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Search for 'O Ring Cord' but check chemical compatibility of whatever material you pick depending on the fluids you're sealing against. Probably nitrile/NBR/HNBR for engine oil but silicone and Viton/FKM may be ok too. If you need to make a continuous loop you can superglue the cut ends together.
The thickness of the material should be more than the depth of the groove by 20-30% but it depends on o ring material hardness and the stiffness/fixing arrangement of the cover.
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it should be possible to just buy a length of the oring material and push that into the groove right, what would you search for though? And is there a generally accepted formula for working out what thickness of o ring you’d use for a given size of groove?
"O-ring cord" should yield search results. Select material suitable for the temperature regime and compatible with whatever fluid you're trying to keep inside the engine. Cut the ends square with a razor blade and join with suitable cyanoacrylate adhesive. The cord diameter needs to be bigger than the groove depth but less than the groove width. If it's just a seal and not for pressure retention, you only need about 10% squish.
I need an oring type gasket for an oblong inspection cover on an engine. I can’t find one for sale but it should be possible to just buy a length of the oring material and push that into the groove right, what would you search for though? And is there a generally accepted formula for working out what thickness of o ring you’d use for a given size of groove?