Mercruiser replacement

jwestjr

Seaman
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Jun 11, 2015
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I have a 1972 Starcraft Starchief with a 120 hp 2.5 litre Mercruiser. It has a cracked head and keeps oil in the bilge. I'm wanting to replace the engine. My question is should I upgrade to the 3.0? I believe it is a more modern engine with more parts availability. I see some of the 2.5 complete engines on ebay for fairly good pricing but am considering the later model. Will it interchange without any major modifications? I am aware of the outdrive shaft being a different length. I just bought an outdrive and foot last year from Stern drive engineering. Maybe I can just change the shaft without buying another outdrive.
Thanks for any input.
Johnnie.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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I would upgrade to a 3.0, however it isnt any more modern

the 3.0 is a stroked and bored 2.5, which started life in 1962 you can interchange them at will.

get a 1990 or older 3.0 and it will bolt in exactly where your 2.5 is. 1991 and later have the larger flywheel, 1-piece rear main seal, etc.
 

kenny nunez

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2 Xs on what Scott said, The only other thing that comes to mind is the drive coupler on the later model engines. You will better off replacing your old engine with a new marine long block. Comes with a flywheel, you will need a new manifold and starter with the staggered pattern, You should be able to re use the rest of the parts from the old engine. These engines have a reputation of thin castings in the water jacket area, the older the engine the more likely the chances of an internal leak with water in the oil pan.
 

fishrdan

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Jan 25, 2008
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Replacing it with an old 140HP would be easier than swapping in a 3.0L. The 140HP and 3.0L are close, but there are a lot of differences that need to be overcome, rear engine mount, flywheel, coupler, manifold and exhaust, carb, shift cables, etc, etc. Check the 3.0L year split Scott mentioned, as the older 3.0L's avoid many of the issues.

The 120HP and 140HP are almost identical externally, few differences that can be overcome easily, but the 120HP and newer 3.0L have many things different, that can be expensive to adapt. Even with a 140HP, I would want the entire engine with manifold, carb, etc. (120Hp and 140HP manifolds and heads are not the same...)

Best solution, find a rotted out boat with an intact running 3.0L and drive, and swap everything over, front engine mount to prop.
 

Scott Danforth

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the old 140hp is a 3.0. different labels, however 3.0 liters / 181 cubes, however you want to refer to it.

over the years, the heads have been swapped by GM..... sort of like Mr Potato head. some of the 3.0's had the same heads that the 2.5's had and visa-versa. it just depended on which plant had capacity to make which head at the time. two plants, two different casting numbers. Tonawanda vs Mexico. Manifolds were up to the marinizer.... Volvo, OMG, or Mercruiser all had their own castings.

find a 3.0/140/135/whatever you want to call it from 1990 or prior and it will drop in. no need to replace the drive, coupler shaft or anything.... in fact, if you strip the motor down to long block, it wont mater if you buy OMC, Volvo, Mercruiser

Note, the 2.5 died when the 2-piece RMS died. after 1991 all that there is is the 3.0
 

fishrdan

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Jan 25, 2008
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That's the fun of doing a a Mercruiser GM 4cyl swap/upgrade, 120HP, 140HP, 3.0L, 3.0LX. Figuring out what will work and what won't. If the 140HPO/3.0L is a complete bobtail from 90 and earlier it should be fairy simple, but there still could be hickups, shift cables, wiring harness.

Long block from 90 and earlier will present more issues, manifold, carb. The old 2.5 manifold will bolt on the engine and run, but the ports most likely are wrong, which affects performance. Some 3.0L engines used the 120HP head (go figure), but most have a different head than the 120HP.

New engine have a Mercarb manifold? Probably won't work with the old Rochester carb, different bolt pattern.

All those little bits add up, and it may be cheaper to buy a complete bobtail engine so you have everything to drop in.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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ports are the same size, gasket is the same part number.
 

fishrdan

Admiral
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Jan 25, 2008
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GM 4cyl marine manifold gaskets.

marine-manifold-gaskets.jpg
 

jwestjr

Seaman
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Jun 11, 2015
Messages
62
Thanks for all the input. Still shopping and looking. Seems the 2.5 is fading.
 
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