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  1. #1
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    Mercuy 2.5 280 Compression?

    Sorry I don't have the serial # but what compression should I see out of this motor for it to be healthy. 2000 power head. Thanks
    2003 Bullet 21XDC with 2012 250 Mercury Sport XS
    1997 Allison 2003 with Mercury 225 Pro Max

  2. Member
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    #2
    ones ive checked 140
    .................................................. ...the scariest thing in life is the unknown ...................................

  3. Member cajunrgfm's Avatar
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    #3
    leakdown is as important on that motor as compression. rule of thumb is to rering them at 100 hours.
    Quote Originally Posted by bjmoss View Post
    Sorry I don't have the serial # but what compression should I see out of this motor for it to be healthy. 2000 power head. Thanks

  4. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #4
    I would say at no more than 100 hours... not to split hairs, but we know folks often will push the "spec".

    Leakdown is very very critical on this engine... and is really the only accurate indicator of the bore's health.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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    #5
    Can you tell me the difference between the 225 vs 260 and 280? I greatly appreciate the help . Thanks
    2003 Bullet 21XDC with 2012 250 Mercury Sport XS
    1997 Allison 2003 with Mercury 225 Pro Max

  6. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #6
    Totally different engines- let's stick to one specific engine when you get the serial number.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  7. Member
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    #7
    Quote Originally Posted by bjmoss View Post
    Can you tell me the difference between the 225 vs 260 and 280? I greatly appreciate the help . Thanks
    225 promax motors are steel bore “laser “ style injection motors.
    porting similar but not quite the same as a 260.
    40 amp electronics switch boxes etc
    20” motors came with 2 piece adapter plates, some 15” had one piece some had 2 piece

    260
    nicasil bore motors, horn style case injection
    16 amp electronics
    20 and 15” all one piece adapter plates.

    280
    nic bore motor, horn style case injected(some later models had optional svs intake)
    Alternator style charging , cdm style ignition(although putting 260/drag style electronics is very common, most have this done)
    20/15” with one piece adapter plates, a little different porting than 260.

    There are some other differences but that’s the jest pretty quick.

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    #8
    I have a 280 ROS that was rebuilt by one of the best in the business. His opinion has been for many years that these 280's can go MANY hours more than 100 if they are not run frequently at very high rpm's. My motor has not been and at 120 hours, compression is 150 on all six and leakdown is 4% on all six. Leakdown is as has been posted far more important than compression on these motors and it is the wise owner who keeps his eye on it.

  9. Member
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    #9
    Quote Originally Posted by catfish123 View Post
    I have a 280 ROS that was rebuilt by one of the best in the business. His opinion has been for many years that these 280's can go MANY hours more than 100 if they are not run frequently at very high rpm's. My motor has not been and at 120 hours, compression is 150 on all six and leakdown is 4% on all six. Leakdown is as has been posted far more important than compression on these motors and it is the wise owner who keeps his eye on it.
    I agree 100 hrs if you are beating on it all the time I know several in the 150-250hr range that spend most of their time between 3/4K rpm with occasional short bursts to 10k

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    #10
    re ring does that mean you have to re ring the pistons at 100 hrs?Coming from someone that has ran older omcs for many many hrs more than that,i think 100 hrs is seriously low for a rebuild

  11. Member silverbullet02's Avatar
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    #11
    Quote Originally Posted by sleeper bird View Post
    re ring does that mean you have to re ring the pistons at 100 hrs?Coming from someone that has ran older omcs for many many hrs more than that,i think 100 hrs is seriously low for a rebuild
    Nikasil coated bores and gold plated rings. If the plating wears off the rings they'll tear up then bore coating. As said above I've been told by the same person as catfish that they can go much more than that, but it's in how hard they're run. All the other motors with steel bore will go for a long long time.
    1995 Allison XB-2003 225 Super Mag. It's slow and I'm ok with that.

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    #12
    now i understand thanks

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    #13
    I ran some land&sea dual plug heads years ago, ultra-high compression. There, I'd be lucky if I ever got to 100 hours. But how ever many house I did get were absolutely screaming fast... :) With the small volume combustion chambers, that motor made ridiculous torque throughout the RPM band. However, that said, the old "whap whap whap" would announce itself given half a chance. Almost had to get a divorce from baking the top half of the block in my wife's oven to release the sleeves... She was NOT amused. Came in one day and said "That had better be a burned turkey in the oven, not a black outboard motor block." I ducked and ran.
    2008 Bass Cat Pantera Classic
    2014 Mercury Pro XS 200

  14. Member
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    #14
    280 engine's vary in compression ,I have seen them with 140 ish and I have built them with fresh plating and new rings as high as 160. As stated a leakdown test done by a proficient builder is the only real test of ring bore health no mater if it's got 200 or 20 hours . A motor assembled with new rings with egged bores will show a high leakdown , also people that don't know how to do a proper leakdown will always show a high leakdown on the starboard side if the motor is turned the wrong direction in rotation doing the test ( port cylinder must be turned clockwise , starboard bores must be turned counter clockwise ) to get proper reading . I build 30-40 of these plated motors a year , a dial bore gauge , bore bending mandrel and proper hones are a must if one wants to extract all the 280-260-drag motor 's power ...

    Jay @ JSRE
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    #15
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    JSRE
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