R12 head
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Thread: R12 head

  1. #1
    1000+ Posts REN TIN TIN's Avatar
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    Default R12 head

    Is this saveable?

    R12 head-r12-head.jpg

    Head height is 71.85mm. According to Mr Haynes this in not the minimum for the high compression head.
    Does it even matter as long as the valve seats are okay.

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    R12 head-img_0178.jpg

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    RTT
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  2. #2
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    Head heights:
    12TL, 810-02/842-01.
    Nominal height – 73.4mm.
    Minimum height - 72.9mm.
    Combustion chamber volume - 37.8cc

    12GL, 810-05/06
    Nominal height – 72mm.
    Minimum height – 71.5mm.
    Combustion chamber volume – 33.8cc

    As for whether it matters or not, it would all depend on the general condition of the rest of the motor, and the various degrees of performance attributes you are looking to achieve.
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  3. #3
    1000+ Posts bowie's Avatar
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    From the other thread Frans indicated he has seen heads shaven beyond 71mm in different applications (noting the depth of the oil hole)

    But I confess i didn't understand what the min height references where published for? Was it just to ensure things were near enough OEM as a reference for mechanics and builders?

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  4. #4
    1000+ Posts Frans's Avatar
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    Everytime the head is skimmed the combustion chamber becomes smaller and the CR goes up. The factory has a minimum and that is what is published. My opinion.

    Frans.
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  5. #5
    1000+ Posts bowie's Avatar
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    There you go Ren. It's ok. it's just giving us free power but making the combustion chamber smaller
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    Works: 1999 Volvo S40 T4, (has Choo Choo's)
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    Need to check many things when going under recommended values.
    Thermal expansion of conrod and piston. Piston wobble especially when worn
    Valve bounce if springs are old or weak
    Check out
    HAMMER PERFORMANCE - High Performance for your Harley Twin Cam, Evo Big Twin, Sportster or Buell! 208-696-1250
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    Hate to rebuild engine and see it destroy itself in the first 20kms when it gets up to temperature
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  7. #7
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    The reason for the factory set specs is that they are the parameters within which the car will perform to its factory designed standards in terms of not pinging, emissions etc..

    Going outside of those specs can enhance the performance, eg higher compression etc, but if you take a mile of metal off the head, the valve timing, ability to run on pump fuel, and emissions are compromised. All of those things concern the factory, hence their specified specs. As has been seen, people like Alan Moore, Ross and Frans have all demonstrated what can be done by going beyond the factory specs. But you really do need to know what you are doing – and what you want to achieve.
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  8. #8
    1000+ Posts REN TIN TIN's Avatar
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    My 12 is one of the early type (not as early as Bowies) GL's so has the 842-01 motor (1251cc).
    I thought this was the low compression motors so according to Simons data the minimum for that should be 72.9.
    (I thought all the 12's with the Solex 32EISA were low compression but I could be wrong).
    But there's no problem with going to the high compression spec of 71.5 I guess.

    But my major concern was the pitting on no.1 cylinder.
    Skimming 0.3mm to bring it to 71.5mm wont remove all the pits but would they really matter?
    They're in the combustion chamber so I'm thinking they won't make any difference but I'm no engineer.

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  9. #9
    1000+ Posts Frans's Avatar
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    To me it will not matter as long as the area where the fire ring of the gasket sits is clean without dents. A little dent or scratch can be the weak spot where the compression can force it's way through.

    According to the photo it looks like it will be good to use even if you have to go to 0.4 to get rid of some of the potholes..

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  10. #10
    1000+ Posts alan moore's Avatar
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    As Frans has said, 0.3-0.4mm would still allow the head to be OK to use in your circumstance. I would get the machinist to just take the minimum to clean up the face. The marks from the broken ring will matter little in the whole scheme of things, with many Sierra motors having the same marks and still running OK. Your comp ratio would still be far from extreme, although I would recommend at least 95 octane fuel.

    I like Chilton Engineering at Wooloongabba for motor machining, although I have mostly used another guy up until last year when he retired.
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  11. #11
    1000+ Posts REN TIN TIN's Avatar
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    None of the dents are in the combustion chamber or where the gasket sits.
    They're just where the piston and the head would be at their minimum.
    Thanks for the tip on Chilton Engineering Alan, I'll contact them.
    The higher compression wouldn't hurt and I only ever use 95RON or higher anyway.


    Cheers
    RTT
    "I cannot help but notice that there is no problem between us that cannot be solved by your departure."

  12. #12
    Tadpole
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    Default R12 head

    Quote Originally Posted by REN TIN TIN View Post
    None of the dents are in the combustion chamber or where the gasket sits.
    They're just where the piston and the head would be at their minimum.
    Thanks for the tip on Chilton Engineering Alan, I'll contact them.
    The higher compression wouldn't hurt and I only ever use 95RON or higher anyway.


    Cheers
    RTT
    I am using an R9 1397 cc engine in my race car and the head has had 2.5 mm skimmed off it for competition use and to achieve a decent CR ( 10.5 : 1 ).
    Following Frans's recommendation I did 3 torqueing procedures after fitting the head and after my first race meeting did it again.
    Three race meetings later and using 7000 RPM as a limit, all OK. Water temp has never gone over 85 C albeit that at my last meeting ambient temp was over 30 C.
    Tough little engines.
    Must add that I use 100 LL Avgas.

    'Till later.
    Arthur.
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