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Thread: looking to increase compression via head skim

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    Spanner Monkey

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    looking to increase compression via head skim

    It looks like the valve stem oil seals have given up on my pinto head. They were fitted a few thousand miles ago along with a HT1 cam.

    From what I have found out the Kent valve stem oil seals aren't the best when using double value springs. Going to get a set ordered up from Victor Reinz (or else where if any recommendations?).

    While the head is off I would like to try and increase the compression to make a bit more use of the HT1 cam

    The bottom end is standard apart from ARP rod bolts and the pistons look like they go level with the top of the block. (I'll see if I can get a gauge to be a bit more accurate)

    The head has bigger inlet and outlet valves, unleaded conversion, ported and bronze valve guides. I have measured the head and its ~94.35 mm which is only 0.15 mm away from what I believe to be the standard thickness.

    I am hoping this is enough information to answer my questions -
    1. what can I safely get skimmed from the head? 1 mm?
    2. will a felpro head gasket be ok to use? (roughly 1.2 mm uncrushed according to Burton) or are there better ones?
    3. can anyone roughly work out the compression ratio? (before and after head skim)
    I believe to get the most from the HT1 cam the pistons require cutouts in them to allow for the compression to be lifted further - unfortunately I don't have the budget for doing this just now.

    Please keep any explanations simple as my knowledge on engines is poor (but getting better) !!

    Thank you
    David

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    Re: looking to increase compression via head skim

    to make an ht1 work properly you need 12.1 compression, which means forged pistons and valve cutouts, so first thing i would do is swap it for something more user friendly like an Rl31. it will give almost as much power, but further down teh rev range and not need stupidly high compression

    if the pistons really are flush, not .5mm down bore then you probably have about 9.8:1 compression. you can safely skim upto 2.5m off the head.

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    Re: looking to increase compression via head skim

    Thanks for the reply Graham

    Seems like a load of work to get the HT1 cam working properly.

    I have a piper 285 cam and associated bits sitting - would that work?
    • piper 285 - 2200-7000rpm (spare)
    • Kent RL31 - 2750-7500rpm (requires purchase)
    • Kent HT1 - 3000 - 8000rpm (installed)

    I am thinking that 1.5mm off the head would do for now, as it would leave me with enough 'meat' if for some reason it ever needed skimmed again.

    The latest rolling road results with the HT1 cam was 147hp at the flywheel. Would I see much of a drop or increase with either the Piper 285 or RL31?

    Edit - I will double check where the pistons sit tonight and see if I can get a measurement - fairly sure I have a clock gauge somewhere
    Last edited by telf; 16-01-2019 at 13:41.

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    Re: looking to increase compression via head skim

    Anyone tried CVH valve stem oil seals (top hat type) in a pinto head?

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    Re: looking to increase compression via head skim

    Quote Originally Posted by telf View Post
    Anyone tried CVH valve stem oil seals (top hat type) in a pinto head?
    no point, just use victor rienze pinto ones

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    Re: looking to increase compression via head skim

    ht1 cam will give low power with low compression, you would of probably got the same power with the 285

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    Re: looking to increase compression via head skim

    I thought that might be the case.

    Going to order a RL31. Middle ground of the 2 cams I have by looks of it but should give the best results.

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    Re: looking to increase compression via head skim

    Quote Originally Posted by telf View Post
    I thought that might be the case.

    Going to order a RL31. Middle ground of the 2 cams I have by looks of it but should give the best results.
    its a good cam, and will work on just 10.5 cr, you can go higher but i would not recommend it with cast pistons

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    Re: looking to increase compression via head skim

    Is there much difference between an RL31 and an RL32? Running say 11:1cr.

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    Re: looking to increase compression via head skim

    RL31 is a magic cam. Piper BP300 and Newman 4,75 are about the same. These cams will give highest power AND torque combination. You can alter the lobes centers if you want, special if it is also a road car.

    Please note, these cams do not fall into what I feel fast road cams. I would use a lower profile and sacrifice some power to get in more torque in lower RPM and paid back by lower fuel consumption. But if you want real power and still be able to drive normall, this is the cam.

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    Re: looking to increase compression via head skim

    To get the compression ratio of 10.5 or close to it, would 1.5 mm skimmed from the head on a standard bottom end get me ball park? Or would I need to take more off? Assuming the pistons are 0.5 mm from being flush with top of the block (still to confirm!)


    The car is mainly used for hillclimbs / sprints / autotests. Seems like the RL31 will do that job nicely.

    Many thanks for the help

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    Re: looking to increase compression via head skim

    Believe me, this cam will satisfy you for all 3 events. Specially if you want to combine all 3 with one engine.

    For compression ratio, you really have to measure the volume of all parts in this proces. You may not start guessing and just take X mm from the head. Anything from 10/1-Plus will work, I would never go over 11/1. It will not add anything with the exception of unwanted head into your engine. Unless you got special fuel like Methanol or Ethanol mixtures.

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    looking to increase compression via head skim

    I love rl31 threads it seems a very highly rated cam for those in the know.

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