cylinder bore
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cylinder bore

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    Senior Member steele211's Avatar
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    Default cylinder bore

    When a block is bored too far is there a location on the cylinder that is the most prone to failure? The reason I ask is if a bowtie block and a regular mk IV block have 4.8 bore spacing why is it ok to bore the bowtie to 4.56 or bigger and I keep hearing .060 to .100 over on a mk IV?

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    Default Bore cylinder

    Bore stability is the main issue. Usually the piston will suffer from an inconsistent bore.

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    gn7
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    Quote Originally Posted by steele211 View Post
    When a block is bored too far is there a location on the cylinder that is the most prone to failure? The reason I ask is if a bowtie block and a regular mk IV block have 4.8 bore spacing why is it ok to bore the bowtie to 4.56 or bigger and I keep hearing .060 to .100 over on a mk IV?
    steele, although Aleexi's post is technically correct, In don't think it address what you're asking exactly. It depends on how far you're boring a particular block.
    Like Alexi posted, if you bore a block too far, but it is intact, you may have bored it to the point that the bores will not remain round under operation. The more power you make, the thicker the bore needs to be to remain stable. A heavily NOS or blown MK IV with a .060 bore can fail. The walls go barrel shape with every firing, and evendually fail, plus you're probably losing power up to the point that it fails. They usually fail on the major thrust side of the cylinder. Either the out side, or valley side of the cylinder depending if its a odd or even numbered cylinder.

    With regards to the difference between production MK IVs and Bowties, or aftermarket, it because of the way the block is cast. If you bored a production MK IV to 4.56 or 4.60, you would have to worry about failure during operation. It wouldn't have a cylinder left to fail. It would be GONE, nothing but water jacket. The cylinder walls of a MK IV just aren't that thick.

    The Bowties, and others have what is called siamesed cylinders. The walls are cast so thick that there is no room left between cylinders for water to circulate. All 4 cylinders are cast as a single block from one end to the other, with no space between cylinders. To give you an idea how thick the castings are, a production MK IV bored .060 has a thinner cylinder wall than a late Bowtie bored 4.625
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    Senior Member steele211's Avatar
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    So if I understand you correctly gn7 what you are saying the main difference is is that the mkIV has a water jacket that surrounds each cylinder and on a bowtie block that space is solid? Makes sense to me thank you

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    gn7
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    Quote Originally Posted by steele211 View Post
    So if I understand you correctly gn7 what you are saying the main difference is is that the mkIV has a water jacket that surrounds each cylinder and on a bowtie block that space is solid? Makes sense to me thank you
    Well, sort of.

    Look at this pic of a Dodge Nascar block. It has an open deck so you can see the water jacket. You'll notice that there is no way for water get between the cylinders, only around the outsides of the them. If this was a MK IV there would be a gap between each cylinder, and the 4 wouldn't be all connected.

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    Senior Member ohflat's Avatar
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    Default bore

    hey gn what about 100 over mark 4 bbc

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    gn7
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    Quote Originally Posted by ohflat View Post
    hey gn what about 100 over mark 4 bbc
    Personally, I would sonic check any BBC after .070

    Some 4.250 bore 427 blocks will go .125 easily.

    Yet I split the walls of a .060 bore this year at Long Beach. Granted its blown and has some pretty serious cylinder pressure, but I never cracked one before so I think this one was a little thinner.
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    Senior Member ohflat's Avatar
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    Default bore

    builder did sonic test it told me i could go 10 more upon wear

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    Quote Originally Posted by gn7 View Post
    Personally, I would sonic check any BBC after .070

    Some 4.250 bore 427 blocks will go .125 easily.

    Yet I split the walls of a .060 bore this year at Long Beach. Granted its blown and has some pretty serious cylinder pressure, but I never cracked one before so I think this one was a little thinner.
    Many moons ago, I had a .125 over 427 (makes it 451 c.i.) blower motor that I bought from a SoCal guy. I was always paranoid with it, but ran it a couple of seasons @ 13 psi of boost. Ran great. It obviously had some decent thick cylinder walls with no core shift. Jocko

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    gn7
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    Quote Originally Posted by jockorace View Post


    Many moons ago, I had a .125 over 427 (makes it 451 c.i.) blower motor that I bought from a SoCal guy. I was always paranoid with it, but ran it a couple of seasons @ 13 psi of boost. Ran great. It obviously had some decent thick cylinder walls with no core shift. Jocko
    Yeah, Chevy wasn't stingy with the iron back then. I have heard of .150 (4.40) from a 427 tall deck marine block. In fact JE stocked 4.40 pistons for them until recently.
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    Quote Originally Posted by steele211 View Post
    When a block is bored too far is there a location on the cylinder that is the most prone to failure? The reason I ask is if a bowtie block and a regular mk IV block have 4.8 bore spacing why is it ok to bore the bowtie to 4.56 or bigger and I keep hearing .060 to .100 over on a mk IV?
    the cylinder is more prone to failure down at the bottom of the bore.this is because that is where the rod angle is at its greatest putting the most side load on the cylinder as it's starting to go up the bore on the thrust side.

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