Engine swaps

[Rranger]

I have a 75 Slickcraft 22' I inherited. Beautiful hull. Has a complete 225 OMC setup in it, only 597 hours on engine runs great. I'd like to switch over to Mercruiser. My understanding is if I can find a 302 Chevy mercruiser donor boat key to prop, I should be able to strip all OMC stuff off the engine, replace with mercruiser parts fix hole and be good to go? Are motor mounts the same and is the switch that easy?
Other option is a complete key to prop switch to a 351 ford mercruiser. Fix hole. Again are motor mounts the same or is stringer/motor mount glassing a issue?

 

[BlueBell]

only 597 hours on engine runs great

Why?
If it "runs great" why fix it?

Pictures?

 

[Rranger]

When I have to make the inevitable repairs, I’d rather be looking for Mercruiser or Volvo parts. And the right donor boat it would be financially reasonable.

 

[C. Dog]

If it's an OMC stringer mount is will have a large hole in the transom sealed with a diaphragm. Because these are mounted to heavy stringers only and not the transom, it is possible that the transom is built too lightly to take a Mercruiser without reinforcement.

Another way out is to modify for an outboard on an aluminium or composite transom add-on.

As regards engines, Ford V8s have long been discarded for marine use because they are neither up to marine loads or direct salt water cooling.

Also GM haven't built a 307CuIn V8 since 1969 and I'd expect any surviving engines to be on excessively oversized pistons for marine. Mercruiser give you one cylinder rebore to 0.030" OS then recommend throwing the block away.

If finances permit look for a V8 Mercruiser Alpha 1 MR or Bravo. They are ridiculously expensive to maintain compared to an outboard and my preferred option (sight unseen) would be to use the stringer while collecting what you need to convert it to outboard.

Good luck with it.

 

[C. Dog]

As to putting MC accessories on an OMC, probably but not certainly, mounts are very different and I'd assume you'd have an hydraulic ram on the front engine mount for longitudinal trim? OMC Stringers were always a nightmare to maintain, not the smartest design and sunk a lot of boats.

 

[Rranger]

If it's an OMC stringer mount is will have a large hole in the transom sealed with a diaphragm. Because these are mounted to heavy stringers only and not the transom, it is possible that the transom is built too lightly to take a Mercruiser without reinforcement.

Another way out is to modify for an outboard on an aluminium or composite transom add-on.

As regards engines, Ford V8s have long been discarded for marine use because they are neither up to marine loads or direct salt water cooling.

Also GM haven't built a 302CuIn V8 since 1969 and I'd expect any surviving engines to be on excessively oversized pistons for marine. Mercruiser give you one cylinder rebore to 0.030" OS then recommend throwing the block away.

If finances permit look for a V8 Mercruiser Alpha 1 MR or Bravo. They are ridiculously expensive to maintain compared to an outboard and my preferred option (sight unseen) would be to use the stringer while collecting what you need to convert it to outboard.

Good luck with it.

I have an outboard engine pod. And a 125hp mercury outboard probably a little small for this Slickcraft boat considering it’s pushing around this 22’er. Do you think the 125 outboard would get a guy around. I’m not racing or anything just heading to a fishing spot to troll.

 

[C. Dog]

Get the propeller right and a desktop fan motor will push you around. I'm in Australia and don't know the boat, but I'd imagine a 125 Merc would get you something like 15-20 knots on a boat that size, assuming correct prop, and depending hull shape and boat weight.

At least once the job is done it will be simple to repower. XL (25") motors go better on pods because the powerhead is further from the drink. The cavitation plate (or anti-ventilation plate) is normally placed at keel level with an outboard mounted on a transom, but pod mounted are generally set around 2" above keel level, as found with a long straight edge.

 

[C. Dog]

If the 125 is carburetted it will not like trolling. Mercury had a moment of genius and left an idle circuit out of the bottom carburettor in the name of low speed fuel economy, and also due to the nature of vertical shaft two strokes, they tend to coke up lower cylinders first at prolonged low speed running. Because they idle on two cylinders they run like a hairy goat and are prone to stalling. I am not a fan of this feature, good strong outboard otherwise and there used to be a kit to fix the idle.

 

[Rranger]

If the 125 is carburetted it will not like trolling. Mercury had a moment of genius and left an idle circuit out of the bottom carburettor in the name of low speed fuel economy, and also due to the nature of vertical shaft two strokes, they tend to coke up lower cylinders first at prolonged low speed running. Because they idle on two cylinders they run like a hairy goat and are prone to stalling. I am not a fan of this feature, good strong outboard otherwise and there used to be a kit to fix the idle.

I’d have a kicker its a 9.9 Yamaha 4 stroke for the trolling pod has a mount for it to.

 

[C. Dog]

Good idea, the 125 is missing the idle air bleed jet on the bottom carburettor from memory, so not hard to fix the idling if necessary. Get one from the top carby on a dead motor and fit to bottom carb on yours. It has been about 30 years since I worked on them however and the memory is a bit dim.

 

[Rranger]

I have the pod but when I get into it after the engine drive removal I will decide on the pod or cutting out a hole to mount the outboard on the transom and adding a splashwell and the appropriate bracing. I've done it before and it might be easier than using a $1500 pod.

 

[C. Dog]

Outboards tend to last longer transom mounted.

 

[Rranger]

I haven't measured the transom but probably close to the same height as the one on the Sangstercraft that I switched from I/O to Outboard, and made it for hanging a 25" shaft engine. I'm wondering if I could get away with cutting out this Slickcraft for a hanging a 20" shaft?

 

[silvestre]

Hi! Switching from an OMC to a Mercruiser engine is definitely possible but not as straightforward as a direct swap. Motor mounts and stringers may need modifications to fit the new engine properly. A 302 Chevy or 351 Ford Mercruiser would likely require some glasswork to the engine compartment and stringers. It's advisable to consult a professional marine mechanic to assess the specific requirements for your Hull. Proper mounts and alignment are crucial for safe and efficient operation.

 

[Gandalfe]

Sounds like the OB conversion is your best bet. Having put a Mercruiser where an OMC once lived before it's a transom rebuild, glass work and mounts. Additionally I/Os are pricey to maintain in salt water even with closed cooling. Properly propped that old Merc will move you along just fine.