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Thank you mtown for the detailed explanation but with my current install I'm pretty sure I will need to remove my engine to fill the bottom holes (1989 Montauk with 1998, 90 hp Yamaha 2 stroke). I have some time this weekend and I'm pretty sure I can do it in my garage with a little engineering.

I'll post a picture later but the bottom mounting on my current engine is a slot as opposed to the top mounting which is 4 holes. Can anyone tell me if the new engines have the same configuration?

If the bottom mounting on new engines is now a 4 hole configuration instead of the slot it adds another level of complexity to make sure I get the bottom holes at the perfect height.

My second minor concern is I estimate the current blind hole is very close to where I will need to drill the new hole. I realize if the hole is filled properly it shouldn't matter but it makes me nervous that the drill bit could drift if they do overlap and I'm boring through two different materials.