120/140 motor question.....

[p3ghost]

Hey all,

I have a '78 starcraft that I'm trying to get back into the water. I got it with a 140 hp merc with a cracked block. Thru my searching I have acquired two 120 hp motors in lots of pieces.

Here's my question:

Can I use the good 120 hp block, pistons, etc and put the 140 hp head, exhaust manifold, and carb on it? I don't have the old 140 block anymore to compare casting numbers. I'm assuming the 120 and 140 blocks were the same, but definitely don't want to make an *** out of myself.

Oh...the block is a 2.5L, 153 CID, GM casting number 3915991

Thanks in advance for any advice you're willing to share

Chris

 

[dozerII]

Re: 120/140 motor question.....

Love these old Starcrafts with the 120/140, but I can't make that call for you. Honestly you will be better off asking in the Mercuiser section, Bondo, Achris, or Don S will be able to help you.

 

[Bwana Don]

Re: 120/140 motor question.....

off to the Mercruiser forum for you.

I think (uh oh) the blocks are different. The bore is bigger on a 3.0 as is the stroke. Let's go to the Mercruiser form eh. Experts live there. Lots of parts interchange.

Get all your info lined up. year, casting #'s etc.

 

[North Beach]

Re: 120/140 motor question.....

off to the Mercruiser forum for you.

I think (uh oh) the blocks are different. The bore is bigger on a 3.0 as is the stroke. Let's go to the Mercruiser form eh. Experts live there.

Get all your info lined up. year, casting #'s etc.

BD, I thought I remembered the only difference was carberator. But I do know the exhaust/water return stuff is different on most of them. I realize I was absolutely no help whatsoever here hahaha

 

[Bwana Don]

Re: 120/140 motor question.....

BD, I thought I remembered the only difference was carberator. But I do know the exhaust/water return stuff is different on most of them. I realize I was absolutely no help whatsoever here hahaha

Bore and stroke are different. Heads will swap out depending on year. They made that motor a looong time and there are a couple of variants. Lots and lots of parts swap. I'm no expert so I recommend the merc forum. Come to think of it I'm no expert on anything, dang it.

3.0 = 4" bore and 3.6" stroke
2.5 = 3.87" bore and 3.25" stroke

 

[Bwana Don]

Re: 120/140 motor question.....

pi r squared (bore) x stroke x 4 cylinders
3.0= ((3.1416*2*2)*3.6)*4=181
2.5= ((3.1416*1.935*1.935)*3.25)*4=152.5

Displacement of the two motors. 3 liter is 30 cubes more!

Carbs are the same, as are manifolds, heads and exhaust.

 

[p3ghost]

Re: 120/140 motor question.....

pi r squared (bore) x stroke x 4 cylinders
3.0= ((3.1416*2*2)*3.6)*4=181
2.5= ((3.1416*1.935*1.935)*3.25)*4=152.5

Displacement of the two motors. 3 liter is 30 cubes more!

Carbs are the same, as are manifolds, heads and exhaust.

Ah ok....so the block is what makes the difference. That being said....the head, carb, and exhaust is what I really want to use from the old 140 engine. Reason being is because of the way the linkages all connect. So if those are the same, I should be golden.

I'll also go ahead and post in the Merc section too just to be sure.

Thanks a ton!

 

[Bwana Don]

Re: 120/140 motor question.....

Ooh. NorthBeach may have something with the exhaust. Mercruiser guys will know. Good luck and let us know how it went.

 

[North Beach]

Re: 120/140 motor question.....

I told you I had absolutely no idea what I was talking about. Maybe it was the 110 to 120 that was the same bore and stroke? Memory is the 1st thing to go. That may also be where we noticed the difference with the exhaust/water stuff to the lower end. I had to customize mine to join the two.

Remember I changed to the 3.0 and kept the MC type 1 outdrive that everyone said couldn't work.

 

[barato2]

Re: 120/140 motor question.....

FWIW---if you laid hands on 3.0 crank rods pistons to bump the stroke, you could probably bore the block out to 3.0 dimensions and have the equiv.

interesting that the 3.0 is essnetially Chevy 350 bore/stroke dimensions. wonder if any internal parts interchange?

 

[starcrafter65]

Re: 120/140 motor question.....

There are probably mate up issues between 3.0/MC Outdrive and and 2.5/Alpha outdrives....what is your outdrive?

 

[Bwana Don]

Re: 120/140 motor question.....

FWIW---if you laid hands on 3.0 crank rods pistons to bump the stroke, you could probably bore the block out to 3.0 dimensions and have the equiv.

interesting that the 3.0 is essnetially Chevy 350 bore/stroke dimensions. wonder if any internal parts interchange?

No internals won't swap with a sbc. Piston wrist pin to top of piston is different heaight. Pistons will go in but will sit way below deck. Compression ratio will probably drop into the 6 or 7's. Valves I think will swap with the SBC. I think that's it though.

 

[North Beach]

Re: 120/140 motor question.....

haha it becomes very evident just how danged mechanically challenged I am.....

 

[p3ghost]

Re: 120/140 motor question.....

I had the outdrive mated up with the 120 already, so I know I should be golden there. Just for my awareness....how do you determine which outdrive it is? I don't see any part or serial numbers anywhere.