Surface Skimming 308 Heads on a rebuild - how much tolerence is there to play with? | FerrariChat
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Surface Skimming 308 Heads on a rebuild - how much tolerence is there to play with?

Discussion in 'Technical Q&A' started by tomoshea, Jun 28, 2005.

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  1. tomoshea

    tomoshea Formula Junior

    Dec 29, 2003
    541
    Ireland
    Full Name:
    Tom O'Shea
    #1 tomoshea, Jun 28, 2005
    Question.

    Does anyone have definitve data on what the standard clearance is between the standard pistons at TDC and the closed inlet/ exhaust valve.

    I have looked in the manual - maybe I am missing something but I cannot see it?

    I am trying to understand what tolerance I have to play around if I was to skim the mating surface on the heads before I put them back together on a rebuild?

    Cheers
     
  2. Dave

    Dave F1 Rookie

    Apr 15, 2001
    2,722
    Little Rock
    Full Name:
    David Jones
    #2 Dave, Jun 28, 2005
    Are the heads warped or are you trying to up the compression ratio?
     
  3. Gary48

    Gary48 Guest

    Dec 30, 2003
    940
    #3 Gary48, Jun 28, 2005
    Skimmed about .004 off my 246 heads just to get a fresh mating surface. Heads were not warped, worked great.
     
  4. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    13,630
    San Carlos, CA
    Full Name:
    Mitchell Le
    #4 yelcab, Jun 28, 2005
    There is a minimum thickness specification for a head. Don't you have that in the service manual, in the heads section? I remember finding it once there.
     
  5. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
    5,379
    NWA
    Full Name:
    Paul
    #5 Artvonne, Jun 28, 2005
    127mm, to 127.2 mm for head height. They dont say anything about it other than the dimension.
     
  6. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
    5,379
    NWA
    Full Name:
    Paul
    #6 Artvonne, Jun 29, 2005
    Some other thoughts though. Metal removed is gone forever, and we have a finite supply of heads. If your head has warpage, and you skim the bottom off to make a flat surface, the top, which is also the cam journal line, will not only be warped also, but now that the block side is flat, it will remain so. Now you would need to line bore the cam journals and recut the upper surface and realign the cam cover, and.....

    I stopped doing all that work some years ago. Unless the head is drastically warped, say in excess of .006"-.010" (judgement call), I would rather just clean the surface and try retorqueing it down. I found a shop some years ago that claimed they could straighten a head by annealing it and then clamping it to a surface plate to cool, sometimes with shims in the low points.

    This is probably why the WSM wants you to torque up the heads and main bearing caps before checking the crank bore for alignment. I would assume then, using that thought, you should maybe check cam bore alignment/head warpage with the heads torqued down?
     

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