95 Mercruiser 3.0L 4Cyl to Diesel Swap?

[kaeden1100]

Hey guys, wanted to pick some brains if that’s okay.
Just pulled the motor out of my 95 Bayliner last weekend. 3.0L Mercruiser 4 Cylinder with Alpha One drive. Turns out the block is cracked and gonna need replaced. While I had it out, my stepdad, who helped me pull it, jokingly mentioned throwing one of the Mercedes 3.0L 4 Cylinder Diesel motors he has laying around in it. I laughed it off at first but then I got to thinking about it, and was just wondering if anyone had ever done anything like this, or if it would even be possible. The Diesel engine is out of a 90s Mercedes sedan I believe. Fully mechanical minus the starter. Any thoughts? Thought it might make for a pretty cool project.

 

[Rick Stephens]

Be a great little boat motor......

Have to make a bell housing or an adapter to fit up to the transom plate.
Must have a heavy flywheel which would need drill and tap to fit a coupler.

Lotsa engineering math to get it right. Might be easier with a jackshaft instead of a coupler. Attaching to exhaust will be another interesting need. And figuring out an interrupt method to get the boat out of gear.

 

[kaeden1100]

Be a great little boat motor......

Have to make a bell housing or an adapter to fit up to the transom plate.
Must have a heavy flywheel which would need drill and tap to fit a coupler.

Lotsa engineering math to get it right. Might be easier with a jackshaft instead of a coupler. Attaching to exhaust will be another interesting need. And figuring out an interrupt method to get the boat out of gear.

Wouldn’t it be possible to just do thru-hull for exhaust? Good point about the interrupt for getting the boat out of gear. Honestly don’t entirely understand how it works on the current motor.

 

[kaeden1100]

Be a great little boat motor......

Have to make a bell housing or an adapter to fit up to the transom plate.
Must have a heavy flywheel which would need drill and tap to fit a coupler.

Lotsa engineering math to get it right. Might be easier with a jackshaft instead of a coupler. Attaching to exhaust will be another interesting need. And figuring out an interrupt method to get the boat out of gear.

Also not super familiar on jackshafts. My understanding is that they’re basically an inboard motor with a shaft that connects the motor to the drive. How would that differ from my current setup? When I pulled the drive off the transom to pull the motor, it looked like it was just essentially a driveshaft that connected to the flywheel through that adapter.

 

[matt167]

You will actually find parts to marinize it in Germany, and you will find parts to convert the bellhousing to SBC from the offroad groups. You will have to run a through hull exhaust, and then deal with a gear ratio that is wayyyyy wrong.. That little diesel does not go much past 3k RPM IIRC, it also only makes about 100hp. The interupt to get it out of gear, only has to pull the shutoff, which is a vac operated system. The same switch that works the gas engines ignition interupt, can activate the shutoff valve with a solenoid

Marinized that would make a killer sailboat engine, and that is the type of setups you actually will find the parts for

 

[kaeden1100]

You will actually find parts to marinize it in Germany, and you will find parts to convert the bellhousing to SBC from the offroad groups. You will have to run a through hull exhaust, and then deal with a gear ratio that is wayyyyy wrong.. That little diesel does not go much past 3k RPM IIRC, it also only makes about 100hp. The interupt to get it out of gear, only has to pull the shutoff, which is a vac operated system. The same switch that works the gas engines ignition interupt, can activate the shutoff valve with a solenoid

Marinized that would make a killer sailboat engine, and that is the type of setups you actually will find the parts for

So you’re thinking I’d actually lose performance swapping to this?

 

[alldodge]

The diesel will weight more, don't know what max rpm would be

 

[matt167]

So you’re thinking I’d actually lose performance swapping to this?

for sure your loosing ~40hp. Mercruiser does actually sell Diesel Alpha One's so you could get the leg from a diesel model and it might work ok.. The caveat is SEI does not sell a lower with the correct ratio, so you will only find it NEW from Mercruiser ( $3,500 price range )

With the 3.0L leg, you will be missing about 2k rpm of the upper end, so it would be like running at 1/2 to 3/4 throttle at full throttle

 

[Rick Stephens]

Also not super familiar on jackshafts. My understanding is that they’re basically an inboard motor with a shaft that connects the motor to the drive. How would that differ from my current setup? When I pulled the drive off the transom to pull the motor, it looked like it was just essentially a driveshaft that connected to the flywheel through that adapter.

The Merc sterndrives are designed to line up and match the bellhousing of mostly Chevy engines. That match is within a very tight tolerance when the motor is directly attached to the transom plate. The only variable then is up and down angle on the front of the motor. Cobbling up a mount/bellhousing to fit would be a bear. On the other hand, if you were to mount the motor a foot further forward, you can then mount the motor independent of the transom and just use a jack shaft with an extra u-joint in it.

Be more fun with an automatic transmission so you could change gear ratio as you go

 

[Scott06]

So you’re thinking I’d actually lose performance swapping to this?

Yes , and as pointed out gear ratio is different for a diesel. Either find another 3.0 or likely the boat was offered with a 4.3 and step up in power.
@Rick Stephens can tell you how to do it as he is the resident 3.0 to 4.3 jackalope ( or did u use a stronger term) expert.

merc makes diesels but more popular in Europe and the workboat market

 

[Scott Danforth]

The reason for watercooled exhaust has been glossed over here. Lots of work and cash to avoid spending $300 on another 3.0

 

[matt167]

If I knew the exact engine model I could find the marine parts. But for instance, the earlier OM617 conversion kit, is about 2,000 euro and it would have to come from europe.. The 1990's brought the OM603 and OM605 and OM606... However for the 3.0L OM606 that I think it is, is 1,316 euro for the manifold alone.. Look up Drinkwaaard Marine

But these are sailboat/ workboat type conversions

 

[QBhoy]

It’s actually something that’s been done often over here. The Merc 3.0 and bmw 3.0 diesel have a very similar rpm range to the gm petrol engines like this. That’s why they are often used. There is a company that specialises in it here and make bell housing adapters to suit. Not sure it would be for me though.