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Options for installing BF90 with a 25-in lower unit on a 20-in transom....

M

mitchbuck

Contributing Member
I have a SeaCat Sl1 power catamaran and this winter I have upgraded the twin Honda BF50s to twin BF90s. I have not installed the BF90s yet as I am waiting till spring up here in the northeast, but I also need to figure out what to do about the lower units since the BF90s have 25in lower unit and I have a 20in transom. I was aware that this was going to be an issue, but for the price that I got the engines I just couldn't pass them up. I see three options here:
1) Buy used 20-in lower units and sell mine.
2) Rebuild the lower unit with a 20-in driveshaft. I hear that you have to disassemble the whole lower unit though which is a PITA.
3) Install manual jack plates and call it a day. The only thing is I may have to jack the engine over 5inches up which is pretty significant.

To complicate things, the lower units on the 90s are the older 1997/1998 Mercury lowers, but from what I've read here I just need to change the shift shaft and some prop shaft hardware to get a newer LU to fit. I really just want to get some opinions before moving forward. Thanks in advance.
 
I am assuming you do not want to modify your engine.....

You could use a jack plate, which normally has some offset or an adapter something like this one. https://thmarinesupplies.com/collec...c-static-jack-plate-5-inch-vertical-extension

I think they are around $200.

You may need to check with the manufacturer on what size motors it will handle.

If you get 20" lower units, changing out the shift shaft is easy, however, you would have to also change the watertube at the base of the powerhead....which is not difficult, but you will have to remove the midsection to get to it.

You might investigate what it would cost to exchange the lower units with Atlantic Gearcase.

Mike
 
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I am not above modifying the engine. So in a nutshell, in order to install a 20-in lower unit/gearcase I have to remove the midsection/leg and cover to replace the water tube, remove the LU spacer, and then replace the shift shaft on the LU along with the prop hardware. That's it? Thanks for your help Hondadude!
 
Pretty much.... You will have to remove the studs that go through the extension and to order the mounting bolts for the lower unit. Your present unit is held on with nuts attached to the studs.

Make sure you get the appropriate shift shaft based on the combination of your engine and the new lower unit.

Mike
 
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