Spring on engine mount.

[Renaud]

Renaud,

A bolt has a threaded portion at the bottom, followed by a plain shank up to the head., unlike a setpin, which is threaded all the way up its length.

A bolt " bottoms out" when one puts a nut on the reaches the end of the threaded portion without putting any tension on whatever the bolt passes through.

Hope this helps (and no, you are not a 'silly frog' - English is the most awkward of languages with lots of obscure idioms!)

Thanks David, always happy to learn those obscure idioms!
So do I understand that "longer bolts bottomed out at the front" means that there is play left under and the nut is not secured on the chassis? In other words the bolt is only there to keep the crankcase in line?
(Still frog but little less silly now maybe)