How to swap out a 3.0l OMC for a 2.5l Mercruiser

rav931

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I have a 1990 Bayliner with a 3.0L OMC I/O that fairly rapidly adds water to the oil. Haven't had a chance to look what's the issue, but I'm sure it's nothing good. I also have a mostly complete mid 80's parts boat with a 120 HP 2.5L mercruiser I/O. I've almost finished rebuilding the 2.5L engine with a friend. Is it relatively easy to swap the 2.5L mercruiser in? I have essentially all the engine and out drive parts from both boats, so using everything from one or the other, or combining parts is possible. I would prefer to keep the out drive parts from the Bayliner as I'm pretty sure they are in better shape, but I'm game for anything. I also have a bunch of parts from a 3.0L mercruiser I/O if that helps with anything. If it's a do able swap, I'd really appreciate any detailed instructions that anybody would care to share. Thanks in advance for all advice.
 

kenny nunez

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If the OMC drive system is still good just re-use the OMC drive coupler and adapter housing. The OMC wire harness should also be swapped. The Merc. Manifold can be adapted with some exhaust hose and creative thinking. The throttle on the Merc pulls verses OMC that pushes. You may be able to reverse the the throttle action at the control box since Bayliner usually used a universal control box. Needless to say run the 2.5 before you install it to check it out and make final adjustments. Good luck
 

rav931

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Thanks Kenny,
I'm a bit of a newbie to all of this, so any advice is great. Trying to get as much info as possible before jumping into this all. I won't have the Bayliner at my place for a week or so yet, but I'll see what control box it has then.
The Merc is in my friend's shop at the moment. He's rebuilt a bunch of Chevy's in the past, so he was willing to help with this one. His business just has been crazy busy lately, so we haven't had a chance to finish the Merc. Will definitely run it for awhile before trying to install it though as you suggest.
 

Scott Danforth

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with the 3.0 sitting next to the 2.5. unbolt everything from the 3.0 down to the valve cover, short-block and oil pan.....move over to the 2.5. you have now converted a mercruiser 4-banger to an OMC 4-banger

I personally would not step backwards from a 3.0 to a 2.5. then again, I would pull all the OMC stuff and replace it with mercruiser or volvo
 

Bondo

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Ayuh,.... Welcome Aboard,.... As Scott says, ya strip the good motor to the Long Block configuration, 'n redress it with the OMC stuff,....

As complete bobtail motors, they are very, Very different,....
 

rav931

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Thanks Scott and Bondo.

I hear ya on switching everything to Merc or Volvo. Just trying to use what I have to get on the lake for a year or two. I figure once I have it running, I can take my time and find something better.

Is there anything I need to watch out for when reinstalling?

Thanks again,
Rob
 

Scott Danforth

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the 2.5 was not a stellar performer in anything larger than a 15' boat. so if you have a 17 or 18' boat, its going to be a dog
 

rav931

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Yeah, I kinda was assuming. At this point though we just would like to be able to cruise around the lake. We'll go for performance another year.
 
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