Logo

3.0 I4 swap to 4.3 V6 - motor mount question

67-LS1

Member
I’m looking at a boat with twin 3.0 liter inline 4 engines with Alpha drives. The engines and drives will need replacing soon but I love the boat.
I notice that the 3.0 engines have a single point front motor mounts. They do not sit on or hang off of the stringers. There is just a small raised block glassed into the hull under the front of each engine. There is no stringer between the engines.
I’m thinking when they do need replacing, and depending on the spacing of the transom cutouts I would like to swap them for 4.3 V6 engines but I’ve never seen a single point front mount for a V6, or V8 for that matter.
Any suggestions, tips, leads ?
Thanks
 
I’m looking at a boat with twin 3.0 liter inline 4 engines with Alpha drives. The engines and drives will need replacing soon but I love the boat.
I notice that the 3.0 engines have a single point front motor mounts. They do not sit on or hang off of the stringers. There is just a small raised block glassed into the hull under the front of each engine. There is no stringer between the engines.
I’m thinking when they do need replacing, and depending on the spacing of the transom cutouts I would like to swap them for 4.3 V6 engines but I’ve never seen a single point front mount for a V6, or V8 for that matter.
Any suggestions, tips, leads ?
Thanks
You would have to glass in the side mounts for the 4.3 s, ormally build up of plywood and glassed in place. I would measure very carefully as the V engines are wider by quite a bit . You would also need an exhaust wye pipe for each transom assy. Drive ratio is different but seems you will be replacing those anyways.
 
I find it hard to believe that a hull made for inline engines could possibly fit V type engines (like a Bertram Moppie, etc). Perhaps a better fit are the I6 engines. You'll still need new mounts fabricated. The Chevy I6 is one of the best engines ever created by man. Super reliable, lots of power and torque, excellent parts availability, smooth, quiet, just superb. Mercruiser 165 is the 250 cubic inch version. You can probably find a couple good used running take outs for thin money. Unsung hero of the boating world.
 
Agreed on the issue of space between engines, the V6 is shorter than the inline 4 but much wider, also allow for being able to change spark plugs, exhaust manifolds and (God forbid lol) the starter on the port side one. If you can get them and related parts, the Chevy inline 6 is an excellent alternative but to my knowledge they were mated to the older style MerCruiser drives, not sure if they can be made to work with later model Alphas, you'd have to aske a vintage Merc expert about that.
 
The 165/3.7 was fit to Alphas. It's not too hard to find complete setups...drive and engine...for under (sometimes far under) $1000. There one near me that is the entire transom cut out of a boat with a 3.7 and drive for $250.
 
The 3.7L was the MerCruiser aluminum 470 junk engine. It is also taller than a 2.5/3.0L. That could cause motor cover clearance issues.

As far as the inline 6 (194ci/140hp, 230ci/150hp, 250ci/160 & 165 hp in that order) were all hooked to #1 drives.
The 165 (1970 & newer) can easilly be adapted to Alpha transom plate with a bell housing & closing plate from an Alpha 4 cyl application.The exhaust riser will need to changed also to the same 4 cyl application with the port direction offset.

Now, here are the sticky points:
1) Throttle cable won't fit as originally routed/mounted along the port side of the riser because of the new riser offset. I got around this by rounting the cable around from the front & machining the mounting studs for the cable in reverse as the holes sre sized differntly. Slack will need to be allowed for the entire cable to move freely.
2) Because the shift cable interface/shift interuptor assembly was on the transom plate of the #1 and engine mounted on Alphas (integral to the valve cover on 2.5/3.0L and never offered with an I-6 inline) you will need to source an assembly from a V engine that had them mounted to the exhaust riser. I mounted it directly to the transom just above the steering tiller and cable looped easily down the the drive.
3) Old (big) starter will not fit thru the closing plate at the front of bell housing. You will need to cut the hole larger.

FYI: The 292 ci was only used in 1969 with a #2 drive, rated at 200hp, 4 barrel carb & manifold. That newly designed (in 1969) manifold with an adaptor plate to 2 barrel was used on the 250ci/165hp starting in 1970 thus replacing the 1967 thru 1969 & some early left over 1970 160hp models.
 
Wow, those all sound like a lot of work and work. Maybe I’ll just replace the 3.0s and motor on.
I see 3.0MPIs with transom assembly, gimbal and an Alpha drive for under $10k.
Or go to a single Merc 425hp, 496/Bravo 1XR for $30k.
 
Wow, those all sound like a lot of work and work. Maybe I’ll just replace the 3.0s and motor on.
I see 3.0MPIs with transom assembly, gimbal and an Alpha drive for under $10k.
Or go to a single Merc 425hp, 496/Bravo 1XR for $30k.
What boat is it in?

It does seem odd to put 2 drive units in for a total of 270 hp unless you need dual drives for docking due to boat size /config.

You didnt say what the spacing is on the existing drives. if the boat manufacturer offered this boat with a 4.3 option it may be not as much work as you think. Certainly converting to a single drive is rework of the transom. If you can do the work yourself its not that bad $$ wise, but if you are paying for it ....
 
Back
Top