I am in the process of replacing a Mansfield 210 flush valve. The replacement valve that I got at Lowe's has a shank nut with a hex shape, while the nut on the old valve was round. My question is whether the gasket between the valve and bowl needs to be specially fitted to the hex shape. I have a brand new gasket that is similar to the old one, but when I put it up on the shank, the nut doesn't quite let it reach the tank bottom, and I'm not sure whether it will be leakproof after it's reattached. Any guidance would be appreciated, thanks!