Hugh is correct. It's a big lump of rubber with a threaded part attached to each end. The bolt doesn't go through the rubber. Just looks a bit like a mushroom stuck onto each end of the rubber.
It looks like this (posted as an example pic only): Mercury Mariner Upper Side Mount 82557M 1977-97 25 28 30 35 HP | eBay
You'll find a schematic of your engine if you go here:
1986 Mariner Outboard 30 [ML] [ 7030226] - Parts Lookup - Crowley Marine
then left click on the brown 'View components' bar. Go down the menu that appears on the left & left click 'Mounting system'. The part you need is No 17 in the diagram.
Unfortunately it does look like a PITA to fit.
(FWIW there are several similarly constructed smaller mounts on the cowl of my Merc 135 & they are a complete PITA as the metal keeps separating from the rubber. As they don't carry much load I just clean the mating faces up & epoxy them back together. Damned if I'm paying £20 odd each! This is unlikely to work with yours though as it appears yours may have a higher load. I do toy with the idea of just drilling though the rubber & substituting a bolt, but this would obviously affect the anti-vibration qualities of the rubber mount.
There is a square portion on each thread which fits the hole in the engine to stop one end turning as you tighten the other so with this type of mount if you can't get at both ends - as would be the case with a bolt - tightening could be a problem as the bolt may just spin unless you can use another way of preventing that. A longer exposed thread on the exposed part could be gripped if you use a normal nyloc nut instead of the acorn you currently have.)
ETA and NB!
On the standard mount the nuts at each end are only intended to be nipped up. Continuing to tighten beyond that point won't do anything to the rubber part but will shear the end off the thread.