Bore size

Discussion in 'Gas Engines' started by Danielsan, Apr 16, 2020.

  1. Danielsan
    Joined: Jul 2004
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    Danielsan Amateur designer-builder?

    need some feedback....
    rebuilding an 165hp mercruiser 250CID 6 in line.
    the specs tell bore size 3.875 (98.425mm)
    when i measure the bore it gets me 98.35 - 98.40mm measured with a bore gauge...
    measured it different ways and always get the same...

    being an mid 70s engine, don't know the usage of it but I would imagine bore size be bigger....

    for info the piston ref number says 3877604 9 GM 6
     
  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Are the cylinders perfectly round or oval?
    Are there signs of piston slap?
    Have you gapped the rings?

    Or, just hone them and put new rings in.

    Sorry, what's your question and why are you doing the rebuild?
     
  3. Ike
    Joined: Apr 2006
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    Ike Senior Member

    The Mercruiser 165 is simply a marinized version of the GM 250. You can lookup the specs in the manual at Boatinfo - Mercruiser Service Manual nr. 2 - 1974-1977 All Engines & Drives https://boatinfo.no/lib/mercruiser/manuals/mercruiser2.html#/0
    for the 165 bore is 3.8745 - 3.8775 or 98.412 mm - 98.489 mm. Specs are in Sect 8 Part B page 683.

    Ps you need to have Adobe flash player installed to view it.

    Sorry that's the link for the earlier models, 74-78 is at Boatinfo - Mercruiser Service Manual nr. 2 - 1974-1977 All Engines & Drives https://boatinfo.no/lib/mercruiser/manuals/mercruiser2.html#/0 But I checked and the specs are the same.
     
    Last edited: Apr 16, 2020
  4. rxcomposite
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    rxcomposite Senior Member

    If the cylinder bore is within specs and not oval, you only need to deglaze and hone it (cross hatching). You can buy manual deglazing tool that can be attached to a drill. If you can't find one, you have to send it to a machine shop.
     
  5. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The dimension 98.425 mm is the nominal diameter, around which there is a tolerance band. Most specs have bore classes, which are mated to suitable piston classes. In this engine size I'd expect an overall tolerance band of +- 0.03 mm. This means your cylinders come out slightly undersize. The reason of that can be wrong temperature at measuring (object and tool should normally be +20 C), or a slight glazing.

    Anyway, I support rxcomposite's recommendation above, good luck!

    Edit: oops, didn't notice Ike's figures; in fact the tolerance band here is +- 0.0385 mm. But the recommendation still holds.
     
  6. Danielsan
    Joined: Jul 2004
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    Danielsan Amateur designer-builder?

    Hi guys, thx for all the feedback. will make short question longer... :) however still try to be short...
    I had myself a Fletcher boat for the price of nothing...
    I decided to strip it completely down.
    now i am starting to rebuild and that is where the engine come in the play also...
    altough the previous owner said they had dome some work on the engine i want to be sure of the state of that engine
    so i did put new head gasket because i removed the valve head to (i opened the engine to see how it looks inside)
    lets say it looked ok could spin it by hand ...
    i closed the engine and built it up again....
    put in oil and primed it till i had oilpressure
    cranked it and it ran in a few moments after fuel got thru the lines....
    after running i tested compression that is where i noticed some problems...
    loss of compression on 3 cyl.
    so i opened the little beast:
    checked for leaks by pouring oil on the pistons and by blowing air into the inlet outlet of the valve head.
    this showed me leaks on most of them some minor some bigger. I polished the valves in their seats with some drill and compound with some positive result at the end.
    No more leaks....
    As concerns the cylinders i had one where the oil dissapeared...
    thats where i decide to take out the pistons.
    in short most look ok but one has oil rings completely seized on the piston.... unable to remove them, heating soaking tapping ... no help....
    so there is tthe moment i say i need new piston(s) and rings
    So to besure of the size and be sure the bore wasn't oversized yet i started measure and that is where my question came from.

    I find smaller bore diameter than the manufacturer specs which does'n sound normal to me.
    should be 3.875 ±0.005 in my case i prefer mm 98.425
    I measure 98.35 - 98.40 so this makes me confused with the specs... temp where engine is is 18°C
    is there a reason for this?

    I also measure the gap of the segment rings: 0.40-0.45mm that seems correct

    I had a small picture with some readings from my compression testing and some coded text :)
     

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    Last edited: Apr 17, 2020
  7. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Yes I find it a little confusing that what you are measuring is smaller than the "Standard" given by the manufacturer. Tighter than usual if you may call it. Most modern engines are manufactured to such close tolerance that the measurement is as spec'd.

    But you cannot assume something is wrong if you have not done full measurements. That is from top (about 25 mm down), middle, and bottom. Two measurements at each position at 90 degree to each other, starting from a position parrallel to the crank pin. Repeat for each position, and repeat for all the cylinders. This will give you the whole story as cylinder liners do not wear out evenly. It wears more at the top than the bottom and becomes more oval perpendicular to the pin.

    A quick check would be to remove the compression ring, place it inside the cylinder and measure the gap at the 3 positions. That will give you a birds eye view of how the cylinders worn out.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    An old engine, I think I'd be more concerned about the state of the drive leg, assuming it is a sterndrive.
     
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  9. Danielsan
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    Danielsan Amateur designer-builder?

    That will be the next thing to worry
     
  10. Danielsan
    Joined: Jul 2004
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    Danielsan Amateur designer-builder?

    Hi guys i am back

    this is what the readings give me they all are way under the nominal specs of the 250CID if you ask my modes opinion...
    or i did something wrong...
    upload_2020-4-18_15-51-38.png
     
  11. DCockey
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    DCockey Senior Member

    What are you using to measure the bore size?
     
  12. Danielsan
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    Danielsan Amateur designer-builder?

    An analog bore gauge 0,01mm readings....
     
  13. DCockey
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    DCockey Senior Member

    When was it last calibrated? How was it calibrated?
     
  14. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Buy oversized pistons and rebore to suit.
    End of problem.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Why are you rebuilding it ?
     
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