Mercruiser 3.0 rods and pistons

[Jussi-T.]

Hi! I am new here but I have been reading this forum for quite some time. Very good forum, lots of information and pros!

I recently bought a -89 Wellcraft 170 Classic with a Mercruiser 3.0 Alpha one. The boat is a big project. The engine needs to be repaired and the deck, stringers and transom are rotten.
When I bought it the engine ran only with two cylinders and water had gone to the engine oil. Well the block was cracked and I bought a OMC 3.0 engine and I am planning to repair it and install it into the boat. The problem is that the Mercruiser is a newer model I think. It has a one piece crankshaft rear end seal and the OMC has two piece rear end seal. The OMC has bad pistons and the pistons are different in these two models. The pistons from Mercruiser are flat and the pistons from OMC have a combustion chamber at the top.
The question is: Can I use the pistons and connection rods from Mercruiser in the OMC block with OMC crankshaft? And if I can, are the connection rod bearings and the piston rings the same in these models? The rings look pretty much the same, but the bearings are a little bit different but I am not sure if this will matter because the connection rods look the same?

I am planning to buy this engine rebuild kit, so if you think this wont work with my configuration please let me know

Mercruiser 140 Chevy 3 0 3 0L 181 Engine Kit Bearings Timing Gears Oil Pump 2 PC | eBay

Sorry for my bad english, I am from Finland so english isn't my native language.

 

[fishrdan]

Re: Mercruiser 3.0 rods and pistons

Without looking at everything as checking numbers, it's hard to say what if any parts are going to be compatible. I'm not sure of OMC, but early Mercruiser 140HP (3.0L or 181Ci displacement) engines had 2 different crank journal sizes. I would get a set of calipers or micrometers and check things, crank journals, rod ends, etc, etc., to see if they are the same. If the pistons are worn out in the OMC engine, you are not going to want to use the Mercruiser pistons since the OMC cylinders will be too worn and need boring, then bigger pistons fitted, .020 or .030 oversized. The dished pistons and flat top pistons are for 2 different compression ratio's, older engines use dished pistons, newer engines use flattops, (something like 8.8:1 compared to 9.3:1).

Beyond the engine, you need to check lots of things to make sure the old 2 piece rear main seal engine will bolt up to where a newer 1 piece rear main seal engine was, rear mount, coupler, flywheel, starter. Also, if the engine has a (freeze) cracked block I would closely check the manifold for cracks too. You'll want to use all the mounts, accessories, head, manifold off the Mercruiser engine.

I bought my Mercruiser 140 HP engine kit from that Ebay seller and was happy with the parts, all name brand stuff.

 

[Jussi-T.]

Re: Mercruiser 3.0 rods and pistons

Today I took a closer look of the rod bearings and piston rings. The con rod bearings are different, but the connection rods looks the same, (the rods don't have a part number stamped...) So I think ( but I'm not sure) the bearings will work if I buy this kit I linked earlier. The piston rings are pretty much the same, but the older piston rings have a bevel at the top, the newer piston rings do not have this bevel... BUT the pistons have exactly same groove...

Fishrdan: Sorry I didn't told more specifically about the pistons, etc... In fact, the OMC has only one bad piston, and they arent worn. Something has gone between the piston and head and the 3. cylinder piston's head is bad. So the block doesn't need to be bored. (It doesn't have a "shoulder"? if you try it with your finger nail.) That's why I want to use the newer pistons. + they give bigger compression, more power, better efficiency, more fuel economy

I have checked the engine block mountings, coupler, flywheel, starter ---> OK! I am only worrying about the rod and piston issue... :/
I was wondering if I just email that Ebay seller and ask if he could do me a mixed kit of older and newer engine parts. Newer piston rings, older connection rod bearings etc...

Btw, What is journal? and what does rod end mean?

 

[ENSIGN]

Re: Mercruiser 3.0 rods and pistons

The older and newer engine parts are not going to interchange.I think you would be better off looking for a used engine that you can retro fit to marine.

 

[Bondo]

Re: Mercruiser 3.0 rods and pistons

The con rod bearings are different, but the connection rods looks the same, (the rods don't have a part number stamped...) So I think ( but I'm not sure) the bearings will work if I buy this kit I linked earlier.

Ayuh,.... The difference could be on the crankshaft journal, 'n not the rods....

You can get into serious trouble, mixin', 'n matchin' like yer doin'....

 

[Jussi-T.]

Re: Mercruiser 3.0 rods and pistons

But if the rods are the same and I put the older model con rod bearings to these newer model rods wich are the same, wouldnt it work then? or I could use the rods and bearings from the older engine and only change the flat top pistons to them... I cant see why it wouldnt work then?

 

[RogersJetboat454]

Re: Mercruiser 3.0 rods and pistons

But if the rods are the same and I put the older model con rod bearings to these newer model rods wich are the same, wouldnt it work then? or I could use the rods and bearings from the older engine and only change the flat top pistons to them... I cant see why it wouldnt work then?

If the journal width, or diameter are different, mixing and matching bearings to make it work will probably get you into trouble.

In some cases, where the crank journals need to be machined due to surface damage, they have over-sized bearings available to fit the original rod back onto the correct crank. But that's working with a crank and rod that are meant to be together.

It's a precision fit, with no room for error.
If you cobble this with out taking measurements, and the bearing clearances are too tight, you will starve the bearings for oil, and will most likely spin one or more bearings. If the clearances are too loose, the bearings will pass too much oil, the engine will have low oil pressure, and you will have a rod knock that will eventually beat it's self apart.

You can get in trouble with ill informed piston matching too. If the combustion chamber (cylinder head) and head gasket you intend to use aren't correctly matched to the pistons you use, the most likely result is you will either build a dog (no power) of an engine, or you may build something with too much compression to be in a boat.

If you really want to know if things can work together, its probably best to get a machine shop involved, so they can take measurements, and tell you what's what.

 

[Jussi-T.]

Re: Mercruiser 3.0 rods and pistons

I am going to machine my crank to next oversize, because there has been a lack of oil. So it will need .010 oversized bearings and of course I will measure the journals etc for correct fit.

I found this site below and if you look the rod bearing part numbers (older vs. newer) the numbers are same!

Internal Marine Engine Parts GM 140 4 Cylinder 181 CID 3.0L LX Model (1 Piece Rear Seal)(1990 and up) Hydraulic Lifter, Gear Drive, LH Rotation (1 piece rear seal)
Internal Marine Engine Parts GM 140 4 Cylinder 181 CID (2 Piece Rear Seal)(prior to 1990) Hydraulic Lifter, Gear Drive, LH Rotation (2 piece rear seal)

The heads should be the same, so I don't think the compression will get too high...

 

[fishrdan]

Re: Mercruiser 3.0 rods and pistons

The heads should be the same, so I don't think the compression will get too high...

Quite possible, but there were 5-6 different heads used on GM 4cyl marine engines, important when mating it to the manifold.

Things might work out OK for your engine, but I think what most of the guys are warning about is, there have been MANY changes with the 4cyl GM engines through out the years. So even though they are very similar, you could run into some issues.

 

[Jussi-T.]

Re: Mercruiser 3.0 rods and pistons

I'm going to take my crank to the machine shop this week, because it needs to be machined (scratches) and I will take one flat top piston-rod set and one dish top piston- rod set and the newer model crank with me. So they can see if it would work... I think that's the wise thing to do now.. Let's see what are they thinking of it =)

 

[Jussi-T.]

Re: Mercruiser 3.0 rods and pistons

Today I took my crank to the machine shop. I also took the pistons and bearings with me. They looked the bearings and they said: No problem with bearings and pistons! They thought that the bearings are a bit different because the bearings are different brand. I checked the cylinder heads also, and they both had the same GM number stamped.