As a follow up, if you are looking to clean up the threads in the aluminum cylinders or manifold block, suggest using a thread chaser vs. a bottom tap. A tap, unless started exactly following the existing threads will cut new threads and probably destroy the part. A chaser is as it sounds, will chase after the existing threads and clean them up in the process. Same process: lots of oil and back and forth.
Earlier this year when I had this issue Lou C recommended a chaser and was glad he did. Here is a picture of them side by side. And here is a post (
link) about cleaning up these treads in a difficult place (manifold block) without removing the sterndrive.
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