Hi Simon,

some of the old hatch designs where not real good. If you can get new Holt Allen hatches I would go with them. The HA's have a coarse thread that doesn't cross thread easily and water can't sit in thread area, so have been leak proof so far <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

But as Pete said don't fasten them down just use a good quality silicone and push them into place and leave to set with hatch screwed in.