I dislike vertical exhaust pipes for that reason; even if you have an effective " flapper" cover over the top of the pipe, water can run down the pipe and seep into any small holes in the exhaust pipe or manifold. My neighbor's John Deere had water in the oil sump, and it turned out it was from rain water getting into the exhaust pipe and connections. When everything is new and clean and perfectly maintained, that should not happen. But, vintage tractors are seldom that perfect. The above comment about a drain hole is interesting , and worth keeping in mind. If you have no indoor parking space, keeping a tarp over the engine and exhaust when not in use is a very good idea. Putting a one gallon can over the flapper cover at the top of the exhaust pipe when not running also helps .