4.375 stroke/ short deck BBC
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4.375 stroke/ short deck BBC

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    Senior Member holorinhal's Avatar
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    Default 4.375 stroke/ short deck BBC

    Would it feasible to build a build a 4.375/6.535 combo, in a production short deck block, that will have longevity?
    The block has been decked and my calculations figure it to be a deck height of 9.78.
    That would need a pin height of 1.057. Are there any shelf pistons with that pin height, out there?
    Any other pin height would put the piston out of the hole, right?
    Would there be an extreme amount of clearancing, needed?
    Reason for asking is that i have come across a nice deal on a brand new 4.375/4340 crank including 6.535 H-beam rods/ arp bolts.

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    gn7
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    Quote Originally Posted by holorinhal View Post
    Would it feasible to build a build a 4.375/6.535 combo, in a production short deck block, that will have longevity?
    The block has been decked and my calculations figure it to be a deck height of 9.78.
    That would need a pin height of 1.057. Are there any shelf pistons with that pin height, out there?
    Any other pin height would put the piston out of the hole, right?
    Would there be an extreme amount of clearancing, needed?
    Reason for asking is that i have come across a nice deal on a brand new 4.375/4340 crank including 6.535 H-beam rods/ arp bolts.
    The shortest CH you are going to get away with is about 1.120 which is used for a 4.250 crank and 6.535 rod in short deck, but just happens to work with a 4.375 crank and a 6.385 rod as well.



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    Senior Member Futs's Avatar
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    Quote Originally Posted by gn7 View Post
    The shortest CH you are going to get away with is about 1.120 which is used for a 4.250 crank and 6.535 rod in short deck, but just happens to work with a 4.375 crank and a 6.385 rod as well.
    Are there any benefits to running a 6.535 rod in a short deck block with a 4.250 crank?

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    Quote Originally Posted by Futs View Post
    Are there any benefits to running a 6.535 rod in a short deck block with a 4.250 crank?
    You could do it with a 1.12 pin height but I don't really see the benefit other than a little lighter piston (about 30g) but a heavier rod, too.
    Last edited by scott foxwell; 01-09-2014 at 03:42 PM.
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    Quote Originally Posted by gn7 View Post
    The shortest CH you are going to get away with is about 1.120 which is used for a 4.250 crank and 6.535 rod in short deck, but just happens to work with a 4.375 crank and a 6.385 rod as well.
    I think 1.12 would put you a little over .100 in the hole with a 4.375/6.385 in a std. deck.
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    Quote Originally Posted by holorinhal View Post
    Would it feasible to build a build a 4.375/6.535 combo, in a production short deck block, that will have longevity?
    The block has been decked and my calculations figure it to be a deck height of 9.78.
    That would need a pin height of 1.057. Are there any shelf pistons with that pin height, out there?
    Any other pin height would put the piston out of the hole, right?
    Would there be an extreme amount of clearancing, needed?
    Reason for asking is that i have come across a nice deal on a brand new 4.375/4340 crank including 6.535 H-beam rods/ arp bolts.
    NO...You need a shorter rod,

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    just a ski boat with bark Carnivalride's Avatar
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    Quote Originally Posted by steelcomp View Post
    I think 1.12 would put you a little over .100 in the hole with a 4.375/6.385 in a std. deck.
    (4.375/2)+6.385=8.5725

    so 9.8-8.5725=1.2275 .0275" in the hole
    or 9.78-8.5725=1.2075 .0075" in the hole

    or did you mean the 6.535" rod ?
    Quote Originally Posted by gn7 View Post
    Carnivalride, you really need to find some other hobby. You have no talent for this boat thing

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    Quote Originally Posted by Carnivalride View Post
    (4.375/2)+6.385=8.5725

    so 9.8-8.5725=1.2275 .0275" in the hole
    or 9.78-8.5725=1.2075 .0075" in the hole

    or did you mean the 6.535" rod ?
    I meant exactly what I said. 1.12, not 1.2.
    (4.375/2)+6.385+1.12=9.6925
    9.8-9.6925=.1075
    I would use a 1.22 pin for a 4.375/6.385 combination in a std. deck.
    Last edited by scott foxwell; 01-09-2014 at 06:29 PM.
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    gn7
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    Quote Originally Posted by Carnivalride View Post
    (4.375/2)+6.385=8.5725

    so 9.8-8.5725=1.2275 .0275" in the hole
    or 9.78-8.5725=1.2075 .0075" in the hole

    or did you mean the 6.535" rod ?
    He's right, my mistake. He was figuring it with a 9.8 deck. With a 9.78 deck a 1.12 CH would put the piston down the hole. I transposed my numbers when I was looking, Its closer to 1.21 not 1.12.

    The problem with the 4.375 crank is, the piston companies don't seem to support it with shelf pistons.



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    just a ski boat with bark Carnivalride's Avatar
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    Quote Originally Posted by steelcomp View Post
    I meant exactly what I said. 1.12, not 1.2.
    (4.375/2)+6.385+1.12=9.6925
    9.8-9.6925=.1075
    I would use a 1.22 pin for a 4.375/6.385 combination in a std. deck.
    Quote Originally Posted by gn7 View Post
    He's right, my mistake. He was figuring it with a 9.8 deck. With a 9.78 deck a 1.12 CH would put the piston down the hole. I transposed my numbers when I was looking, Its closer to 1.21 not 1.12.

    The problem with the 4.375 crank is, the piston companies don't seem to support it with shelf pistons.
    Sorry guys I misread the 1.12 my bad
    Quote Originally Posted by gn7 View Post
    Carnivalride, you really need to find some other hobby. You have no talent for this boat thing

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    Quote Originally Posted by gn7 View Post
    He's right, my mistake. He was figuring it with a 9.8 deck. With a 9.78 deck a 1.12 CH would put the piston down the hole. I transposed my numbers when I was looking, Its closer to 1.21 not 1.12.

    The problem with the 4.375 crank is, the piston companies don't seem to support it with shelf pistons.
    Nice thing with JE...they will move a pin as much as .050" for 10.00 ea. if it's a shelf stocking forging.
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    gn7
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    Quote Originally Posted by steelcomp View Post
    Nice thing with JE...they will move a pin as much as .050" for 10.00 ea. if it's a shelf stocking forging.
    Good to know. I will have to remember that.



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    just a ski boat with bark Carnivalride's Avatar
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    So back on topic with the OP thread how short of piston skirt can you go before piston rock becomes an issue? When I was researching whether or not to use my 4.25" stroke crank I heard from a few places the I should stay with the 6.385" long rod in the short deck for street/endurance use because the pistons for 6.535" long rod had shorter skirts and tended to rock more with the increased side loading and required more frequent inspections. In your opinion was there any truth to that? In hindsight it seems like the would only be .150" shorter so how short is too short on a 4.28"-4.31" bore and 4.5"-4.625" bore?
    Quote Originally Posted by gn7 View Post
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    Quote Originally Posted by Carnivalride View Post
    So back on topic with the OP thread how short of piston skirt can you go before piston rock becomes an issue? When I was researching whether or not to use my 4.25" stroke crank I heard from a few places the I should stay with the 6.385" long rod in the short deck for street/endurance use because the pistons for 6.535" long rod had shorter skirts and tended to rock more with the increased side loading and required more frequent inspections. In your opinion was there any truth to that? In hindsight it seems like the would only be .150" shorter so how short is too short on a 4.28"-4.31" bore and 4.5"-4.625" bore?
    I draw the line at 1.27 CH, which happens to be the CH of a 4.25 crank in a low deck and a 6.385 rod. I can't see the benefit of using a 6.535 rod in that combo. I have built longer rod 4.25 engines, but in a tall deck.
    Its not just the short piston, its the short piston in combination with the tight ring pack.
    Needless to say, I am not a huge fan of 4.375 cranks in low decks.
    If the rods the OP has came together with the crank, they are from a tall deck.

    BTW, Mahle has shelf pistons for a 4.375 crank with a 6.385 rod



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    Last edited by gn7; 01-09-2014 at 07:48 PM.

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