330 to 260 swap

[Jeff8330]

Hello all, I have a 1981 310 sedan Bridge. She's a great boat. Haven't had her long, but the more I tear into this boat, the more I love it, and the build quality.
Anyway, she's powered by twin 330 (454) mercs. Mated to B&W velvet drives. The motors are very tired, probably will need rebuilds soon. I'm looking at my options. I have access to a pair of early 80s 260 mercs. I know these are just gm small blocks, and I can get them for almost nothing from a very good friend who will have to dump them soon anyway due to storage issues. They are low hour rebuilds that were just pulled out of the boat that had rot issues. This boat isn't THAT heavy, and I know two small blocks would pull it out of the water nicely. My transmissions are fine, bullet proof as we all know. What would I need to do to have a set of small blocks bolt up to my current transmissions? Bellhousing issues?? I'm new to the world of inboards and am currently learning the differences in transmission setups. However, not new to marine engines. What am I looking at here? I'm assuming it would be almost a direct swap. My 87 Chris craft COMMANDER 333 had two merc 270s in it. That boat was so much better on fuel and it was more than enough power for me. It was just as heavy as this boat, and very comparable as far as size and vintage. Anyway, thanks in advance...... Wheels in my head are turning so you might get alot of questions if I proceed.

 

[iBoat Skipper Doug]

Jeff,

I will continue following this threat in hopes of learning through you.

Just as a bit of background, both the 454 cubic inch V8 and 262 cubic inch V6 started their lives as GM blocks. The 454 was a 7.4 liter GM long block V8 and the 262 was a 4.3 liter GM small block V6.

After the above, I am now guessing from memory and my memory ain’t great, and I’ve only 2nd had experience from people going the other way, replacing a 5.7 liter with a 8.1 liter. Engine mounts on the 7.4 and 8.1 vs 5.9 and 5.7 are 4” further away and the exact same width.

If I was doing this swap, I might head to a automotive forum or two and find someone who shoehorned a 7.4 or 8.1 liter GM motor into a car with a GM 4.3, 5.0 or 5.7 liter motor and ask them how things lined up.

 

[Jeff8330]

I thought the 260 was a 350 (5.7)?

Either way, I'm sure I can make it work, and I'm sure it's been done, I just want to hear from someone that's done it. Or, has enough experience with this motor/trans combo to know from experience what will need to be done. Thanks for the response Doug. We will both be learning. But yeah my 330s are 454 big blocks, pretty sure the 260 is 5.7 350. Either way, the 230 (5.0) is externally exactly the same as a 350 so it doesn't matter much.

 

[Bt Doctur]

are both330`s standard rotation or is one reverse rotation

 

[Jeff8330]

I believe one is reverse rotation.