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Thread: Skimming a 2.25 head to 9:1 compression

  1. #1
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    Skimming a 2.25 head to 9:1 compression

    How thick should an unmodified 8:1 compression ratio 2.25 litre Series 3 petrol engine head be, and how many mm would need to be skimmed to raise the compression ratio to 9:1?

    I put this question to the Facebook page of this forum and two people suggested, without referring specifically to this engine, that to raise compression ratios by that much I should be looking at domed or high crown pistons to avoid valve clearance or timing issues. But ACR in England skim these heads to 9:1 and I've heard of other people doing it successfully. I just don't know precisely how much needs to be taken off.

    Thanks

    Marce Cameron

  2. #2
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    Quote Originally Posted by Marce View Post
    How thick should an unmodified 8:1 compression ratio 2.25 litre Series 3 petrol engine head be, and how many mm would need to be skimmed to raise the compression ratio to 9:1?

    I put this question to the Facebook page of this forum and two people suggested, without referring specifically to this engine, that to raise compression ratios by that much I should be looking at domed or high crown pistons to avoid valve clearance or timing issues. But ACR in England skim these heads to 9:1 and I've heard of other people doing it successfully. I just don't know precisely how much needs to be taken off.
    Do ACR just 'skim the heads' a set amount or do they measure the capacity inside the combustion chamber at TDC and then work out how much to remove to get to 9:1 ?
    Not sure about valve to piston clearance because I haven't played with the 2.25 motor but it seems a common modification so should be OK.

    9:1 compression - Series Forum - LR4x4 - The Land Rover Forum

    You could use this
    Milling Calculator



    Colin
    '56 Series 1 with homemade welder
    '65 Series IIa Dormobile
    '70 SIIa GS
    '76 SIII 88" (Isuzu C240)
    '81 SIII FFR
    '95 Defender Tanami
    '58 Series II (sold)
    Motorcycles :-
    Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C

  3. #3
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    I had my 8.1 head skimmed when I had harden exhaust valve seats put in, and it pressure tested.
    Not sure if Bob Garner Engineering skimmed the suggested amount to take it up to 9.1.

    Land Rover FAQ - How to tell a 7:1 head from an 8:1 head

    whitehillbilly

  4. #4
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    Thanks Colin and Whitehillbilly.

    I'm buying an upgraded 2.25 head from ACR so they were kind enough to tell me the following:

    "A standard 8:1 Cylinder head measures 3.585". To take to a nominal 9:1 we would remove 0.095" from that
    figure. Looking at your cylinder head, its clearly been reworked at some time, so I would expect it to have been skimmed before, so aim for a finished thickness of 3.490".

    Note that all these measurements are in inches.

    Marce Cameron

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    Remember Bob made the comment, if he took to much, he would have much room to skim it at a later Date, if needed
    So may end up, Un usable, if that was needed.
    Have read some comments, on OS LR websites, saying a double cork Rocker Cover Gasket, may have to be added, as the Tappet screws may contact the Rocker Cover.

    whitehillbilly

  6. #6
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    Quote Originally Posted by Marce View Post
    To take to a nominal 9:1 we would remove 0.095" from that
    figure.
    Interesting that they state 'take it to a nominal 9:1"
    I was always under the impression that you checked the volume of the combustion chamber (the stroke is known) and then you can work out how much to remove. Time consuming (i.e. expensive) so normally more the realm of racing engines.

    Compression Ratio Theory and How to Calculate in Powersports

    Colin
    '56 Series 1 with homemade welder
    '65 Series IIa Dormobile
    '70 SIIa GS
    '76 SIII 88" (Isuzu C240)
    '81 SIII FFR
    '95 Defender Tanami
    '58 Series II (sold)
    Motorcycles :-
    Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C

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