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Thread: Identifying TDC

  1. #1
    LS Swapper Josh's Avatar
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    Identifying TDC

    I could figure this out on my own, but i figured I would ask first and potentially save me some effort.

    I am currently indexing my 36-1 trigger wheel for MS. I am running it off the crank pulley, so i need to identify where TDC is. It would be easiest to locate it in relation to where the keyway sits. I currently assume the keyway is facing upwards at TDC, as shown i the picture. If someone could confirm or deny this (and tell me where it actually is supposed to be) that would be awesome.

    trigger wheel alignment.jpg

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  2. #2
    One of those purists you keep hearing about. sdg3205's Avatar
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    Quote Originally Posted by Josh View Post
    I could figure this out on my own, but i figured I would ask first and potentially save me some effort.

    I am currently indexing my 36-1 trigger wheel for MS. I am running it off the crank pulley, so i need to identify where TDC is. It would be easiest to locate it in relation to where the keyway sits. I currently assume the keyway is facing upwards at TDC, as shown i the picture. If someone could confirm or deny this (and tell me where it actually is supposed to be) that would be awesome.

    trigger wheel alignment.jpg
    Josh,

    I found TDC using the method whereby you remove the plug under the water pump and find the rectangular slot. Is your water pump off the car? If so it's super easy and quick.
    Dave

    Here, somewhere.


  3. #3
    LS Swapper Josh's Avatar
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    Quote Originally Posted by sdg3205 View Post
    Josh,

    I found TDC using the method whereby you remove the plug under the water pump and find the rectangular slot. Is your water pump off the car? If so it's super easy and quick.
    It was a week ago, darn.

    Something that came to mind is since the 2.8 is oddfire and the 3.0 is even, the TDC may be different?

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  4. #4
    One of those purists you keep hearing about. sdg3205's Avatar
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    Quote Originally Posted by Josh View Post
    It was a week ago, darn.

    Something that came to mind is since the 2.8 is oddfire and the 3.0 is even, the TDC may be different?
    Yeah, i hadn't thought of that. good call.
    Dave

    Here, somewhere.


  5. #5
    Senior Member
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    The best way to find TDC is to get #1 cylinder on the compression stroke and, using a dial indicator, rotate the crank until the piston is at TDC.
    David Teitelbaum

  6. #6
    LS Swapper Josh's Avatar
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    Quote Originally Posted by David T View Post
    The best way to find TDC is to get #1 cylinder on the compression stroke and, using a dial indicator, rotate the crank until the piston is at TDC.
    yep! That is the plan if I have to figure it out myself.

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  7. #7
    Senior Member WelmoedJ's Avatar
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    Quote Originally Posted by Josh View Post
    yep! That is the plan if I have to figure it out myself.
    Use a timing light, or an old spark plug (not the one in there) and plug wire for cylinder 1.
    Should be a quicky that way.
    Welmoed
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  8. #8
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    Quote Originally Posted by WelmoedJ View Post
    Use a timing light, or an old spark plug (not the one in there) and plug wire for cylinder 1.
    Should be a quicky that way.
    And with those items how do you mechanically locate TDC? The OP was not looking to time the ignition system, at least not yet. Before you can do that you need to know where TDC is. TDC (Top Dead Center) is where the piston in cylinder #1 (or whatever cylinder you are trying to time) is at the exact top of it's stroke and is between the compression stroke and the firing stroke. I believe he is trying to find this point so he can set up a crank sensor for an alternate ignition system. Locating TDC (or just verifying it) is a basic technique in preparation to setting ignition timing. It is not good to just assume the timing marks are accurate. They are not always and can be off a little and are not usually adjustable. This is a mechanical procedure and does not involve a running motor or even an ignition system.
    David Teitelbaum

  9. #9
    LS Swapper Josh's Avatar
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    Quote Originally Posted by WelmoedJ View Post
    Use a timing light, or an old spark plug (not the one in there) and plug wire for cylinder 1.
    Should be a quicky that way.
    I am also curious about this method, the assumptions David made are correct.

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  10. #10
    "Former Delorean owning Guru" Spittybug's Avatar
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    Quick and dirty......

    Remove plug in #1. Insert a pencil, eraser side down, slowly and carefully until it hits the piston. Put car in 4th gear and gently push/pull car until the pencil sticks out the greatest amount. This means you are either TDC on either the power or the exhaust stroke. To find out which, look to see where your rotor is pointing in the distributor. If pointing to #1, you are on your power stroke.

    "Pencil" is a metaphor. Depending on the depth of your plug hole (I don't recall if I've used this trick on my Delorean, but I certainly have on other cars) you may need a long plastic straw or something.... Just make sure it is soft and that you pull it back out again when done!
    Owen
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