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  1. #1
    Join Date
    May 2009
    Location
    West Virginia
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    102

    Question Piston/cylinder wall clearance

    Recently had a "new" powerhead put on my Merc 2001/225HP/EFI. Have had problems getting out of the hole and general sluggishness, since this installation. I am going to put a set of Simon worked heads, new reeds, and a reworked flywheel from Eric Simon. Tore it down to install above and checked clearance between piston and cylinder wall. All were .45mm, and seemed to me, "a real novice" that the pistons were rather loose, but I don't know. I do know there has to be some expansion clearance, but is this .45mm within tolerance?
    Last edited by Powmer; 05-24-2009 at 04:24 PM. Reason: more info

  2. #2
    Join Date
    Feb 2009
    Location
    Edgewater,B.C. CANADA
    Posts
    4,244

    Piston fit.

    JMO but .45mm = 0.018" seems like to much to me, I'd think something like 0.010" or .26mm would be better #s for forged piston max. Sure one of the engine builders will give better #'s.
    Dave

  3. #3
    Join Date
    Apr 2001
    Location
    Old River , Texas ( 35 mi East of Houston )
    Posts
    10,970
    Cast pistons to steel bore would be close to .005" and forged piston to steel bore would be .008" clearance to be "normal".....Dunno know the specks on 3 liters BUT that seems REAL loose to me...

    my .02
    Jay
    Jay @ JSRE

  4. #4
    Join Date
    May 2009
    Location
    West Virginia
    Posts
    102
    I can, (the heads are off) rattle the pistons with my fingers. I would think that the rings would hold the piston snug in the cylinder. If the pistons to cylinder gaps are over tolerance, what are my best options. Never had better that 100 psi. This 3.0L block has pressure relief slots, will this make for low psi?

  5. #5
    Join Date
    Apr 2001
    Location
    Old River , Texas ( 35 mi East of Houston )
    Posts
    10,970
    Relief slot motors WILL yield low compression with a low piston speed in testing ( as with a battery ) The window of opportunity to make compression is less with the slot open to compression loss. The faster the piston speed the ring goes by the opening quicker thus the compression will have less time to escape thus the compression will be higher. 100 lbs cranking for a relief slotted 87 octane rated motor would be in the ball park at cranking speeds...

    my .02,
    Jay
    Jay @ JSRE

  6. #6
    Join Date
    May 2009
    Location
    West Virginia
    Posts
    102
    Just examined the pistons again. I can literally shove the piston to one side and see the top ring with a bright light. Worst case is .40mm on two cylinders to best case 30mm on the rest. Have tested compression on warm engine running and got 105psi, this is tops. I have at least 25 hours on this engine.

  7. #7
    Join Date
    Sep 2002
    Location
    Atascadero,Calif
    Posts
    1,211
    I take it your measuring at the crown of the piston not the skirt. The crown needs to be looser because of the expansion through heat. To find out you'll have to pull the piston and measure bore and piston. Seazeco pistons in a OMCeaze will move what seems like a 1/32" or more!
    Gary

    I don't know enough, to leave well enough alone.

  8. #8
    Join Date
    Apr 2001
    Location
    Old River , Texas ( 35 mi East of Houston )
    Posts
    10,970
    The tighter points of the piston to wall clearance is low on the skirt not high...To know the EXACT clearance you would need to remove the piston and measure below the wrist pin and then measure the bore and compare measurements..

    Jay
    Jay @ JSRE

  9. #9
    Join Date
    Sep 2002
    Location
    Atascadero,Calif
    Posts
    1,211
    Quote Originally Posted by Jay Smith View Post
    The tighter points of the piston to wall clearance is low on the skirt not high...To know the EXACT clearance you would need to remove the piston and measure below the wrist pin and then measure the bore and compare measurements..

    Jay
    Unless it all comes out as a molten mass.
    Gary

    I don't know enough, to leave well enough alone.

  10. #10
    Join Date
    Apr 2001
    Location
    Old River , Texas ( 35 mi East of Houston )
    Posts
    10,970
    OUCH !
    Lets hope not !


    Jay
    Jay @ JSRE

  11. #11
    Join Date
    May 2009
    Location
    West Virginia
    Posts
    102
    I can see on the piston .015. I take this as being .015" over sized. At TDC on each piston they all rattle easily. I will pull a piston and check both bore and skirt.

  12. #12
    Join Date
    Sep 2002
    Location
    Atascadero,Calif
    Posts
    1,211
    Ok. There is a difference between the piston rocking on the piston pin and the piston moving in the cylinder bore. I know on my engine you can see the piston ring move, or actually the piston move over the ring. If I rock the piston by pushing on the left and then right side of the piston, that is the piston rocking back and forth on the pin. If I push up and down, sliding the piston on the pin, this is the clearance in the bore. This movement is very slight.
    A piston is tapered from top(smaller) to bottom like a cone. Also it is cam ground, wider across the skirt then across the pin, like an oval. The piston grows to size across the crown and across the pin boss aera.
    Gary

    I don't know enough, to leave well enough alone.

  13. #13
    Join Date
    May 2003
    Location
    SOUTH OF BOSTON
    Posts
    2,685
    some pistons step in at the ring landins but are wider at the skirts ware ya need it.

  14. #14
    Just being able to move the pistons in the bore is just guessing. You need the true skirt clearance to be even close to an educated guess at what the bore to piston clearance is. The pistons are not round either so you have to check for clearance at the widest area on the piston.Sounds like you expected a big increase in compression and power from a standard rebuild???
    www.eagleoneperformance.com


  15. #15
    Join Date
    May 2003
    Location
    SOUTH OF BOSTON
    Posts
    2,685
    hears a shot of 3 pistons, they all fit the same Moda, the one on the left is forged, the crown is under cut, sounds like what you are seein. if they was all at .020 clearance you would hear them
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