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Thread: This Can Not Happen

  1. #21
    mmoyle's Avatar
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    Default Re: This Can Not Happen

    Quote Originally Posted by Steve Pierce View Post
    Have not seen open ended push rods. I have a good collection of pushrods to choose from. Got lucky on Thursday and the two push rods removed left the correct amount of clearance between the rocker and the valve after cylinder change.
    Dang.......I think I’m kinda stuck and have to shorten the valve stems to get the correct clearances...even then I think we’ll need longer push tubes for a couple intake valves... will swap around push tubes until I’ve reduced the number of valves to shorten....and hopefully not have to wait for more parts...Marv needs to get back to Arkansas...


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  2. #22
    Administrator Steve Pierce's Avatar
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    Default Re: This Can Not Happen

    Guess I am not aware of the practice of shortening the valve stem to gain the correct dry tappet clearance.

  3. #23
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    Default Re: This Can Not Happen

    Quote Originally Posted by Steve Pierce View Post
    Guess I am not aware of the practice of shortening the valve stem to gain the correct dry tappet clearance.
    It’s a common practice on many automotive applications. Small block ford 302 or 351 C...one of them.....for instance...the rocker arms are not adjustable..they are at a set height requiring the valve stem to be ground shorter to some height...don’t recall that number... Some engines like Perkins have a very narrow range for the valve depth below the flat surface of the head and the valve lash is adjustable. There are also maximum spring heights that require shims under the valve spring to get the correct spring pressure when the valve is closed. Whole bunch of ways to skin a kitty... and I didn’t find any minimum seat thickness in the Lycoming manual...so..


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  4. #24
    Administrator Steve Pierce's Avatar
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    Default Re: This Can Not Happen

    Lycoming manual says adjust the clearance with different length push rods. I think I have posted the service bulletin here before.

  5. #25
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    Default Re: This Can Not Happen

    Yeah...looked up the dimensions for the different length push tubes. I’m kind of surprised the overhaul manual doesn’t say anything about cleaning up the valve stem when it galls like a lifter face does. Nor does it give minimum valve length. Which leads me to believe a rebuilder uses what was and may still be a standard practice of shaving off the end of the valve stem to clean it up..and if the machinist is nit picky....hit the tight tolerance to insure maximum lift if a lifter starts to bleed down at some point.


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  6. #26
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    Default Re: This Can Not Happen

    Be careful trimming valve stems or grinding rocker arms for that matter to gain proper clearance. Doesn't take much and you can throw off valve train geometry, then the rocker will side load the valve and prematurely wear the valve guides out.

  7. #27
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    Default This Can Not Happen

    Quote Originally Posted by JPerkins View Post
    Be careful trimming valve stems or grinding rocker arms for that matter to gain proper clearance. Doesn't take much and you can throw off valve train geometry, then the rocker will side load the valve and prematurely wear the valve guides out.
    Thanks...yup...if I could clean up the rocker arm where it contacts the valve stem it would only be a few thousandths of metal removed to square up the contact area... I’ll post up a picture..


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    Last edited by mmoyle; 06-23-2018 at 11:18 PM.

  8. #28
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    Default Re: This Can Not Happen

    Steve,
    Found the part numbers and length on four of the push tubes. 11.909” and 11.936”. There is one shorter at 11.882” which should give just over 0.020” lash. Do you know the length of push tube with no part number stamped into the tube?


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  9. #29
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    Default Re: This Can Not Happen

    No idea. I usually compare them in length and start with the shortest one and start installing and checking dry tappet clearance. Would be kind of a pain to remove a cylinder, grind a valve tip and reinstall and check. Order the right parts. Won't matter a month or a year from now. You do have the rotator cup installed?

  10. #30
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    Default Re: This Can Not Happen

    Are the tips of the valves heat treated or other to reduce wear. Many are and should not be ground. The angle of the rocker arms are as important as the valve clearance. Proper ratio can be affected and valve side loads can increase dramatically. Valve pressures are low on lycomings so they are more forgiving of less than ideal valve train geometry than a race engine but wear will increase on valves and guides.

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