TDC -Compression stroke?
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Thread: TDC -Compression stroke?

  1. #1
    Senior Member dwil4's Avatar
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    TDC -Compression stroke?

    Okay I feel kind of dumb asking this cuz I feel like I know the answer and I'm sure it's been asked a hundred times but I couldn't find it. I know when adjusting the valves it says engine should be top dead center on compression stroke. Is there a top dead center exhaust stroke? When both valves are closed then that should be TDC on compression stroke correct? It's kind of hard to see the position of the cam lobes so I can't really go by that.

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    Member Fast Eddie B's Avatar
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    You're on the right track.

    Yes, there's a second TDC on the exhaust stroke.

    One easy way to tell is to put your finger over the spark plug home. On the compression stroke, air will escape past it. When it stops, you're near TDC on the compression stroke. If you need to be exact, a wooden dowel in the spark plug hole can verify when the piston is at the very top of its stroke.

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    Senior Member GCFishguy's Avatar
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    There's TDC at the end of the compression stroke and TDC on the end of the exhaust stroke. They are one turn apart on the crank. (The crank turns two complete revolutions to complete all 4 strokes)
    Watch the adjusters opening and closing the valves to see which TDC you're coming up to.
    Remove the plug, put something like a drinking straw into the cylinder to feel on the top of the piston to see when it's at the top.

    Or....eliminate all the guesswork and remove the camshaft sprocket cover and use that mark. If the mark is at the top, that's TDC on compression, because the cam makes one complete revolution for all 4 strokes.

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    Senior Member elime's Avatar
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    The easiest best way is to use the timing mark on the cam sprocket. As I have said before, get it right the first time every time.

    Timing Mark.jpg
    Long live the internal combustion engine!

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    Senior Member dwil4's Avatar
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    Quote Originally Posted by elime View Post
    The easiest best way is to use the timing mark on the cam sprocket. As I have said before, get it right the first time every time.

    Timing Mark.jpg
    That's what I thought but I was second guessing myself into thinking that it could line up on exhaust also.

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    Senior Member dwil4's Avatar
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    So there are 2 marks and then a "T" (I guess) with 1 mark to the left of it. Should the crank mark should be on the mark to the left of the T ?

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    Senior Member dwil4's Avatar
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    So there are 2 marks and then a "T" (I guess) with 1 mark to the left of it. Should the crank mark should be on the mark to the left of the T ?

  9. #8
    Senior Member elime's Avatar
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    I believe so.

    A little hard to see the "T" but it is there. If you use this mark you could be exactly wrong so be careful.

    Crankmark.jpg
    Long live the internal combustion engine!

  10. #9
    Senior Member dwil4's Avatar
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    I could be exactly wrong?

  11. #10
    Senior Member elime's Avatar
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    Quote Originally Posted by dwil4 View Post
    I could be exactly wrong?
    Yes!

    With the crank mark in this position the mark on the camshaft sprocket could be either at 12 o'clock or 6 o'clock.

    If the camshaft mark is at the 6 o'clock position, the exactly wrong position, both valves are open at the same time.
    If you adjust the valves with the camshaft sprocket at the 6 o'clock position when you start the engine it will make a lot of noise.

    If the camshaft mark is at the 12 o'clock position, the exactly right position as pictured, both valves will be closed and all will be well.

    Both valves being open at the same time is called "valve overlap". The greater the overlap the higher performance the camshaft and the "lumpier" the idle.
    Long live the internal combustion engine!

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