Mercruiser 120 Swap for Bigger Engine

[MadsBoat]

Hi Guys, I have an 18' 1973 Sierra Cuddy Cabin. Its a really cool looking little old cuddy cabin. It currently has a Mercruiser 120 in it that does run. I am restoring the boat and replacing the stringers, transom, bulkheads and interior. In the process of the tear-down I got the idea that this boat needed to be a bit of a sleeper and could use a bigger engine. What is the most reasonable engine swap? I have considered a 4.3 Vortec for the boat. Maybe something even bigger? In this situation, will it be necessary to swap out the outdrive too. I am open to any and all suggestions on how to do this. Also, if you have any ideas on where to get a reasonably priced engine (preferably used), I would appreciate that too.

 

[Scott Danforth]

I would not hesitate putting in a 4.3 liter. yes, you will need a different drive as well. I would look for a donor boat and get the 4.3 complete from carb to prop. look into some of the 205hp or 220hp versions. Plenty of boats available with good motors and bad hulls. just look at craigslist. they will be listed as "needs tlc"

then sell your 120 to recoup some of your costs.

 

[Bondo]

In this situation, will it be necessary to swap out the outdrive too.

Ayuh,.... Welcome Aboard,.... The only other motor that'll run right with that outdrive is the 3.0l,....

Any other motors, need a matchin' drive ratio,....

 

[Rick Stephens]

I'm doing a full swap over right now to a V6 with a complete outdrive. Find a donor boat with everything or it is too costly to contemplate.

 

[MadsBoat]

Thanks guys! If I find a donor boat and pull the engine and outdrive, will the bolt holes through the transom be different than what is there? Also, if I do the full swap is there any specification that will help me to align the new engine/outdrive properly?

 

[Rick Stephens]

There are a couple more holes for newer transom plates. No problem making things work, just have to lay them out and drill away.

On alignment, the rear of the flywheel cover/engine sits on the transom plate. Same for inline and V engines. Everything else works off of that. Going from a crossmember nose mounted 4 cylinder to a stringer mounted V engine means you'll be fiberglassing in new mounts for the side motor mounts. Depends on which year V6 you get as to how tall those are in relation to the transom plate. Mine are 2.5 inches lower than the transom plate mount points. This means I am building up my stringers by something a little over 4 inches. The V6 I got out of a donor boat has liquid motor mounts, I have no clue if the solid mounts are a different height or no.

4 cylinder is lengthier than the V6. But the V6 is considerably wider and somewhat taller with exhaust risers being about 29 inches side to side. THat way you can rough in whether it fits inside your doghouse.

Alignment is getting the height of the new sidemounts close and then using the motor mount adjusting nuts to raise and lower the fron of the motor to align to the gimbal. That's it. Nothing special of 'spec' about it. It has to be the right height, period.

RIck

 

[Bondo]

Going from a crossmember nose mounted 4 cylinder to a stringer mounted V engine means you'll be fiberglassing in new mounts for the side motor mounts. Depends on which year V6 you get as to how tall those are in relation to the transom plate. Mine are 2.5 inches lower than the transom plate mount points. This means I am building up my stringers by something a little over 4 inches. The V6 I got out of a donor boat has liquid motor mounts, I have no clue if the solid mounts are a different height or no.

Ayuh,.... The way I find that point in space, is,.....
Mount the transom assembly to the hull,....
Hang the motor on it's rear mounts on the transom assembly, 'n slide in the alignment bar,...
Align it perfectly, 'n with the motor mount adjustment screws 'bout centered in their travel,.....
The bottom face of the motor mount pad where the lags go though, That's the top of the hull's motor mount blocks,....

 

[Rick Stephens]

Ayuh,.... The way I find that point in space, is,.....
Mount the transom assembly to the hull,....
Hang the motor on it's rear mounts on the transom assembly, 'n slide in the alignment bar,...
Align it perfectly, 'n with the motor mount adjustment screws 'bout centered in their travel,.....
The bottom face of the motor mount pad where the lags go though, That's the top of the hull's motor mount blocks,....

Yup,

That'd work.

 

[MadsBoat]

No Title

Ok guys, I found a 1998 4.3 Engine that came out of a Regal 202SE - Supposedly rebuilt and only has 15-20 hrs on it. I can get the engine with the shifter, transom plate, gimbal, steering, trim pump and outdrive for like $900.00. It comes with a Yamaha outdrive. It seems like most guys prefer the Mercruiser. Will there be added complications making this work where a mercruiser outdrive once was? If I buy just the motor, I assume that all the mercruiser parts would just bolt right up if I could find the stuff for the 4.3? Is $900.00 for all the parts a reasonable price?

 

[Bondo]

Ayuh,.... No way in 'ell I'd put a Yamaha anything into an I/O boat,....

Yamaha was imported only a couple of years, then they abandoned the North American market,....

Nothin' on that Yamaha dressed motor will work with a Mercruiser drive,....

The only thing a Merc 4.3l, 'n Yamaha 4.3l share in common is the Long Block,.... Everything added to the Long Block, is different,....

 

[Rick Stephens]

+++ to what Bondo said. Not a good choice for your needs..

 

[MadsBoat]

that's kind of what I thought. I have tinkered with some mercruiser stuff and really have no desire to mess with Yamaha or Volvo.

 

[Scott Danforth]

Volvo still supports their product. I would have no problem using either Mercruiser or Volvo.

 

[fishrdan]

What you're looking for is a complete engine/drive from a rotted hull, one that you have to pull the engine/drive out, or break out the sawzall to cut it free from the transom. This will give you all the small bits and pieces needed to install it back into your boat.

If you buy one that someone else has pulled, they could have left behind parts that you need, left behind parts that will be expensive to replace. Plus, if you pull it, you can document where everything came from and where it needs to go back.

 

[Rick Stephens]

if you pull it, you can document where everything came from and where it needs to go back.

That takes all the fun out of guessing where that little black doohicky goes.

 

[fishrdan]

That takes all the fun out of guessing where that little black doohicky goes.

Oh.... You'll still be guessing where that little black doohickey goes, no matter how well the uninstall is documented...

 

[MadsBoat]

There is always a doohickey, OR TWO, that make it to the glove-box for later installation - - once you discover what it was for... I have been able to locate several older straight 6 engines locally. its amazing what you can find when you take a drive looking for junk. I would prefer a 4.3 v6 for sure, however, I keep coming across these older straight 6 engines. what are your thoughts here? won't the straight 6 be compatible with my existing mercruiser outdrive? Bondo mentioned that the 3.0 engine would work, and I assume that the straight 6 is the 3.0 engine?

 

[fishrdan]

The 165 HP 6 cylinder will bolt up to your existing transom shield, but the drive's gear ration will be wrong. In your vintage of boat, the "3.0L" was a 140HP 4 cyl 181CI engine, pretty much the same engine you have now, just bored and stroked,,, and uses the same drive ratio.

That vintage of engine were crank rated, not prop rated like modern engines, so a late model 130-135HP 3.0L will have more HP compared to an old 140HP. Your old 120 HP probably puts about 100 to the prop, and the old 140HP would put 120 to the prop, while a modern 3.0L puts it's rated HP to the prop 130-135HP. Sooo, if you swap to a modern 3.0L, it should net you about 30HP over what you have now. If I were to swap to a modern 4cyl, I would make sure to get a LX engine, or at least not the lower performance engine with the siamesed exhaust port head. This family of engines came with many different configurations, head, compression, etc., designed to raise or lower HP. Along those lines, if you get a modern 200HP V6, you'll be doubling the boat's current HP.

FWIW, there is also a 3.7L 165HP 4cyl engine, but it's a problem ridden failed design, ditched early on by Mercruiser, so avoid this one.

 

[MadsBoat]

I appreciate all your help guys. I have spent a lot of time looking for a good 4.3 V6. So far I have been unsuccessful in finding something that isn't too expensive. I will keep on it. is there a specific range of years that I should be looking for in the V6? I have stumbled upon a few V8's... Comments??

 

[GM165]

The 165 HP 6 cylinder will bolt up to your existing transom shield, but the drive's gear ration will be wrong.

Isn't he swapping the outdrive as well? I would think a 165 with outdrive would be a bit easier swap than a 4.3L and outdrive. Just need to make the doghouse a bit longer, but not any wider. If he finds a good 165 6 cyl, why not go for it?