[JMFL36]

I had a similar problem on my '54 Merc 256 Y-block. I removed the rubber grommets then put a regular nut (not acorn) and washer on and tightened the valve covers down. I then removed the nuts and washers, and replaced them with the rubber grommets and bushings. This had compressed the cork gaskets enough to get the acorn nuts started, and I tightened them down as needed. Be sure to coat the cork gaskets on both sides with gasket sealer. These Y-blocks are known to leak oil from the valve cover gaskets.