BF90A gearbox repairs

[Big Blue]

Good afternoon.

I have 2 BF90A's on my boat and have somehow managed to damage a gearbox. I stripped the box, and found the pinion gear was chipped badly.
It's a 13 tooth tapered helical gear. I am having trouble lo ating spares here in South Africa. Would I be able to replace the entire lower unit with a mercury one, and if so, which one? Are the parts interchangeable?
Any help would be greatly appreciated. I ha e read a number of posts here, and see that Hondadude is the man. Hoping to reach this wizard! 🤘

[Alan Hicks]

First, you need to realize there are 2 completely different lower units used on the BF90A's. There's an early one used through 98 (I think), and the other on later engines. Most easily identified by the rear most bolt that holds the back of the lower unit to the housing above. The early one is inserted from above, the later from the bottom.

The internals are completely different between them. The water pumps are even different with the earlier using a metal housing, and the later using plastic. They are NOT interchangeable.

All that said, for the price of a used lower unit, compared to what you are going to spend buying gears and the time you're going to have invested in setting them up correctly, that would be how I would proceed. Just make sure you are talking about the correct one when selecting a replacement....

[Big Blue]

First, you need to realize there are 2 completely different lower units used on the BF90A's. There's an early one used through 98 (I think), and the other on later engines. Most easily identified by the rear most bolt that holds the back of the lower unit to the housing above. The early one is inserted from above, the later from the bottom.

The internals are completely different between them. The water pumps are even different with the earlier using a metal housing, and the later using plastic. They are NOT interchangeable.

All that said, for the price of a used lower unit, compared to what you are going to spend buying gears and the time you're going to have invested in setting them up correctly, that would be how I would proceed. Just make sure you are talking about the correct one when selecting a replacement....

Alan Hicks

Many thanks for your response. At a bit of a loss here.
The last bolt, at the rear on my motors, is inserted from the bottom. I have however had the legs off, and the water pump housings are stainless steel.

[Alan Hicks]

Alan Hicks

Many thanks for your response. At a bit of a loss here.
The last bolt, at the rear on my motors, is inserted from the bottom. I have however had the legs off, and the water pump housings are stainless steel.

Yes, sorry. Apparently memory not what it used to be. It's the rear bolt, but not the one that holds the lower unit. It's the one that fastens the trim tab. Early goes in from the top, late goes in from the bottom.

See# 23 here
https://www.boats.net/catalog/honda/...case-gear-case

It's not there on later motors, nor is the metal water pump housing. -Al

[Big Blue]

Ok. Reading you loud and clear. Trim tab bolt goes in from the top.
Where does that leave me?
Many thanks

[Alan Hicks]

As mentioned, faced with replacing those gears, I would be looking for a used lower unit. When looking at those, you're looking for a picture that shows that bolt, or a hole for it, whatever.

When you do searches looking for a BF90 lower unit, you're going to see both the new AND old styles. Now you know how to identify which is which, and the one you need.

Worth mentioning maybe, is how I learned this lesson - after receiving an old style lower unit to replace the new style (2002) I needed. That was an expensive lesson for me and the very understanding fellow I bought it from. We BOTH thought all the BF90's were interchangeable..... -Al

[Big Blue]

Hi Alan
Thanks. I shall be checking things out properly!
Do you know which mercury lower units will fit? Mercury spares are way more plentiful here. If I could fit a mercury lower unit, or even build over the parts, that wo2be an absolute win. Hence my question. Do you know which will fit on my motors.
Many thanks

[Alan Hicks]

I'm sorry. That's a move I haven't tried (yet). I know Merc made these for Honda and a few others (Yamaha and Suzuki?), but I don't know if there's a direct replacement available that's used on a production Merc. engine. You would almost need them side by side, and be prepared to do a LOT of measuring....

Oh. last thing, the BF75's are the same (model year for model year), in case you didn't already know that. -Al

[Big Blue]

Many thanks Al. Much appreciated.Let's see how far I get. I hope to have 2 merc boxes soon, to measure and fit

[Alan Hicks]

First thing I would be looking for is that bolt....

[Alan Hicks]

I'm always looking for dead Hondas to buy and repair. Ran across this a few minutes ago. Thought it interesting as he does mention Merc/Honda interchangeability - and this price seems pretty reasonable for a lower that doesn't appear to have much corrosion. He mentions wrong splines, but I don't know if he's talking about drive shaft splines or shift shaft splines.

In any case you might want to get in touch with him....


https://gainesville.craigslist.org/b...267814883.html