bolts in the corners?

The corners are ok if the hatch cover is small / stiff enough. If there's a gasket to compress or the cover is large, I've used 2 per side spaced appropriately.

Good luck.

Pete Keillor

A little detail would be good. Is the cover a piece of sheet metal or a piece of cast iron? Three inches long or three feet? What sort of forces will be trying to open it? That sort of thing.

Greetings Unk, I machine the fastener holes in hundreds of cast aluminum hatch covers and their associated frames each year. The largest of these has an opening in the frame of 10 x 14 inches. The frame has a gasket channel cast in and the cover has a lip that extends into this channel to compress the gasket. The fastener holes are two on each long side and one at each end. The centers of these holes are approximately 12 x 16 inches. I do so many because they are so popular, they work. As an aside, I also machine O-ring grooves in the heads of flathead stainless steel screws for these hatches. As near as I can tell nobody is doing screws the way I do them. The groove is at right angles to the the 82 degree angle of the screw head. This retains the O-ring beter than any other method as well as allowing larger clearance for the threaded portion of the screw. Furthermore, it is much easier to control the finish of the countersunk surface than it is the drilled hole. Eric