Is it a low rumble knocking that you can almost feel or a higher pitch tap knocking? The former indicates main bearings, the latter rod bearings. The noise you hear is before oil reaches and lubricates the bearings and cushions the noise as the crankshaft turns. Because the mains receive oil from the top via holes and passageways through the block running from the valve galley, they are already primed. It is the rods that will usually make tapping noise upon start up (especially in cold weather) until the crankshaft rotates many times and scoops up oil into the rod cap dippers, thereby oiling the bearings. Then the engine becomes quieter, right? So, I'd still be inclined to believe it's the rods that need adjusting more than the mains. You should be able to do the job in less than two hours, even as a first-timer. Drop the pan, remove an equal amount of shims from each side of each rod (sometimes peeling through the layers is necessary to obtain the correct thickness), tighten the nuts until the crankshaft cannot be turned over by a hand crank or starter. If the engine locks up, replace one thin shim on each side of the rod (if possible) and try again. If the engine now turns over, you are good to go. Install cotter pins, new pan gaskets CAREFULLY installed, add NEW oil and start the engine. This is an abbreviated description of the procedure, but you get the idea. If you were able to remove several shims, that was the problem = loose rods.
The leaking rear main is a common Model A problem and can be caused by many things. The most likely cause, however, is loose center and rear bearing caps. It is far more difficult to adjust the main bearings than the rods because so much of the engine has to be disassembled before that can be done. Plus, the rear bearing nuts are a BEAR to loosen because of their location and tight quarters. For a first time procedure, it would be best to find someone who has done this before. Watch what he does so that you can do it yourself down the road. These engines are not a one-time adjustment and good to go for the rest of their lives. They need periodic maintenance, such as attention paid to bearing adjustment. The good news is that the rods are much easier to adjust than the mains and they will be the ones more likely needing to be adjusted. Piece of cake.
Marshall
