OMC to Mercruiser Conversion

[Jrobbs3]

I have a 1977 OMC 235 (Ford 351W) w/stringer outdrive set-up that I pulled from an older tri-hull boat, it is in rebuilt and working condition. I've been looking for a boat to put it in to and I have come across another older tri-hull boat (which is in great shape for it's age). This boat has a 1975 Ford 351W but with a Mercruiser outdrive. The engine needs a rebuild, but the outdrive is supposedly in working condition. I'm trying to determine if either of the following options are possible:

1. Remove blown engine, drop in my rebuilt engine, and modify as necessary to work with the Mercruiser system.

2. Remove all Mercruiser components and replace with all of my OMC components.

Any advice on whether either of these options are possible or how difficult either of these would be is appreciated.

 

[Bondo]

I have a 1977 OMC 235 (Ford 351W) w/stringer outdrive set-up that I pulled from an older tri-hull boat, it is in rebuilt and working condition. I've been looking for a boat to put it in to and I have come across another older tri-hull boat (which is in great shape for it's age). This boat has a 1975 Ford 351W but with a Mercruiser outdrive. The engine needs a rebuild, but the outdrive is supposedly in working condition. I'm trying to determine if either of the following options are possible:

1. Remove blown engine, drop in my rebuilt engine, and modify as necessary to work with the Mercruiser system.

2. Remove all Mercruiser components and replace with all of my OMC components.

Any advice on whether either of these options are possible or how difficult either of these would be is appreciated.

Ayuh,...... The quickest, easiest sequence would be, strip yer 351 of all of the OMC stuff, 'n swap just the Long Block into the new boat, usin' the Merc accessories from the blown motor,.....

 

[kenny nunez]

2Xs on Bondo’s suggestion. As far as the original Mercruiser engine I would give a lot of thought about the manifolds. They may be the reason the that something is wrong with the Mercruiser. I suggest new manifolds.
You can keep your OMC distributor because some Mercruisers also used either Mallory or Prestolite. The throttle action on the Mercruiser pulls where the OMC pushes to open. Use the complete Mercruiser wiring harness On the new engine. The OMC should have a 4 barrel Holley and the Mercruiser will probably have a Rochester 2 barrel. Needless to say keep the Holley set up.

 

[Jrobbs3]

Ayuh,...... The quickest, easiest sequence would be, strip yer 351 of all of the OMC stuff, 'n swap just the Long Block into the new boat, usin' the Merc accessories from the blown motor,.....

Good idea. I should probably confirm the condition of all of the Merc accessories from the blown motor, or I could be on the hook to buy all new. I just bought all new for my OMC, would hate to do that again. I might have to walk from this one.

 

[Jrobbs3]

2Xs on Bondo’s suggestion. As far as the original Mercruiser engine I would give a lot of thought about the manifolds. They may be the reason the that something is wrong with the Mercruiser. I suggest new manifolds.
You can keep your OMC distributor because some Mercruisers also used either Mallory or Prestolite. The throttle action on the Mercruiser pulls where the OMC pushes to open. Use the complete Mercruiser wiring harness On the new engine. The OMC should have a 4 barrel Holley and the Mercruiser will probably have a Rochester 2 barrel. Needless to say keep the Holley set up.

I'm pretty sure the manifolds are the issue on the Merc engine. Seller tells me there's water in the oil and he can't figure out why. I had the same issue with my OMC and it was the manifolds.

Can I still use my Holley set up from the OMC even though the throttle action is different? Also wondering if it's ok to use the same intake manifold off my OMC?

 

[kenny nunez]

The Holley to me is a better set up and the engine should perform good with the Mercruiser. There is a hole on the upper part of the Holley throttle lever to attach the Mercruiser throttle cable. You may have to make up another bracket to anchor the cable.