John, I've never personally seen or even heard about a mower with a screw-on blade plate. The blade plate is normally held by a boss that is pressed onto the crankshaft. Either the boss has tapped holes to retain the blade plate, or there is a tapped hole in the end of the crankshaft and a set screw holds the blade plate against the boss. All threads are right hand.
If you have already removed the blade plate and are now trying to remove the boss from the crankshaft so that you can take off the timing cover (oil sump, crankcase cover, choose your favourite name for it), in most cases the boss is a moderate press fit on a parallel part of the crankshaft, and you remove it with a puller after dowsing it in penetrating fluid for a day or so. In a few cases (Victa 2 stroke for instance) the crankshaft is tapered and you use a different technique to remove the boss.
If you post a picture of the parts you want to dismantle, we can probably be of more help, but it sounds like a normal Briggs & Stratton, that is pressed on and can be pulled off with a gear puller provided you soak some penetrating fluid into it ahead of time.
