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Thread: TR-3A Compresson check hot vs cold

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    TR-3A Compresson check hot vs cold

    This morning a ran a compresson check on my 1959 TR-3A - COLD, I was 120# on all four - HOT, I was 100# on all four - I don't understand - I thought it would be exactly the opposite - The motor has 22,000 miles on it - ????? Mid- Night
    miles crail

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    Luke Skywalker
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    Re: TR-3A Compresson check hot vs cold

    The good news is that your compression is consistent although very slightly on the low side. The gauges are not always that accurate and could account for the very slightly low numbers. Certainly no reason for concern at this point.
    As for the hot and cold readings, cold oil might provide a better seal than hot oil, outweighing the effect of expansion of the hot metal. I've never done mine hot.
    Tom
    1960 TR3A TS73117 (under restoration year 8, owned since 1964)
    1959 TR3A TS58023 (in case I never finish the one above)
    1969 Triumph Herald 13/60 (a whim)
    1970 Lotus Elan Plus 2 /0023/N
    1992 BMW325IC (fun to drive)

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    Re: TR-3A Compresson check hot vs cold

    Too many variables to know. Is the starter up to speed hot? Throttle and diaphragm blocked open on both tries? Valves set loose enough? Gage seal consistent? Battery charged fully and in good shape? I general I would expect the pressure to be the same or slightly higher too when hot, but there are a lot of other variables that can account for it. In the end...like Tom says, if it checks by the book cold, don't worry about the hot numbers.
    John

    1955 TR2

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    Jedi Knight
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    Re: TR-3A Compresson check hot vs cold

    I would be concerned the valves were to tight.You could pull the cover while hot and just make sure there is some wiggle at tdc.
    Tom

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    Re: TR-3A Compresson check hot vs cold

    Thanks for the comments - everything is good - battery, starter, valves, ect. - I just thought the hot cold differental was odd - the motor only has 20,000 miles on it - it runs good - like every TR I have been around it has piston slap that goes away once the motor is warm - but, sense I am anal, I am changing out the piston and sleves this weekend - boredom - midnight
    miles crail

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    Luke Skywalker
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    Re: TR-3A Compresson check hot vs cold

    Certainly will do no harm, other than to your checking account, and you might want to check and lap the valves while you've got the head off, although that will add another couple of hours to the job.
    Tom
    1960 TR3A TS73117 (under restoration year 8, owned since 1964)
    1959 TR3A TS58023 (in case I never finish the one above)
    1969 Triumph Herald 13/60 (a whim)
    1970 Lotus Elan Plus 2 /0023/N
    1992 BMW325IC (fun to drive)

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