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Thread: T.D.C

  1. #1
    Apprentice MountainMike's Avatar
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    Default T.D.C

    How do I get to Top Dead Center? Talk me through it like I was a Idiot cos I can screw up the obvious.
    Mike

    1997 Defender 110 300Tdi



  2. #2
    Apprentice MountainMike's Avatar
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    Default Re: T.D.C

    Forgot to mention T.D.C. on No 1 cylinder, you see, I'm an idiot!!

    1997 Defender 110 300Tdi



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    Consultant Marc Lurie's Avatar
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    Default Re: T.D.C

    An easy way to find TDC is by removing the injector, and inserting a length of thin steel rod into the hole until it touches the piston.

    Then you slowly rotate the engine by using a spanner on the crankshaft pulley nut. MAKE SURE YOU DISCONNECT THE BATTERY BEFORE YOU DO THIS.

    You will see the rod rise and fall. when it's at its highest point, that's TDC.

    However, there's two times in each revolution of the engine when a piston is at TDC. The first time is when it compresse the air/fuel mixture and ignition takes place, and the second time is when it ejects the burned fuel through the exhaust valve.

    If you need to find TDC at the time of firing, then you must remove the tappet cover and look at the tappets above the cylinder you are checking. You will see that they open and close as you rotate the engine. When a tappet presses down on the valve stem, the valve opens. When the valve is closed, there will be a small gap between the tappet and the valve stem.

    If the piston is at TDC and one of the valves is open, then that cylinder is in its exhaust stroke.

    If the piston is at TDC, and both valves are closed (neither tappet is pressing against a valve stem), then the cylinder is at TDC on its compression stroke, and would ordinarily be firing at that point.

    I hope that isn't too confusing.

    Cheers,
    Marc
    Last edited by Marc Lurie; 11th Jul 2007 at 08:05. Reason: I didn't look at Mike's signature. If I had, I would have seen that it is a 300Tdi.
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  4. #4
    Defender of the Forum. TEMPL4R's Avatar
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    Default Re: T.D.C

    Quote Originally Posted by Marc Lurie View Post

    If the piston is at TDC and one of the valves is open, then that cylinder is in its exhaust stroke.
    You will find they are "on the rock", Marc.
    The exhaust valve is closing and the inlet starts to open just before TDC.

    A quick way to find TDC is turn the engine while you touch the rockers with your fingers, when you feel one start to move down as the other is coming up, then rock the crank, you will feel the point the valves pass each other. Get them even and the engine is a TDC.

    The more accurate way, as you say, it to take a glow plug or injector out, but you can still have the crank a couple of degrees before or after TDC. You are best using a timing mark on the cover or in the flywheel.

    Chris
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  5. #5
    Apprentice MountainMike's Avatar
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    Default Re: T.D.C

    Thanx guys, I need T.D.C on 1 so that I can remove the vacuum pump, does it matter if its on the exhaust or firing ?

    1997 Defender 110 300Tdi



  6. #6
    Consultant Marc Lurie's Avatar
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    Default Re: T.D.C

    Quote Originally Posted by TEMPL4R View Post
    You will find they are "on the rock", Marc.
    The exhaust valve is closing and the inlet starts to open just before TDC.
    You're right of course Chris, but I thought that would confuse the issue. Also, I said that the cylinder would fire at TDC, but that's not necessarily true (particularly in petrol engines).

    AFAIKR it won't make a difference to Mike whether he's at either TDC if it's just the vacuum pump he's replacing. I don't even think it's entirely necessary to put the cylinder at TDC in any case, it just makes reassembly easier.

    Marc
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    2006 300Tdi Kalahari S/W - White and muddy

    1970 SIIA 88" - Semi-matte dark green

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  7. #7
    Engineer Jode's Avatar
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    Default Re: T.D.C

    I don't think it matters; I think the point of the workshop manual insisting that you find TDC at firing is that you can then insert a dowel into the injection pump (by removing the small round timing cover to the timing belt housing) and lock its position, and at the same time fit a pin into the bottom of the flywheel and fix ITS position. By doing so, you know that the valves and fuel system parts are EXACTLY aligned, so you avoid what might be a costly mistake arising from misalignment when starting the engine later.

    Cheers.
    Mosweu ko nageng

    1995 300TDi 110

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