3.0 Mercruiser pistons, flat top or dished?

[1987sylvanoffshore]

I have a 1985 Mercruiser 181 CID (3.0L) (140HP) 4 cylinder GM engine. Does this motor come from the factory with flat top pistons? I am doing a complete rebuild this winter and am getting conflicting info on the pistons. I know if I don't replace with correct pistons will change the compression, that is NOT what I am after! When I took my engine apart it had flat tops but all the "rebuild kits" are dished?

Thanks

 

[Alpheus]

Re: 3.0 Mercruiser pistons, flat top or dished?

Check these guys out. They got everything you need...

http://bpi.ebasicpower.com/mm5/merc...&Product_Code=HAS3493H&Category_Code=MER6U-IB

 

[zbnutcase]

Re: 3.0 Mercruiser pistons, flat top or dished?

$48 EACH for a Chev piston? They are nuts.

 

[Alpheus]

Re: 3.0 Mercruiser pistons, flat top or dished?

Do you have any better ideas or are you just posting to get your numbers up like usual???

 

[fishrdan]

Re: 3.0 Mercruiser pistons, flat top or dished?

I'd pull it apart and see what it looks like before ordering anything, then get a complete engine rebuild kit. You need to know what size bearings and pistons to get,,, and without tear down, mic'ing and inspection you will be guessing. Pistons come std, .20, .30, .40 and you will need to know which you need before ordering.... Buy .20's and the cylinders might not clean up. Buy .40's and you might be taking off too much cylinder wall to clean it up.

I bought the kit for my 73 140HP off Ebay (around $450) and was pleased with the parts, everything name brand. greenbaypartsworld.com is another place to check out.

Check with your machine shop and see how much they will charge you for the engine kit, might be worth the small difference in price, might not.

Couple of tips, don't get a high volume oil pump and get the 1 piece oil pan gasket. I would pop the extra $100+ to have the engined balanced too.

 

[1987sylvanoffshore]

Re: 3.0 Mercruiser pistons, flat top or dished?

The connecting rods and crank are at the machine shop now, I will know what bearing size to order for both by next week. The cylinder walls are in good shape and just need honed. The guy took one look at the pistons (which are flat top) and said "detonation huh". I'm still not sure if the previous owner put the wrong pistons in OR if Mercruiser used different pistons On the early models (1985). All of the rebuild kits I find online are with the dished pistons. My motor had the flat top pistons in them when I took it apart. The newer 3.0LX use the flat top I think but again mine is a 1985 far before the 3.0LX. Thanks for all the help so far!

 

[Alpheus]

Re: 3.0 Mercruiser pistons, flat top or dished?

Put the dished pistons back in it. If it shows signs of detonation some hot rodding car engine builder probably put those pistons in looking for more HP and made the compression to high for pump gas.

Keep it as low performance and as stock as you can. It will start every time you turn the key and if the maintenance is maintained it will last a long time...

 

[coheej]

Re: 3.0 Mercruiser pistons, flat top or dished?

The 85 3.0l and the 89 3.0l that I have had apart both had the dished pistons.

 

[GLENN M]

Re: 3.0 Mercruiser pistons, flat top or dished?

3.0l and 3.0lx use differant heads,all info i could find was just generic ie.valve size, flat top or dish,only thing differant reach plugs,do you have 3/4 in reach or 1 in reach . did you have problem before, i would be tempted to run what it came with if i didnt have problem,lower compression to much youll be paddling.lx has 9.25 to 1 i would guess {never guess} flat top

 

[1987sylvanoffshore]

Re: 3.0 Mercruiser pistons, flat top or dished?

So all my parts from the machine shop are finished. I still have not figured out what was going on with the pistons that were in there. The machine shop said the engine had never been bored before. If I called Mercruiser and gave them the # on my block do you think they would be able to answer my question? I would hate to spend all this time and money and just guess, that would be just dumb! Any more ideas

 

[coheej]

Re: 3.0 Mercruiser pistons, flat top or dished?

So all my parts from the machine shop are finished. I still have not figured out what was going on with the pistons that were in there. The machine shop said the engine had never been bored before. If I called Mercruiser and gave them the # on my block do you think they would be able to answer my question? I would hate to spend all this time and money and just guess, that would be just dumb! Any more ideas

Why don't you have the machine shop cc the combustion chamber and calculate your compression ratio with each piston; then make your decision. That way you will know and not be guessing.

 

[1987sylvanoffshore]

Re: 3.0 Mercruiser pistons, flat top or dished?

thanks coheej I may have them do just that, here is an old post(not my post) of what my pistons look like. They look like flat top to me. It doesn't say what year it is.
http://forums.iboats.com/showthread.php?t=287724

 

[zbnutcase]

Re: 3.0 Mercruiser pistons, flat top or dished?

So all my parts from the machine shop are finished. I still have not figured out what was going on with the pistons that were in there. The machine shop said the engine had never been bored before. If I called Mercruiser and gave them the # on my block do you think they would be able to answer my question? I would hate to spend all this time and money and just guess, that would be just dumb! Any more ideas

Calling them won't do any good, as they buy the complete, assembled long block from GM...