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Thread: 94mm HC Piston Identification

  1. #11
    Join Date
    Aug 2007
    Location
    Cornwall S.W. England
    Posts
    5,195

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    Wossner pistons are top quality kit, I have them in my 970cc Bevel twin.
    I have PISTAL'S in my 955cc DESMOQUATRO motor.

    As for skimming the barrels instead of the heads??
    I'll have to agree to disagree with Steve Hillary aka OUTACONTROLLA...
    If you take 1mm off the barrels it's likely that the pistons at TDC will be close to the top of the barrels, if not actually above them?
    Skimming the heads won't alter the deck height of the pistons but shortening the barrels certainly will, possibly then needing to modify the pistons.
    It's 100% OK to skim the 851/888/916 heads by 1mm without the need to sink the (standard 32/28, 33/29mm) valves.
    I set the squish clearance on the 92,94 and 96mm DESMOQUATRO motors to 0.8mm, to be on the safe side.
    Anything more than 1.00mm isn't going to achieve what a squish should and any less (0.75mm or under) is running the risk of the pistons hitting the heads at high rev's.

    Not wanting to start an argument here, rather just stating what I know works

    Steve R.
    Last edited by Mr.R; 01-17-2020 at 02:35 PM. Reason: poor spelling

  2. #12
    Join Date
    Apr 2007
    Location
    bakersville, NC
    Posts
    2,839

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    I tend to agree with Steve R

  3. #13
    Join Date
    Jun 2007
    Location
    Melbourne, Australia
    Posts
    1,415

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    the old fibre head gaskets were about 1.3mm thick, so you needed the piston 0.3 - 0.4mm up above the top of the cylinder to get the squish up. std the piston was pretty much level with the top of the cylinder. leave the original 0.3 - 0.4mm base gasket out and you were about right.

    the later steel shim head gaskets are thinner, so if you are using old cylinders and pistons you generally need to lift the cylinder to effectively lengthen it to get the correct piston to head (squish) clearance. later engines tended to have 0.6mm base gaskets too, so cylinder lengths varied as well.

    the issue i have with shortening cylinders or skimming heads is that you bring the cams closer to the timing shaft, which can lead to belt tension (lack of) issues. so you need to pursue methods there to bring the tension back up, like the gold or silver belt rollers that are larger as used on later engines. even leaving the base gaskets out can give issues with otherwise all std parts. we saw some all std bikes not able to get the correct belt tension over the years.

    the purpose of skimming the head is to reduce the chamber volume and thereby increasing the compression. machining the cylinder doesn't change the compression from where it would be if you were setting the squish anyway, the squish face on the head surface limits where you put the piston.

    imo the 748 head is a great starting point over an 851/888/916 head if the valve sizes suit what you're doing. lot smaller chamber. i even think the 748 ports work better at 850cc level than the 851 ports do - 94mm bore kitted 748 make very nice torque and power (and compression) for the capacity.
    Brad The Bike Boy

    www.bikeboy.org

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