the stress to the cases without a bit of "give" may not be wholly desireable!
We eventually solved the problem by machining the cases to allow a 3mm plate to be clamped behind the selector box. Which was then fixed to the swingarm pivot outrigger plates, which brace to the subframe. All the twisting forces were then spread over more than twice the width. Result - no movement and no broken cases. An easier solution however, would have been to use a widecase
Regard Nigel