<font color='#000080'>Don't use the silicone sealant you would use in your bathroom as silicone cures by the exchange of moisture with the air so would not cure down inside the threads. You need something that will either cure anaerobically or a 2-part system.
You could use Araldite as it's 2 part then when you need to get the screw out again, heat it up.
Alternatively, use a good thick wrap of PTFE thread tape (from any DIY store) that plumbers use. Ideally it should be used on a tapered thread but by a little perseverance you can get it to work on a parallel thread.
The best solution, if possible, would be to use a Dowty washer under the bolt head (sometimes called a bonded seal). Have a look at RS components
RS Components who have a wide range. Much more reliable than O rings. In fact, they should be designed into the A clamp to get rid of those poxy O rings.