OMC 120 to Mercruiser 470 conversion

"I have a restored 1984 Harris

"I have a restored 1984 Harris 28' pontoon boat powered by the original OMC 120 (GM base 4 cyl) I/O. It runs, at best, 18 mph. The engine has a lot of blow by and rather than overhauling the 120, I am seriously considering changing the driveline over to a Mercruiser 470. The 470 would give me additional power, and the ability to trim up the outdrive whereas the OMC drive knuckles will not. I am wondering if the 470 will fit into the space (aluminum pod) the 120 is in, and if anyone has an opinion about the comparison of maintenance requirements of 120 vs 470. I can do the swap myself including welding the aluminum. I'm just looking for more power with ease of maintenance and good durability. Your opinions are appreciated."

"Ayuh,... You'll be jumpi

"Ayuh,... You'll be jumping from the frying pan,... Directly into the Fire...

That's just a Dumb Idea..."

"Sir: this is a bad idea and i

"Sir: this is a bad idea and i will try to explain why.

The 170,470,is a terrible motor. It has a open deck design.

Also hole for the stringer will not allow the merc drive to match up, i don't know what the hole is thru cause you didn't post any pic's.

However if this were a cobra drive the hole would be suitable.

My suggestion is get a boat with the motor, drive setup you want and sell the one you have."

Easy Chiefalen the 470 imo is

Easy Chiefalen the 470 imo is a very nice motor If all steps were taken to preserve it over time. you are right there is very little respect for the 470 because they are a dukes mixture of parts but let me add this i have a 1984 merc 470 and it has been properly serviced from day one and thats how it continues to run today the 470 is just like any other motor treat it rough and rough you have i myself have to stand tall on my 470

"steve never meant to insult y

"steve never meant to insult you or your motor.

You bought it new?

You took care of it?

You put the alternator fix in?

You changed out to a 4" pipe?

You never overheated it?

You never had a head gasket go?

Good for you, for the size, weight, can't be beat at that time it was made, i know you ain't getting that hp at the time it was made in that size package.

BUT: What about parts NOW ? Is the original poster gonna get a perfect motor to drop into his boat.

I have a stringer drive, could have switched over to a merc i passed. I understand people bad mouthing stringer's all over.

See in the right hands and understands the stringer drive it is superior to the merc of that time.

470 in the right hands such as yourself, that understands it will last forever also.

PS the alpha 1 is in my opinion is superior to the alpha 2."

"I'm having trouble gettin

"I'm having trouble getting my reply to post.
This is the third attempt:

I am a little surprised at the response considering how many boats have the 470s.

Chiefalen, please explain what is the "open deck design" motor and what problem does it cause?

I tried to upload a picture of the stern but my pics are too large. They are 3.0mp

The stringer mount is the large oval rubber that the stringer mounts in. I was planning to weld the opening closed with an appropriate aluminum plate.

I was wondering about the size comparison between the 120 and the 470. Considering they are 4 cyl motors I was wondering if the 470 would drop into the 120 engine bay.

Thanks!"

"i will assume you have the 47

"i will assume you have the 470 and drive from a donor boat correct? You will need everything, motor back to the prop.

The 470 is wider i believe. It has the closed cooling system.

It might, just might work but is it worth it to YOU ?

You will have to weld it close and get the templite jeez i spell like chit. Never could spell correctly my whole life. thank g-d i can count got me this far.


And drill the holes for the drive. May i ask where you are located. if not to far i can come look at it.

You do know that the 3.0L / 140 motor is the exact size as the 120 / 2.5 motor. Will give you between 15-20 more hp. And you can add a trim to the motor that will allow YOU to in essense lift the prop bye tilting the motor for the omc you already have.

Blow bye is no big deal to fix if, thats a big if that is the problem with the motor.

You did do a compression check on the motor.

Bye the way i have a perfect head for the 120 in my basement i have been holding for the right person who needs it.

Could you be the ONE."

I would consider installing a

I would consider installing a 150 HP refurbished outboard and get the space back taken by the I/O. It will also reduce the overall weight. Since you can weld it should be a simple job.

"[b]"I am a little surpris

""I am a little surprised at the response considering how many boats have the 470s."

Ayuh,... I agree with Guy,... Put an Outboard on it....

The 470s aren't all That popular,...
And,...
They've been Out of Production for over 20 Years...
Part #s are now coming up as NLA..."

Bond-o look out for steve.

Bond-o look out for steve.

"Okay gentlemen, I think I&#39

"Okay gentlemen, I think I'm getting the picture.

I don't have a donor boat with a 470 but being a gearhead with some current 460 race experience I liked the thought of having one when I heard the 470 was half of a 429/460 and it has an aluminum block. I live near Dallas and often see boats at charity auctions. I'm sure one with a 470 - 488 will eventually show up for me. But, now that I've done a little research on the 470 and gotten your responses I'm thinking I'll be lucky to find a good 470 in a charity boat. Then, when I find one some significant alterations or fixes will likely need to be made.

Steve, I think I would be lucky to find a boat at a charity auction that has been as well taken care of as yours. Otherwise, I think a complete driveline guaranteed to be in good gondition will probably cost me at least $2500. Then I have a lot of work to do to install it.

I was not aware the 3.0 140 was an option. My OMC 120 is okay except for needing rings (it has serious pressure coming out of the oil fill hole and after a hard run the dip stick is pushed up but it does not smoke, so I know the compression rings are worn out) and can only push my 28' barge about 18 - 20 mph depending on wind direction. The 120 has only used about 4 ounces of oil in the 50 hours I've run it. I bought the boat in sad shape last Feb. (the carpet kept me from falling throuh the floor) and only ran it for about a half hour. It had 40 lbs oil pressure and didn't smoke. I thought it was great. I rebuilt the boat right and from the pontoons up with flooring actually from Harris (tongue and groove marine plywood that I could not find anywhere else). At this point, I'm stuck. I can't dump the boat and go find another one with more power. I like the I/O because I'm familiar with car engines and I can fix it. I've never worked on outboards.... Of course I have not had a stringer apart. Anyway my new live well sits above the outdrive now, and I've extended the floor 2 feet back on each side for the downriggers and that would interfere with the outboard in turns. I'm quickly coming to the conclusion the 140 block is the way to go maybe with a hyd. roller cam. I wonder if there is a 4 barrel intake for one? I guess I'll check with Edelbrock.

A bigger stretch would be a Mercruiser with V6 but I'm sure I will have to widen the engine bay meaning some serious alteration to my nice new vinyl covered floor and beams.

Thanks again for all of your input. I appreciate your wisdom.

Dave"

"Good move glad you decided to

"Good move glad you decided to go with a 140.

I had a 120 / 2.5 in my 1986 Citation cuddy, was a real pig.

Make a long story short i had to replace it and i found a 87' boat that had a good motor but bad everything else i bought the whole boat with the trailer, the guy wanted it out of the driveway.

I installed the 140 in my boat and i couldn't be happier. That 20 extra hp is everything to me.

Make sure you get the same series motor, 3.0 / 140 up to 90' will do you nicely.

Good luck !"

LMAO @ Chiefalen! nah buddy im

LMAO @ Chiefalen! nah buddy im just partial to my 470 and i have picked up 2 spares just incase i have any problems bondo is right many parts are listed NLA so therefore i have some spares also to add to your earlier response mine does have the 4inch exchanger and believe it or not the original charging system that still performs well but ive always taken the balancer off in the sring and cleaned the rust from it and the stator as well the biggest fault with them was the heat from it being right on the block and you scared me about the head gasket becaause mine has yet to let go so its got me wondering if while shes laid up for the winter if i should throw a new one on just for precautions that part is still availible lol

"I got your back Steve I have

"I got your back Steve I have a great running 470 for 28 years old but I made sure when I bought the boat that it had been taken care of. I think the idea was great; a light weight high horsepower motor fuel efficient motor. The execution could've been better from the complaints on this board but Steve and I have had pretty good luck, knock on wood. I would rather have my 470 than a 188 OMC! Roughly the same HP, half the weight, and way better fuel economy.

That being said I would not buy one to put into another boat for the fact as stated that they aren't in production and parts are hard to find. I would go with a 4.3L instead. I think we can all agree to throw the OMC to the curb!"

"[b]"I would rather have m

""I would rather have my 470 than a 188 OMC!"

Ayuh,... Tough call,...

'spose atleast the drive could be salvaged from the 470,.. "

"Steve the head gasket maybe w

"Steve the head gasket maybe won't go if you don't overheat it.

I have the last of the stringer drives, in a 86' cuddy, and i simply love it.

And i also am gathering spare parts as i come upon them, mostly free HAH.

Those 470, 170 motors are all over here on the side of the road. Couple of boats have trees growing thru them no kidding.

Interesting that you left the old charging system.

Your 470 is hard to start? Everyone i talked to that had one says it's a byatch to start cold.

Good luck guys as they say we are all in the same boat."

"There are some structural iss

"There are some structural issues in going from a stringer mount OMC to a Merc stern drive. The thrust from the prop on the OMC was transferred to the motor mounts on the stringers - thus the big boot in the transom. The thrust on a Merc is absorbed by the transom - that's why the installation specs require a 2 1/4" muiltiple layer plywood thickness. Without a stiff transom and transom support members the thrust will cause the transom to flex, cause mis-alignment and coupler failure. Not to mention the boat coming apart."

"Interesting that you left

"Interesting that you left the old charging system."

Why fix what isn't broke? Mine works fine and see no reason to replace it until it goes out.

Re: "Okay gentlemen, I think I&#39

@460racer, did you ever make this happen? I know I'm over a decade late to the post, but I'm in a similar boat as you now... yes, puns. Sorry.

I have a 1978 24' I/O Harris Pontoon with an OMC 120hp 400 outdrive. I bought it this spring and have it running amazingly, between new alt, plugs, wires, cap, electronic ignition upgrade, lots of fluids, and a new fuel pump. I actually love the way it runs, and 21mph in this lead sled is nothing to scoff at, but I want more!

I have a donor boat in the driveway, just got it last week, the fiance might kill me over. It has a 1989 Mercruiser 165hp (3.7 4cyl) with Alpha1 outdrive. The intention would be to rip and replace from the prop to the console. I don't think I need to remove the outdrives from the motors (straight drop into pontoon and Sawzall to the fiberglass on thedonor boat if needed).

What isn't going to work? How will my mounts like up? Is the hydraulic trim a pain? Power steering now?
What will work?

Myzerman, you have posted to a thread that had been started back in 2009!

RicardoMarine said:

Myzerman, you have posted to a thread that had been started back in 2009!

Click to expand...

I'm aware and acknowledged that. But in searching the internet for reference on this topic, this is the only thread I've found anywhere that this has been discussed. Which, in itself is concerning to me, but I am hopeful.

Many people talk about I4 to V6 or V6 to V6 or V8 to V8 power swaps, but this is the only one addressing OMC120 to Merc I4.

In addition to more power, I want to get away from a ball gear sterndrive.

Do you have any real insight, other than timestamps on posts?

You should start a new thread. You'll have to redo the transom to get the Mercruiser to fit. Will require filling the old 400 hole and cutting out for the Alpha. Probably new engine mounts. This is what I had to do on my old Allmand. I'm putting a b3 on it with a big block (one of these days ugh) I doubt that the juice is worth the squeeze tho. 20 more horses on that barge won't make much dif. It's just not the kind of boat that's able to take big horsepower due to the design. You really need v8 power and there's just no practical way that one will work. The stern will squat due to the added weight for one thing. The other is that the pontoons are not true planing surfaces. Yes, when you push them along they rise up but that's about it. The rounded bottoms just won't get up on top of the water the way that you want them to. That said, I've seen people do some insane stuff with pontoon boats but big hp on one of those scares the crap out of me.

Oh man, I'm not sure what you're reading. I'm talking about OMC 120 to Merc 165 and I don't have a "hole" for the 400 on a pontoon. There is a rubber boot, but it's just for aesthetics; as it's essentially connected to nothing on the top side.

I'll take your advice though and create a new thread and then link it to the bottom of this one.

Thanks.

Ugh. Well, then you have only the option of trying to fit the larger engine to the 120 drive. The Alpha needs a solid transom to hang from unlike the 120 that just sits there.
Your challenge will be the exhaust of course. Not sure if the OMC elbow would fit on the merc engine. If not then you might have to do something creative there. Personally, I would just get the 120 going best you can, enjoy the boat for a few then sell it on. Nothing wrong with stringer drives as long as you keep on top of maintenance.