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Old 07-19-2010, 02:40 PM
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Default mercruiser rebuild kit?

i am beginning a search for a rebuild kit for my mercruiser 120. i want decent quality and a reasonable price, the motor is over 30 years old and im guessing it has never had an overhaul. i believe it is time!! just did a compression test and the highest pressure was around 101 and the lowest was 64 pounds. she has been getting a little weak over the past 2 years. how are the kits i see on e bay??
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Old 07-19-2010, 02:46 PM
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Find out if you just need a valve job first.

I like the FelPro marine gaskets, and I would shop for original GM or Merc parts other than gaskets.
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Old 07-19-2010, 02:48 PM
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i have noticed considerable blowby lately. i mean when i pull the dip stick while the engine is running there will be air blowing out of the tube. could this be caused by valves?
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Old 07-19-2010, 03:04 PM
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Probably not. You could redo your compression check adding a little oil into each cylinder first. If the compression comes up considerably, your piston rings are probably worn out.
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Old 07-19-2010, 03:04 PM
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thank you sir
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Old 07-19-2010, 03:53 PM
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the compression increased dramatically with a little oil in the cylinder. as you said the rings are worn out. any advice on where to get an overhaul kit or do i just need to tear it down and replace the rings?
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Old 07-19-2010, 03:59 PM
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It appears that is the venerable "iron duke" 3.0 from GM. If so, any rebuild stuff for the GM 3.0 will work.
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Old 07-19-2010, 04:05 PM
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it is the 2.5L 153 CI 120 hp mercruiser
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Old 07-19-2010, 05:03 PM
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standard 4.030 or .030" over slugs polish or machine down the crank cam bearrings main and rod bearrings new rings regrind the valves check the springs and slam her together . roughly a machine shop would charge $20-30 a hole for a new .010 .020 .030 or choke .040 over bore . Depending on the hrs of the motor might be able to get away with a hone and over sized rings ie .010 over . though i would just start new and have her bored over the minium amount possible then go from there . if its raw water cooled go .030 over( though it would drop the compression ratio a few tenths , its not like its going to over heat lol
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Old 07-19-2010, 05:10 PM
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I've never seen one of these actually wear out or blow up. The parts usually rust off them first. They are hard to hurt. If you go to an automotive shop, make sure they install brass freeze plugs and use a good quality marine head gasket.
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Old 07-19-2010, 05:30 PM
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Find a good local old school auto machine shop. Ask around and see who auto shops are using.
I have been using a very good local shop for years (Precision Crankshaft). They are reasonable on there machining and I have them supply my parts which are always reasonable too. It helps them with machining when they have the new parts to perfectly machine your old parts to match.
My first Mercruiser rebuild years ago was a 140hp. I needed two pistons, dealer wanted $52. each plus rings. I checked with local shop and he told me they were regular 4" Chevy pistons. He sold me 4 new pistons with rings and changed the rods to the new pistons (pushed the pins) for me for $79.!!!!!!
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Old 07-19-2010, 05:40 PM
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I'm not a fan of rebuild engines. I had wasted to much time in the past. You might look into a new short block. TMC engines do have new,not rebuilt blocks that come out of Mexico that has a 2 year warranty, unlimit hours. You should be able to get a new block for way under $500.

Is your boat ran in saltwater? The block will need to be boil out to clean water passage ways.
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Old 07-19-2010, 05:42 PM
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this is going to be done over the winter. i will pull the engine before i store the boat and rebuild it in my garage therefore it will not be a rush job. i think what i may do is get it apart and see how everything looks before i buy a kit. if i have to bore it out .030 over then i will buy the kit accordingly. it is raw water cooled. should i get a kit with a new crankshaft? is that normal or usually unnecessary? as you probably know that adds a few hundred to the price of the kit. money is definitely an object as i have had to change careers recently. i know boat stands for bust out another thousand but i would like to do this as cheap as possible but still have quality work done.
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Old 07-19-2010, 05:55 PM
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I would just tear it down, have the block heat tanked and have whatever shop you choose for the machining check the crank. They can grind the journals if necesary and that way they will be able to fit it to the bearings.

At 30 years old I am guessing that if the block checks out OK you will mostly be looking at machine cost. Boring the holes, align boring the mains, decking the block and heads and the valve job. Don't forget to buy all new fasteners. Rod bolts and head bolts stretch and will be weak if used again. Don't skimp on it, after this long it needs everything.

It may be cheaper to buy a new long block and then you could order the 3.0 instead of the 2.5, (I don't even know if you can get it in 2.5 any more), since they are the same on the outside and you will have more power and torque that way. Great motors that last a long time, as you can see.
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Old 07-19-2010, 05:55 PM
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fresh water engine. the boat is over 30 years old. i just dont know about buying a new engine for a boat that old.
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Old 07-19-2010, 08:42 PM
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shouldnt need a main hone/bore unless the main bearrings spun . have the crank checked probably needs a polish and standard bearrings worse case scenario it needs cut down and overs sized bearrings . i highly doubt it needs decked . i would spend the money on checking the rods and rod bolts for stretching (though highly unlikely they stretched) just too be sure . the cam and crank are gear too gear (no need to worry about that unless you want to press new gears on the cam and crank) . if you need help when the time comes shoot me a pm i will be more then happy to help you out . brass freeze plugs you can pick up at auto zone for .60 cetns a piece or so dont worry about that . just have them check the springs out to make sure there with in spec and the rods , i had my crank polised in my first camaro and it had close to 150k on the engine . as long as you never starveds the engine for oil or over heated the pi$$ out of it the crank and block should be fine like i said feel free to pm me any questions you have
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Old 07-19-2010, 09:21 PM
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Of all the years I was a tech at a GM dealer, I don't believe I've seen one single 2.5 or 3.0 come apart for a repair or rebuild.

As for engine rebuilds, here's my .02 - spend $500 + few hours rebuilding it and get X years out of it.

Spend hundreds of course cleaning every nook and cranny and thousands machining, xraying, checking it and get another 20% of life out of it under the same circumstances.
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Old 06-26-2011, 01:40 PM
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Default Omc 2.5

Hello, I just rebuilt two 2.5 153 engines that are rated at 120 HP but I am unable find the torque specs. Has anyone ran across these? A torque curve would really be nice.

Thanks,

LN
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Old 06-26-2011, 07:39 PM
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Hi, Anthony, and welcome to THT.

A little advice. You will get some answers on this old thread if others see it, but you would be better off starting your own new one. I would just copy your whole post above, and then go to the first page of the Boating Forum (or any of the forums, but this belongs in "Boating) where in the upper left corner of the page you will see a button, "New Thread." Just hit it and you are on your way. Give the thread a title that reflects what you are after, "need torque specs for 2.5 153 engine" or something like that. You'll probably get more action. Good luck.

(And if I knew the answer, of course, I'd give it. )
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