Switching MCM 120 to a 3.0

[BillysBoat1986]

Hi all, I have a 1986 Thompson bowrider 16 ft with a 2.5 liter MCM 120. It has a loud knock so I'm debating on going the lazy way out and replacing it with a 3.0, preferably a 140hp crate. Will it fit on my existing mounts and will all my old accessories fit? Does this seem like a good idea to you all? Thanks for the help!

 

[Bondo]

Ayuh,.... Sure,.... It's a plug, 'n play swap,.....

 

[Scott Danforth]

with a caveat, stick with a 1990 or older 3.0 for plug-n-play in 1991 they changed the block and crank for a 1-piece oil seal.

3.0 long-block crate motors are in the $2000 - $2800 range.

 

[harleyman1975]

Bolts right in!

 

[BillysBoat1986]

http://www.michiganmotorz.com/marine-engine-1968-1989-p-942.html
This is the one I'm looking at. Does this one have the same flywheel... will I be able to use the same engine to drive coupling?

 

[Scott Danforth]

Yes

 

[BillysBoat1986]

Sweet! Thanks guys... looks like this won't be too difficult. The hardest part will be spending that 2k.

 

[BillysBoat1986]

One more question... been shopping around. Will the 2.5 liter head fit on a 3.0 short block? Looks like they use the same gasket.

 

[Scott06]

http://forums.iboats.com/forum/engi...06-mercruiser-4-cylinder-3-0-head-vs-2-5-head

Some info on head interchangeability.

One caution, if you reuse your head- you had enough wear or lack of lubrication to create the knock, you want to have the head gone through. Last engine I reused a head on a new short block- had a valve hang up and ruin the new shortblock. Wasn't a money saver in the end.

 

[Scott Danforth]

I would have your head redone if you want to reuse it, however by the time you pull it apart, have it cleaned, checked, machined, etc. and re-assembled, you can almost have a new head.

save money by eating bologna sandwiches vs skimping on motor work.

 

[mickyryan]

don't use the 120hp head on a 3.0 they are poor breathers compared to the 3.0 litre head, also best to just get long block but you will most likely need to get a new exaust/intake manifold because more then likely you have a 120 head that's got smaller exaust ports. the 3.0 has a harmonic balancer where as a 120 doesn't so make sure that's sorted out that you get a balancer on whatever engine you buy. there are flywheel differences with those older engines if i can find the info I will post here but its on the internet. only other snag you might hit is with starter there was some starter differences back then but I haven't ran into that so i cannot elaborate.

 

[BillysBoat1986]

Thanks for the input and helping me to make a decision. I'm going the long block route. It's going to cost more up front but hopefully it will give me a lot of good seasons of service. I also won't have that nagging anxiety feeling when I'm miles from the dock. Maybe I can have some mustard on my bologna sandwichs though.

 

[BillysBoat1986]

Mickyryan, if the crate has a different flywheel would there be balance issues if I installed the old flywheel? Would that even be an option?

 

[mickyryan]

Yes if crate is a 181 and you put a 153 non harmonic it would destroy engine eventually im not even sure the crank will accept hub from 120 onn181 but 18q has balancer the 120 153 cu inch dont

 

[BillysBoat1986]

Ok thanks. So I need to look into getting a new coupler also.

 

[mickyryan]

well coupler for flywheel to shaft no but harmonic balancer has its own coupler where as a 120 has a coupler then the belt pulley attaches to it
as far as flywheel and coupler to sterndrive goes you should be fine just make sure if its a 2piece or 1 piece real seal because those flywheels are different .

 

[Scott06]

This Michigan motorz add details what you need as far as flywheel and coupler for older engine at the bottom of it.

http://www.michiganmotorz.com/base-marine-engine-1967-current-replacement-p-8871.html

 

[Scott06]

FYI there's a 1989 complete 3.0 on eBay asking $800 at the moment, allegedly runs well...