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checking TDC

Discussion in '206/246' started by daviekj, Nov 21, 2008.

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  1. daviekj

    daviekj Formula Junior
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    May 9, 2008
    486
    UK
    #1 daviekj, Nov 21, 2008
    Last edited by a moderator: Sep 7, 2017
    Before setting up the ignition timing, i thought it best to clear up and check what the markings next to the crank refer to. For the timing, i could rely on the exisitng 6deg advance marker, but feel more comfortable checking.
    With a dial guage on piston#3, TDC ocurs fractionally to the right of the timing indicator lower peak. If i mark (as in the photo), equal inceraments back from the 10 and 6deg marks, then this lies closely to 0.5deg adavnce. Given the error in judging TDC from the dial gauge, this could well be 0deg.
    My questions are:
    1) do i have my markings in the picture correct, with numbers indicating degrees of advance from TDC?
    2) what does the "PM" mark mean. Is it short for latin Post Meridian, this implies after, not before TDC?
    3) what does the "3-4" mark mean below the "PM". Not obvious in the photo, but those who know the answer, will know what the question is.
    4) right in front of the crank timing mark is a big water pipe to the oil heat exchanger. It provides an obsticle to sighting the timing marks. Its very easy to get a parallax and be at least a few degrees out. i managed to unbolt the heat exchanger and pull it down on it engine mount using a nylon tie, without un plumbing. Still not easy to get a good line of sight, specially for the dynamic timing. Anyone found a way around this?

    Kevin
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  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,846
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #2 Steve Magnusson, Nov 21, 2008
    You need to think in Italiano not Latin ;)

    PM 3-4 = punto morto 3-4 = "dead point" 3-4 = TDC for both cyls #3 and #4 (which occur simultaneously, but are in different parts of the 4-stroke cycle).
     
  3. daviekj

    daviekj Formula Junior
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    May 9, 2008
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    #3 daviekj, Nov 21, 2008
    Steve,
    bravissimo, molto grazie,
    So is the peak of the lower comb between my "0" and "2" in the timing plate supposed to be TDC?
    kevin
     
  4. DinoDriver

    DinoDriver Formula Junior

    Mar 14, 2005
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    Leesburg, VA
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    Bill Ebert
    #4 DinoDriver, Nov 21, 2008
    Kevin, Ref. your Q #4: I don't know your hose/oil cooler configuration, but on my '74 U.S. Spec Dino w/ A/C, I use a tool you probably have in your kit . . . a 2 1/4" mirror mounted on a swivel on an extendible rod by Craftsman. With my head deep in the wheel well, the engine at idle and no caffeine in my system, I carefully thread the mirror thru/around the obstructing hose and belts to a position directly above the timing marks. I note about how many degrees it needs to be moved, bring my head and the mirror out, rotate the distributor mounting plate the amount I guess it needs to be moved and repeat the mirror trick. If I'm doing this alone, it usually takes me no more than three tries using pencil reference marks on the distributor housing plate. With a knowledge helper, one try. Eight degrees is the magic number for my '74 w/ all the EPA stuff removed. You have 8 degrees correctly marked. Hope this helps.

    Bill
     
  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
    26,846
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #5 Steve Magnusson, Nov 21, 2008
    I think so, but you never quite know where they assumed the observer's line of sight is located (and being a stamped sheetmetal piece it may not be that uber-accurate itself). The way you have it drawn (with your added arrows) assumes the observer is always looking down a radial line that passes thru the center of the crankshaft -- maybe they didn't assume this (based on your description in the difficulty of getting a direct view).
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,846
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #6 Steve Magnusson, Nov 21, 2008
    Actually, is this correct? I'm having a hard time thinking how two cylinders in different banks of a 65 deg V-6 block could simultaneously be at TDC? Did they "fudge" a little on this one?
     

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