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Thread: Maximum overbore

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    Jedi Knight
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    Maximum overbore

    What is the current thinking of the maximum over bore on a TR6 (1972)engine. It is presently bored at .040 over and needs to be bored out yet again. I would guess that this block is near the maximum, but don't know.
    Charley
    Charley
    1962 TR4
    1963 TR4
    1959 TR3A A work in progress.

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    Great Pumpkin TR3driver's Avatar
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    Re: Maximum overbore

    I don't know either, but sounds like it might be time to have it sleeved back to standard.

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    Yoda glemon's Avatar
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    Re: Maximum overbore

    I don't know either, but manufactures will generally only make pistons/rings to maximum overbore size, so checking some catalogs might be helpful (and if you can't get the proper parts for the overbore anyway...)

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    Darth Vader Rut's Avatar
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    Re: Maximum overbore

    .060 over looks like the max standard over bore.
    Rut
    https://www.bpnorthwest.com/triumph/...tr250-tr6.html
    Rut, '60 Bugeye, '70 MGB, '62 TR4, '66 TR4a IRS, '67 TR4a IRS, '68 TR4a IRS, '72 TR6

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    Yoda glemon's Avatar
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    Re: Maximum overbore

    I am too lazy to do the math, but last time I did I seems to recall that an .060" overbore adds a little bit of displacement to the motor, more than a couple of CCs anyway.

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    Yoda poolboy's Avatar
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    Re: Maximum overbore

    As well as raising the compression ratio.
    DRIVE 'EM IF YOU GOT 'EM

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    Jedi Knight
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    Re: Maximum overbore

    Thanks for all of the input.
    It looks like we are going to explore the "other" option of trying to find a replacement block.
    Anybody know of a 72-76 TR6 block located in Northern Calif or Southern Oregon? Preferably with standard size ( no overbore) ?
    Charley
    Charley
    1962 TR4
    1963 TR4
    1959 TR3A A work in progress.

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    Yoda TOC's Avatar
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    Re: Maximum overbore

    Core Shift. Not sure if TR's had the issue, but it can limit (or increase) amount of bore.
    Machine shops can check through water ports to outside on cylinder walls from bore surface.
    One of the engine designs I work on can go 180 with no core shift.

    Oddly, some of the ones I have worked on came factory sleeved. Knock worn sleeves out and none, it's .o83" over, and they make (made?) pistons and rings that were.....083" over.
    Longevity-wise for you, if it takes more than .020 to clean it up, finding slugs bigger without using Egge Machine (if they're still around) may be a trip.

    My Jag was .040 over, had broken rings, slugs okay, power honed okay....but nobody had .040 rings for it. Did .060 rings, gapped the ends, runs fine.

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