Mercruiser 120 and Mercruiser 140 intake interchange?

killerkawi713

Recruit
Joined
Aug 23, 2017
Messages
3
I have a 1985 Chaparral 178 and it has the mercuiser 140 3.0ltr.. when I got it the motor was popped.. I have found a mercruiser 120 which I now know is a 2.5ltr.. I know its a drop in hp.. which isn't a big deal since I never ran it with the 3.0 I wouldn't notice difference.. now here is my question.. Can I take my intake/exhaust off the 140 and mount it on the 120 so it matches up for the throttle cables and exhaust outlets.. now what about putting the 3.0ltr head on the 120.. would that increase any hp in the motor performance? I am an experienced mechanic in the car field.. but new to the boat motor thing.. and don't know a whole lot about the mercruiser motors.. any help is appreciated... thank you..
 

mickyryan

Rear Admiral
Joined
Apr 18, 2016
Messages
4,210
i would keep head and the exaust/intake matched , i put a the 140 on my 120 no problems. but i matched it and kept the exaust from 140 so head matched the exaust ports. the 140 head has bigger exaust ports/intake valves if i remember right so yest maybe some slight performance but the 3.0 is punched out where the 153 /120 is just smaller bore
 

Bondo

Moderator
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Joined
Apr 17, 2002
Messages
70,469
now here is my question.. Can I take my intake/exhaust off the 140 and mount it on the 120 so it matches up for the throttle cables and exhaust outlets.. now what about putting the 3.0ltr head on the 120..

Ayuh,.... Welcome Aboard,..... You can do that, No problem,....

I'd bring the head to a machine shop for a rebuild 1st though,....
Flatten it, sharpen the valves,...
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,309
the 18' boat would be a slug with the 3.0, not sure it could get out of its own way with a 2.5

the 3.0 is extremely common and easy to find cheap. sometimes you have to dispose of the rotten hull its in.
 
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