I recently removed the inspection hatch on both my diesel and water tanks to clean a replace the fittings.
Time has come to re install the hatches.
They are 5mm steel and are mounted onto a steel flange with about 30 nuts and bolts holding them.
Iím looking for a product to place between the hatch and flange to seal them.
I have rather large inspection hatches made from fiberglass pipe fittings designed to sustain hot, caustic fracking liquids used in oilfields. I used Nitrile Buna-N gaskets. I purchased from McMaster Carr, but here is some additional tech information I scraped from a Google search: