Powerhead swap between different sized outboards?

[scout-j-m]

Particularly on the makes that are 30hp and under, on model groups that share identical part numbers for the lower unit, gearcase, and cowling, can the powerhead as well as fuel delivery and electrical systems be swapped? Seems like since you swap out everything that makes that model unit unique it would work fine. Also is the statement I have read regarding modifying an outboard hp being illegal by the EPA true? I searched for it online but all I found were posts on forums making the statement. I'd like a link to see it myself if someone has it.

An example of what I'm talking about is say you have 15hp tohatsu or mercury with a blown powerhead. If you found all the parts and a powerhead for a 20hp of the same year on eBay could you put those on the 15 and be good to go but now have a 20hp instead.

 

[MTboatguy]

Should be able to, what years are we talking about? Although I don't want to break any laws, the EPA is the least of my worries when I am out on the lake enjoying a day of fishing.

 

[scout-j-m]

I didn't have any specific years in mind although a few of the ones I checked for part compatibility were from the last 7 to 8 years. I'm not in a situation like this. I'm just contemplating getting a new boat which is a small duck/fishing combo and rated for a 25hp. I'd like to get the boat newish and pair it with a used 15 or 20hp. Just thought I could potentially build my own motor in a sense if the right parts were available. I could probably afford a new 9.9hp but that would be pretty lacking on a boat rated for 25.

 

[ondarvr]

It is possible with some makes and models, but it's typically easier and cheaper to buy what you actually want and need, than to build it after the fact.

 

[scout-j-m]

It is possible with some makes and models, but it's typically easier and cheaper to buy what you actually want and need, than to build it after the fact.

I know and I agree. It was really more of a hypothetical question I guess. I was just curious in the unlikely event I did just run across what I described above.

I am about to attempt my first rebuild on a 70hp I have and, if I can do it successfully, I will be feeling pretty confident about my mechanic skills lol.

 

[Silvertip]

Even if the interchange is possible physically, the gear ratio may not be the same thus creating another performance problem.