I suggested using a couple of 3/8" fender washers and a bolt with a piece of rubber gasket material under the outside washer. Then cross-drill the nut and bolt for a roll pin or a cotter pin to make sure the nut stays tight. A freeze plug and some brush-on Permatex sealer would be simpler.

If you want to incorporate an oil filter drain into that hole, just drill and tap the freeze plug for a 1/8" NPT thread and insert a 1/4" tubing compression fitting into the plug before you drive it into the block.
Jerry


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