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120 Mercruiser issues (possible removal/diagnosis)

funny_pooh67

New member
Hi...I'm entirely new to boats, but have a general knowledge of automotive mechanics. The boat I just purchased (75 Invader tri-hull) has a 120 Mercruiser that has a pretty loud knock that I'm assuming is a rod knock. Question I have is what do I have to pull to remove the engine? I have removed the lower drive (prop/piece attached with 6 bolts). How do I go about separating the engine from the boat? I don't see the typical exhaust like a vehicle so I'm a little lost in this regard.

Also, the block itself has a crack (6" or so) down the side with the manifold/carb on it, just below the freeze plugs and runs horizontal. The manifold also has a small crack on the bottom outer edge, just by the drain plug. Would these be acceptable to fill with something like JB weld or is it a bigger concern? I'm not trying to be the backyard mechanic, but I don't know what direction would be the most cost effective.

Biggest concern I have is getting the engine out. Does anyone have a step-by-step to follow so I'm not just guessing? Engine was built between 74-77 based on info I found online, if that matters at all.

Thanks in advance and sorry for the very newb-like questions...first time owner of a boat...and I got hosed on this purchase....
 
You need to remove the elbow on the port, left side looking at the bow. Maybe the rubber hose that holds the elbow to the exhaust tube. Has maybe one or two clamps holding it on.

Then you have to remove any cables and, unhook the main barrel plug.

You then have to remove the rear 2 nuts on the motor mounts, and the 2 blots on the bottom of the forward motor mount, The neg cable ground to the battery.

Pos battery cable at the starter.

Then hook up a chain hoist and up goes the motor and bell housing all in one unit.

To be clear the drive and shaft must be off to allow the motor to move.

Ok that block it garbage buy any make 3.0 up till 90' will fit with no problem.

Guy on ebay has one complete right now.
 
And that I appreciate... I'm sorry I'm not a jack of all trades and know everything about boats. This is the first boat I've ever owned...the drive system confuses me a little and I didn't know if after pulling the lower unit I would have a free engine. I don't have any manuals of any nature on this boat for reference, no mechanically inclined friends, as I just moved...so I wanted to play it safe rather than end up breaking things. I work more on auto electrical, so that part I have no concerns with, but I have gone through college on auto repair. Again, with a nearly 40 year old engine/drive system, I wanted to guarantee I wasn't breaking things.

I have found a guy locally here that has a 3.0 merc. that's willing to sell for dirt cheap...apparently off a boat he wrecked. I'll be checking into this and firing it before buying. Any common issues with the 3.0 I should be looking for, other than the obvious block/manifold cracks? I appreciate you guys' time and sorry if I'm asking the most basic questions.

And for clarification, when the rear drive is pulled (with the shaft that's attached) the rear will be clear to go when I get the motor mounts out? Thanks again, guys, I'm hoping to gain some knowledge as I go here...maybe one day I can have the knowledge you guys do!
 
Yes drive off then it's free the motor.

Check compression on the new motor, look on the starboard side of the block for the year of the motor, or under the valve cover for a date.

Check oil, check for leaks around the oil pan lip, and the front bottom of the motor, the timing gear cover.

Leaks there are common and not a deal breaker. Casting plugs pushed out are a deal breaker.

Doesn't matter make of motor but the same make as yours would be nice with the riser on, if not the same make as yours then the mani' off your old motor will have to be moved over with your riser.

Put water to the motor shunt in a garden hose to the t-stat, if the motor is in a boat watch the temp gauge.

Check engine oil after you run it to see if milky or overly full, and smell the oil.

I'm sure i missed something, oh bye the way welcome aboard, we all have to start some where.
 
77 now thats uncalled for. He doesn't need me to defend him, i'm sure he can handle himself just fine. You could have sent him a private message.
 
funny_pooh67:

Check the floor, stringers (floor joists) and transom for rot or waterlog before doing anything else. Floatation foam under the floor probably is wet; maybe not.
 
yeah..the floor/engine mount areas are seemed solid when inspecting. Transom is fiberglass in the areas visible to me...so no visible rot there.

Bt....do you have a link to the section on removal/installation or the entire manual? The link only pulled the table of contents, basically. Or, do you know where I can get the manual online or whatever?

Again, I really appreciate the time and sorry for the super-newb questions. Wouldn't have asked if I'd found while searching. Apparently 120 is too short of a search term...and mercruiser brings up a heck of a list! Thanks guys!
 
Thanks for the updated link! Definitely helps more than the TOC haha. Just a quick question before I head out and tear into this thing...guy I bought this boat from says his "boat buddy" says a 4.3 v-6 is identical for bolt pattern/fitment in my boat. Is there even a shred of truth to this, or is he my "typical" backyard mechanic kind of guy? I appreciate your time, guys....this should be the end of this thread. BT, thanks again for the updated manual and link!
 
Tell your friend that a 4.3 is nothing like in any way shape or form to a 120 / 2.5. A 120 is almost a twin brother to a 140 / 3.0. All externals mani, riser, belts, starter,alt,flywheel,coupler,pulleys,oil pan,timing cover is the same,the difference is in the pistons, and such. The inline 4 is in what way close to a v 6 motor ?
 
Guy's not a "friend"...so at least my thoughts were correct on it. Apparently he said the mounts were identical...sounded way too good to be true to me, as well. I've really bit the bone on this purchase.... Thanks again for you guys' help!
 
inline 4 has the front mount v6 has side mounts, the only thing might be the same is the bell housing mounts but i think they are different also.
 
"Transom is fiberglass in the areas visible to me...so no visible rot there."

Any rot in the transom will be below the waterline. Sound the transom from the top down. Drilling 3/8" holes from the inside of the boat near the drain plug will tell you if there is rot. Sawdust will be wet and black mush. If all is good then fill the holes w/an epoxy and sawdust plug.
 
Tomorrow, I'm swapping a mercruiser 120 engine (circa 1980) with the same from another boat.

I have the service manual but it does not talk about engine removal. It does describe in detail about rebuilding the stern drive including some special tools like a "puller kit" that lookls like it removes the bearing cartridge in the gimbal housing. Another special tool mentioned in the manual is an "engline alignment tool". No need to tear down the stern drive as I had it rebuilt recently. I just want to change motors.

Here are my questions:

1. It appears that I can remove the stern drive (which will leave the bell housing fastened to the transome) and the drive shaft will slide out with the drive. Is that what you mean by "remove the drive" before you remove the engine?
2. The same size engine is going in. I expect that it will bolt into the same place as the orginal engine and the stern drive to slide back into place. Is an "engine alignment proceedure" necessary? Special tools? It looks like plug-and-play to me.
 
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Thanks for the speedy reply BT.
I thought this thread was dealing with my motor type but not so sure now.
Back to square 1. My engine is marked:
Mercruser 120 hp
Eng No. V03121BA
SN No 24/8299
4 cyl, inline.
circa 1968
 
Funny,
My 2cw. This is my third boat. It is a 1967 vintage with a 120, 2.5L 4cyl. It took 20 months to rebuild.
First, get the manual(s) for your barge. I ended up with two manuals and the on-line manual to get my project finished. If you are good at wrenching the books and the folks on this site can get you through any questioins.
Second, autos are autos and boats are boats. They each have specific settings for their respective environment. Look at your boat as a different animal than your car.
Third, the transom may look solid but the fiberglass you are looking at may be hiding some much needed repair. Search the internet. There is a great deal of information concerning checking and repairing the transom. the infomation posted above is good stuff. All three of my barges needed transom work to some extent. The latest project was a complete replacement of the transom wood and new glass.
Fourth, as you say you are new to boating. Locate your local Boating Safety course. Take the class. This course is like drivers ED when you were 15. It will teach you the rules of the road and help you have a safe boating experience.
Five, don't forget the trailer. Make sure your trailer is in top notch working order. Breaking down on the way to the ramp or on the way home puts a real shadow on what could have been a good day.
Good luck with your project.
TxGalz Dah
 
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