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  1. #1
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    Merc factory reman 2.5 compression variation- 20 PSI port to starboard

    Got 2.5 Merc with factory reman powerhead (or at least it has the sticker) 1/3/5 have 148 to 144 psi, 2/4/6 have 130 to 127 psi- about 120 hours reported- I have put 20 on it. Idles a tad rough- but not bad- 20 psi variation bothers me. Plenty high but a lot of variation? ??

  2. #2
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    hey may have resurfaced one head. Your average Merc will put 120-130 on the gauge. 148psi is on the high side for me but with premium fuel should be fine. You could have the low side head resurfaced to balance it out.

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    I have another 2.5 - XR6 with 145-148 all across- I have always run 87 - should I be running 93?

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    2.5 what? IMO dont go resurfacing anything until you decipher WHY??? Do a leakdown on all cylinders first to get a base line on internal engine condition. Then pop the heads for a look see. If you dont have a burette to check the heads CC's you can lay a straight edge across the bowls and measure to see if there is any variation from one head to the other. 1/64th would = .015 ths. 1/32nd = .032 ths, 1/16th = .062 ths. You might also want to check the head numbers to make sure they are a factory matched set.

    The likeyhood of mercury milling one head and not the other is almost 0%. Unless someone else has tampered with the engine.
    Bud Conner "Heathen" "Defending Our Constitution"

    FOR ALL ENGINE APPLICATIONS
    DRY Film Lubricant for Piston Skirts & Cranks + Thermal Barrier Ceramic Coatings for Piston Tops, Combustion Chambers, Valves etc !!



  5. #5
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    On 140 + 93 is recommended unless someone retarded the timing.

    Quote Originally Posted by CircleHook View Post
    I have another 2.5 - XR6 with 145-148 all across- I have always run 87 - should I be running 93?
    Bud Conner "Heathen" "Defending Our Constitution"

    FOR ALL ENGINE APPLICATIONS
    DRY Film Lubricant for Piston Skirts & Cranks + Thermal Barrier Ceramic Coatings for Piston Tops, Combustion Chambers, Valves etc !!



  6. #6
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    Someone may have just done a head gasket and used the thin one on that side too.

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    Quote Originally Posted by s10gto View Post
    Someone may have just done a head gasket and used the thin one on that side too.
    That my guess
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  8. #8
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    Ok this will be my final post on this issue. I'll lay out the possibles and NON possibles.

    First if a leakdown doesn't tell you something which it should, the possibilities are as follows.

    Possibles: Its a Merc reman so here's whats possible.
    (A) One side decked and the other not? Check TDC on port ans stbd bank with a depth mic.

    (B) Different Pistons on one bank than the other. Some pistons have the ring land location higher or lower in relation to the crown (Piston Top).

    (C) Different sleeves installed on one bank. (Different Port height/timing).

    (D) Two different heads (Heads mismatched) Check the head numbers. Large combustion chambers vs small combustion chambers.

    Whats not possible unless some has tampered with the motor:

    (A) Head gasket thickness. From the thinnest to the thickest .030 to .040 ths WILL NOT!!! give you a 18/21 PSI compression variation. Merc does make a .075 ths gasket But that is HIGHLY unlikely!!!

    (B) Head Milling??? It would take .060 ths difference to give a 18/21 PSI compression varriance.

    NOTE: On a 2.5 motor NON drag HEADS. For every .010 ths milled off you deminish the CC by .7cc's. A basic head is 38 cc's from the factory (foam imbrelled head) which yeilds 120 ish compression (Unless its a low compression fishing motor with 42 cc heads.

    So on a fresh motor with 120 ish compression you need to remove approximately (as heads do vary) .060 ths to gain 20 PSI in compression.
    Bud Conner "Heathen" "Defending Our Constitution"

    FOR ALL ENGINE APPLICATIONS
    DRY Film Lubricant for Piston Skirts & Cranks + Thermal Barrier Ceramic Coatings for Piston Tops, Combustion Chambers, Valves etc !!



  9. #9
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    Quote Originally Posted by s10gto View Post
    Someone may have just done a head gasket and used the thin one on that side too.

    .75 to the thick 1.25 gaskets are exactly 10 lbs diffrence on a 2.5 200.

  10. #10
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    It is a 2001 merc 2.5lt 150hp - SN # says it is O-ring heads- so no gaskets.
    I emailed with merc tech folks- they insist re-mans are straight factory spec. They were not very helpful other than to say it is per the book spec.
    Head bolts appear to be un-touched since re-paint at re-man.

  11. #11
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    "(D) Two different heads (Heads mismatched) Check the head numbers. Large combustion chambers vs small combustion chambers."
    Little help- what numbers am I looking for? I found a 6 digit that matched - a few 3 digit that do not match and I have an extra head off a similar age 2.5 with same 6 digit number but it is not the same casting as the "do not re-used head bolts" is in a completely different place.
    I ordered a leak down tester today- popping heads is about $100 "look see" with o-rings and gaskets.

  12. #12
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    Leakdown won't tell you anything about the heads. If the cast letters (letters really) should be the same on both sides....there are several different 858281 series heads. You really need to remove them and cc them to tell what you have. One good thing is that there aren't many "cut" oring heads out there as there is a lot more to cutting them than the older style heads.

    Those "factory" remans are not what they are cracked up to be....I was told they are done by outside contract builders, not mercury. One 300X quicksilver reman powerhead I disaasembled (shortly after it was purchased) did not even have a factory crank or rod bearings. Aftermarket crap....
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  13. #13
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    most likley what happened was they resleeved one or more holes in that bank and decked only that side i had one that twas 18 thousandths more decking on one side then the other cc the heads first if the same then this will be not to hard to check use a dial indicator thats off the deck a 1inch or so by a spacer check the tdc with the dial indcator it will tell you how much the piston is above the block

  14. #14
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    head gaskets cost me 14 each, orings i reuse the spaghetti on the side and buy new cylinder seal about 2.00 each, if its a oring block which it probaly is ive found lots of surprises in Mercs remain grab bag if gasket heads casting numbers dont lie 488-4 is the number to be on the block 150 or 200 sometime 488 5 on the 200

  15. #15
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    Heads off to measure decking?
    OK- I did not follow that- it has been a long day-
    You re-use the "seal" /spaghetti ? and buy new o-rings for $2.00 ? where - cheapest I have seen is $7.

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