What happens if you over bore an engine too far?
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    Official RnS Addict zephyr's Avatar
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    What happens if you over bore an engine too far?

    What happens if you over bore an engine too far?

    The walls get thin and what? Overheating? Porosity?

    Can you push the envelope by ensuring the cooling system is bigger/better? Would you run a cooler thermostat?
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    if the cylinders are overbored, you can loose too much wall thickness & end up with alump of scrap metal because the walls may give way on the compression stroke, the head gasket sealing rings won't seal & you'll go through head gaskests & other problems. you may not be able to get production head gaskets, pistons & rings that fit the bore, which would mean 1 off productions of each, creating more expense.
    it wouldn't matter what size coolong system you had, or how efficent it was, if the engine isn't strong enough, you're wasting your time fitting it.
    only ever rebore to the 1st + size that will give you a clean new bore, as long as pitons & rings are available (same with mains, crank & big ends) ... once the engine is at it's maximum + size, then depending on wall thickness, you can have it over bored & lined back ot std, but this can be very expensive, so most blocks are normally thrown away

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    "call 0-800 Apocalypse" satan's Avatar
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    agreed with all of the above 100%, is that what you were refering to,? or do you mean for example overboring say,,,a 283ci to 4.00 to make a 301ci in which case you can get away with if
    a,you got the right year block.
    b,you have it checked,xrayed, for core shift ect,

    its quite an old school way of engine building/tuning, and these days there are more options to achieve better results.
    And I heard a voice in the midst of the four beasts, And I looked and behold: a pale horse. And his name, that sat on him, was Death. And Hell followed with him.

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    Official RnS Addict zephyr's Avatar
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    I bought a 350 Chevy that was in bad shape. Chap that sold it is in the trade and guaranteed it to be a workable block - i.e. that it would clean up ok. Machinist had to take it to .080" to get it straight, but assures me it will be just fine. Both are reputable sorts and the block is NOT going in a race car.

    I'm hoping that if it all goes wrong, the engine selling fellow will give me another block and the machinist will machine it for me, after all both have said it will be fine.

    Internet is full of people saying .080" is too much and others saying the opposite.
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    Official RnS Addict WB54's Avatar
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    depends how much money you're putting into the engine and what the intended purpose is. If it's just a stock rebuild and it won't see high rev's, then it'll probably be OK, but obviously cylinder walls get thinner and closer to water jacket et, for racing or fast road action it's beyond the accepted norm for what most people would class a decent base on which to build a short block and bolt in all their expensive goodies. I assume you got pistons before you bored it.......

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    Official RnS Addict mygasser's Avatar
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    In the case of a 427 fe you cant overbore more than 30 thou'. Bigger and you have to have the block sleeved. So maybe sleeves would be the answer if you have overheating issues.
    need a job done on your project? i may be able to help.

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    Official RnS Addict zephyr's Avatar
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    Had it bored first to see what size I'd need. 'Only' a .030" lip at the top, so I thought I'd be well within a .060" overbore, but the wear was assymetrical, necessitating an .080" overbore. Pistons in that size are available from several sources, which made me think it was almost a normal thing. Machinist who's been in the game for years reckoned it was ok too, but every time you google it you find people saying it'll never even fire up.

    It's going in a cruiser, not a racer so revs will never be high, but even so if there were any measures I could take to give it the best chance of working I'd consider doing it.

    I know with a Rover block you're looking at £600 to have it linered. Is it any cheaper with an iron block?
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    the difference with the rover (& other ally block engines) is that they have seperate steel liners (ally would be too weak) usually pressed into the block, which is why you should always clamp the liners when the heads are off before attempting to rotate the crank once pistons are fitted.
    i'm not sure how much it would cost to liner an iron block, as you need to have the cylinders bored, but not all engines can be linered, it all depends on the wall thickness.
    you'll always have 2 planes of wear in the cylinder (+ the lip around the top) due to thrust.
    the bore will always have ovality ... measuring across the bore, it will always be more than measuring lengthways, caused by thrust. the bore will also taper along the length of the stroke, again caused by thrust of the piston movemtnt within the cylinder.
    if your engine had a lip of .030", then it was either long overdue an overhaull, or if it'd been rebuilt, it was a poor attempt. a .030" lip (where the #1 compession ring on the pistin reaches the top of it's stoke) is more than 1st or 2nd rebore, which would suggest the piston was slopping around like anything. can't imagine there would've been much compression, it would've been quite smokey & run poorly.
    over bored engines will always start & iff done too much, would fail very quickly while being run in, even on a cruiser. if it's still within the manufacturers tolerences, then it'll be ok.
    you've got to remember that the more an engine is bored out, (even to within max tolerences), the weaker the cylinder walls (& effectively the engine) becomes, so the less stress applied to larger oversized bores, the better. it's the reason most race engines may only be rebored to 1/2 it's maximum, before either being scrapped or sold for rebuilding for non race applications.
    not wishing to offend anyone here that run machine shops, but most are just that, they don't build the engines, so they may say it's ok, but they'll probably machine umpteen different makes of engine. the company i did my apprenticeship at was 75% ford pinto & kent, 20% rover 'k' series & 5% other makes. if we had a yank v8 for example, te'd find out the max tolerences it could be machined to etc before doing anything ... it's worse to blindly overbore than to double check what you can & can't do.

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    Official RnS Addict beach57's Avatar
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    Get another block ,then it's not in the back of your mind will it blow,+how much extra is it for the 80thou bits

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    Official RnS Addict zephyr's Avatar
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    The machinist builds engines too and 99% US V8s, so I took his advice. I have the pistons (no problem with availability or price). Reading stuff on the net now has me wondering, although he's still adamant it'll work fine.

    And where do you get a proper 350 from these days? When I was looking for a block, there just weren't any out there, which is why I'm where I am now.....
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