I just fitted 5 new Blakes seacocks to my Nicholson 35 after replacing the old ones which were starting to fall apart after 17 years hard service.
The key question is whether you are seeing any seepage past the conical plug? With mine, the retaining plate is loose enough to wobble with light finger pressure. No water comes in so I am happy...
If you overtighten the plate you will cause undue wear on the cone and eventually the valve will seize up.
Graham