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finding TDC with cylinder head on.

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  • finding TDC with cylinder head on.

    In preparation for the assembly of a 7A engine with skimmed head, i have been looking into how to find TDC.

    The marks pressed into the belt guards never seem that convincing to me as for one - the guard could conceivably move if not bolted in place as it came off and two - the mark is quite wide and bulbous compared to the crisp notch in the damper pulley.

    I tried and found using a dial gauge is awkward so looked into the method of 'positive piston stop tool' and spent more time looking on the internet trying to buy one than i did this evening making one out of bits of scrap in the workshop.

    The photos show the simple idea borrowed from youtube and modified to fit deep recess 7A head.

    When it comes to the final install of head, cam and belt i will have a timing disc mounted on the damper and will make a crisp pointer somehow to point at it rather than guess which bit of the original mark is right.

    My test this evening using a red marker shows that the mark on the damper pulley was marginally retarded. not by much at all and it didn't seem to affect the marks on the cam pulley but, with so many points to align and taking into account the possible error introduced by a skimmed head, bent belt guard etc this will show up if i need to buy an adjustable cam pulley. I suspect i will...


    Make tool.
    Insert tool into sparkplug hole and screw all the way home.
    Adjust threaded bar so it protrudes slightly into path of piston as it approaches the top of its compression stroke.
    Turn over engine slowly and gently until the piston meets tool and stops.
    Make mark on damper pulley against the TDC mark on belt cover (or whatever sharper TDC mark you end up making)
    Turn over engine in opposite direction until the piston meets tool and stops.
    Make mark another on damper pulley - this will be a little way away from the first mark as the piston hasn't traveled a full revolution.
    Half way between these two marks is ACTUAL TDC.
    The red ink marks are guesstimated in my photos but for final assembly, timing disc will help clear up the numbers.
    Check cam timing mark 'dot' on pulley lines up with triangle in end of rocker cover...
    weld M8 nut on other end of steel tube - this will serve as spanner flats for screwing the tool into the head. smash the ceramic out of plug and tap M8 thread in hole. weld spark plug remnants onto end of steel tube which has been cut to length so it clears the 7A manifold radius one end of threaded bar and screw through the thread in spark plug and nut welded on other end of tube Make marks. note the factory notch on damper slightly towards the left hand mark insert tool into head. adjust protrusion of threaded bar using large nut welded onto end of threaded bar

  • #2
    Nice work but over kill. A wooden dowel will accomplish the same thing. BTDT.

    Pick one, any one.

    RS2'd 93 UrS4 5 spd sedan
    94 UrS4 V8 6 spd manual avant


    • #3
      Good work. I use a piston stop too. What I did with the original pulley TDC mark was set TDC exactly off a degree wheel, then added an additional mark on the lower cover, so as to line up all 3. Works well, no ambiguity
      Panthero Coupé quattro 20vt
      Indigo ABY coupé
      Imola B6 S4 Avant


      • #4
        Love that. I'm going to make one this weekend.


        • #5
          I think you're overthinking this. The only mark you should be using is on the flywheel plus the cam pulley mark at the top. The damper and belt guard marks should never be used for timing.


          • #6
            I agree the belt guard marks are unreliable but while my engine is on the stand, I can't get my flywheel on let alone the gearbox the mark works with! At least the tool allows easy access for setting up cams etc before it all goes in the car.

            @ urs4 boy - I could find any m14x1.25mm dowels so had to scrap that idea! seriously though, I did look initially for the correct threaded bar but it just doesn't seem to exist which is a shame as that would gave been a lot easier.