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piston valve pockets 7A

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  • #31
    The collection of parts is growing and in no small part to forum members who have posted up advice and tips or sold parts they no longer need.

    It is time for an update as this evening i have done a dry build to check and set cam timing prior to checking piston to valve clearance, the results are interesting.

    This whole thread started off with my question over piston to valve clearance and the effect a maximum head skim might have on this figure, also; will there be anything to worry about in terms of collisions and how much will it affect the cam timing like some have suggested?

    I got hold of a Tommis Billet cam pulley and this evening fitted it all up and measured from TDC on piston #1 - set using my home made piston dead stop tool and a temporary pointer.


    #1 TDC set
    Cam sprocket circle marks pointing at each other and intersected by half by the top edge of head
    Cam pulley notch lined up with the V groove in cam cover

    To achieve all this i had to advance my pulley by just over 4 degrees. (I've read each mark is 2 degrees but that wasn't specific to Tommis pulley but you can see for yourself in the pic what it is)

    It seems that a max head skim can have quite an effect on timing. I'm sure it would have run and probably very well but there is something satisfying knowing that now it is set EXACTLY as the factory intended.

    Well happy and the next part is the crucial bit - seeing how much a plasticine snake lying on top of my piston gets squashed after a couple of rotations...


    • #32
      The up turn in weather has made my unheated garage a more bearable place to be and this afternoon I have been checking the all importance clearances...

      Many thanks to Error404 for his help in this - top bloke with skills to match.

      The plasticine snake went on top of piston 1.

      I built up the head with 1 inlet and 1 exhaust tappet made 'solid' with parts supplied by Alex, tweaked the head down a little bit but not torqued properly so will need to subtract a small amount from any figures.

      Cams fitted and caps tightened, engine timed up.

      I confirmed that the 'solid' tappets gave no lift when the heel of the cam was on them and they didn't so i knew that the valves were being pushed as they should by the cam and not by any mistakes in my measurements for the tappet blanks.

      2 crank revolutions to make sure both cams had done a complete turn and then lift the head off.

      Interesting results!

      Snake compressed into valve pocket left a lovely witness mark that measured to be 3.4mm (as best as you can measure plasticine with a dial gauge)

      I did a second test with my adjustable pulley turned back to zero advance just to see what difference that made and I got pretty much the same result in the valve pocket but, this time I paid attention to the bit of snake near the edge of the cylinder and i found the piston gets rather close to the head. I had been focusing on the valve pockets and not the other bit.

      The piston / head gap is less than 2mm so i'm planning on a skim off the top of the piston to sort that out, strange how the pockets are well clear - where did the original marks on the valves come from?!?

      oh well, a skim and then on to the balancing step!


      • #33
        As per my PM mate, ~2mm piston crown to head clearance is fine, 1.2mm I run on my build, so I think there’s no need to skim the pistons
        Think you can crack on now and get it built
        Panthero Coupé quattro 20vt
        Indigo ABY coupé
        Imola B6 S4 Avant


        • #34

          Nice one! I need to deduct whatever it was for gasket crush but based on another member measuring his old head gasket v my new one, that figure was only 0.04mm so not enough to worry my pistons if you are running 1.2mm!

          Thanks Alex, I'll make the call to the balancing people...