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  • valve clearance

    Does anyone have any data on how valve clearances vary with temperature. e.g. if you set .010" hot what would you find when the engine was stone cold?

  • #2
    Ron,

    The thermal coefficient of linear expansion of steel is 7.2 x 10-6.

    So roughly.....

    Ignoring cam lift expansion and assuming the engine runs at around 190°F and cold is 60°F.

    expansion would be 0.0000072 x 130 (delta t) x length of pushrod (guess 7" + length of valve stem guess 2").

    so.....0.0000072 x 130 x 9 = 0.008"............so your 0.010" would very roughly end up as 0.002". Plus whatever the block and head grows which could be around 0.008" as the length between the cam and the valve top is the same/similar.

    There are lots of guesses....best way is to measure cold and hot. My guess is that it would not change a lot.

    Why do you want to know?

    Dick.

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    • #3
      Dick,
      Many thanks for your post. Indeed I have done that. Got the engine up to 87C, set one valve to .006" and left the car for 24 hours. I then remeasured the same valve without the engine being moved and this was still .006" so on my particular engine build, and in its current state of wear, it has all balanced out so I feel happy to set the clearances cold.
      I am afraid that I am a lazy sod and it avoids overheated fingers!! In cold weather, with thermosyphon cooling, it avoids having to hurry or repeated bouts of engine running to maintain the desired hot engine temperature.
      Incidentally the procedure I used was that recommended by Michael Marshall of removing all plugs, setting the desired valve at max opening and then rotating the engine one full turn of the starting handle before setting the gap. Nice and simple and works on any engine!
      Gordon.

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      • #4
        Hi Gordon,

        I suspect that the "maintained" clearance is to allow splashed oil to get between the sliding surfaces of the cam and follower.

        Sorry about addressing yo as Ron........senile moment.

        DICK.

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        • #5
          Hi Dick,
          I think the point about ensuring sufficient oil gets to the sliding cam and follower surfaces is a good one. Thanks for that. No need to apologise - no offence taken at all.
          Gordon.

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          • #6
            I tried out a method for checking the valve clearance with the engine at tickover. Say you have just set uo your clearances to .006". Using this thickness of feeler gauge I could insert it carefully - if the clearance was less then it would not gob in. If the gap was too wide then you could move the feeler easily. If the gap is correct there is a little friction to moving the feeler. When I did this I found one clearance to be excessive and when checked statically an .008" feeler would not go in. Result is engine now ticks over with no clicking. The oil from the rockers disappears down the pushrod hole and doesn't build up so must work its way past the followers and so should lubricate the cam/follower interface

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