Deck clearance

Discussion in 'Fox 5.0 Mustang Tech' started by rtmx, Apr 29, 2008.

  1. Gentlemen!
    I have a 5.0 (1991) Mustang with stock pistons. According to Technical Reference & Performance Handbook, the piston deck clearance on this stock motor is .0135 above deck. Which head gasket should be used with aluminum heads, such as Windsor Jr, Edelbrock Perf. etc.
    The Fel-Pro 1011-2 is for alu heads, but the compressed thickness is .039, makin the piston to head clearance an issue. How have you done it? Should I buy the expensive special gaskets, or what?
  2. Moved to 5.0 Tech.

    I'm currently running the 1011-2 gaskets with my TFS heads on a stock shortblock. I haven't experienced any problems whatsoever with my combo, but I'm not winding it past 6250 RPM either.
  3. As far as I recall, the OEM piston to deck is IN the hole appx .010". And I assure there will be variances of +/- .007 on a used block.
  4. There are variances as stated. It can range .01 below and .01 above the deck.

    The stock gasket is .041" compressed IIRC.
  5. I've measured mine to .013. This would give enough clearance with the stock .047 gaskets, but give only .026 with the 1011-2s. Stock gaskets should not be used with Windsor alu heads.
  6. If your piston is "out of the hole" .013"... it is not a stock OE block, or stock OE block with stock OE internals.

    I would like to know how you measured your piston to deck?
    It would be incorrect to measure anywhere other than the center/center of the piston top. Some will say it's okay to measure one edge of the piston perpendicular to the pin to deck while pushing down on the opposite end, then measure the other side you were just pushing on using the same procedure.

    Within the engine performance/rebuild world, it is an acceptable practice to keep a minimum .035" of clearance between the piston and squish pad.
  7. That should be BELOW the deck...
    This isn't something you can look up in ANY book... (and if that is the book I think it is, you shouldn't trust ANYTHING printed in it...)
    This needs to be measured, and re-measured...

    Be more careful to specify either 'In the Hole' or 'Out of the Hole'

    .035" Quench Height does seem to be the *magic* number most often thrown around...
    and is what I aimed for on my 408w... haven't remeasured after final surfacing though. :shrug:

    I have used a stackup of paper/cardboard to center the piston in the hole before measuring deck clearance, then measured from an edge just over the pin.
    I still checked both sides (fwd/aft) of the piston though...

  8. Thanks for your inputs. The book was only mentioned as a reference. The listing in the book would tell that the deck level is .0135 above piston crown, then?
    I've measured the height at pin centerline, both sides, and it will give me .0135 out of the hole.
  9. Just to clarify... when you take a straight edge and run it down the top of your block, it hits the piston?
    Has someone replaced parts on the rotating assembly?
    Is that the same measurement you get on all eight holes?
  10. Most stock decks vary as mentioned above... you have to measure from the center of the piston, and also measure them all. If it is out up front it may be in out back, like a twisted deck. Afterchecking all 8 you should deck the motor accordingly.

    Here's the math

    half the stroke so 3" becomes 1.5" +
    rod length 5.09" +
    compression height of piston stock is 1.600"

    makes for a 8.19", the factory block should be close to 8.20". I've seen them out of the holes only once and that was +.005"

    What are the comp height of your pistons?