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  1. #1
    Welly's Avatar
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    AGU pistons - undersized. But why?

    Can anyone shed light on this? I have googled and search till now blue in the face.

    The pistons I have removed from an AGU engine are .05 mm undersized. This is marked in the normal way on the piston face with 80.95 -.

    Does anyone know why this would be the case in an 81mm bore?

    Doesn't really matter I grant you, but I am curious.
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  3. #2
    Prawn's Avatar
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    At a guess I'd say surely all pistons will be fractionally undersized in the bore? if you had an 81mm bore with an 81mm pistons it would be an interference fit wouldn't it? there has to be some give which is taken up by the rings, to take into account the piston expanding at a greater rate than the iron cylinder when it gets hot.

    Are your old S3 pistons stamped 81mm?
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  4. #3
    Welly's Avatar
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    No, thats the weird thing, they aren't stamped at all. I can't see why the AGU pistons would be stamped if the others aren't - it seems to be information of some kind.

    Checked VagCat and came up with this:

    APY and AUL pistons have this part no: 06A107065AB

    AJQ,ARZ,APP,AQA,AGU pistons have this part number: 06A107065J

    Obviously the suffix letter is different.... Both pistons are listed as 81mm - but I guess this is to fit an 81mm bore - not the piston size.

    I'll be fitting the AGU pistons anyway, I just wondered why they would go to the hassle of stamping them 80.95-...
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  5. #4
    Welly's Avatar
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    Have also found this: http://www.mjmautohaus.com/catalog/p...oducts_id=3982 which corresponds to the AGU piston part number.

    Clearly says piston is 81.01mm.
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  6. #5
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    Don't quote me on this but I remember seeing that AGU pistons are made by a different supplier to other 1.8T engines. Pretty sure ETKA cataloque mentions Mahle as the manufacturer. Might account for differences in numbers stamped and where.

    Check out Mahle's website perhaps?

  7. #6
    RIP S3dave

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    they are not undersized.. they are for the stock 81mm bore
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  8. #7
    Andrew@A.L.D's Avatar
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    The bore is 81mm

    The pistons are smaller as they need room to expand ( Skirt/piston clearance ) with the heat of the turbo
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  9. #8
    S3 Nattie's Avatar
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    There is also a tolerenace of 0.05 +/- mm for the piston. If the bore is 81mm measured then the piston will be slightly smaller as you have the piston rings.

  10. #9
    Welly's Avatar
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    Just struck me as odd that a parts supplier listed that piston's part number as 81mm or whatever it was.

    That's OK anyway, mystery solved!
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