Scorps, it sounds as tho one of the bolts has been over tightened/crossthreaded and has stripped out the casing, meaning it cannot apply enough pressure to seal the case at that point.

Easiest fix is to helicoil it. In case yoyu don't know what a helicoil is, it's a Stainless or steel thread inserted into aluminum threaded holes to strengthen the connection on the inside for attaching a screw/bolt.

If you have not done one before it's a good idea to have someone around who has.

http://www.workshopsupplies.co.uk/en-gb/dept_269.html