How to mount toroidal transformer in metal chassis
A bolt placed through the middle of the transformer effectively acts as an extra secondary. This secondary can produce very little voltage but potentially very high currents.

As long as this parasitic secondary is open circuit there is no problem. So you can mount a toroid in this way. However if the parasitic secondary is shorted through a low resistance path (e.g. the case) things could start to get very hot.

Depending on the application it may be considered wise to fit an insulating plate to the underside of the top of the case in which the transformer is mounted so that even if the cover is crushed it can't short the parasitic secondary.