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Discussion Starter #1
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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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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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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steelcomp was here
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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.
 

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steelcomp was here
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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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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, :wink2:
 

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just a ski boat with bark
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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 ?
 

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steelcomp was here
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(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.
 

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(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
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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.
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
 

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steelcomp was here
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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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just a ski boat with bark
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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?
 

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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?
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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just a ski boat with bark
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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. I wasn't thinking about that, I did notice some run .043/.043/3mm and other thin rings
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
I get hung up a little on rod/stroke ratio but my current engine combination has me rethinking that some. I'll know better after I tear it down for inspection. So I assume the Mahle pistons must have a CH of around 1.208"? the 1.459 r/s ratio still makes me leery but in a production block with a 4.31" bore you'd go from 496 to 511 cid so maybe it would work in some applications.
 

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I get hung up a little on rod/stroke ratio but my current engine combination has me rethinking that some. I'll know better after I tear it down for inspection. So I assume the Mahle pistons must have a CH of around 1.208"? the 1.459 r/s ratio still makes me leery but in a production block with a 4.31" bore you'd go from 496 to 511 cid so maybe it would work in some applications.
Not lot worse than a 4.25 crank with a stock 6.35 rod at 1.44 and look how many dummies have built thousands of those. I obviously does work. I think the majority of them are built by people that the idea of a longer rod is something requiring voodoo magic to pull off. I have had a similar discussion with people about using an off the shelf 1.52 or 1.27 piston in a 427 3.766 stock BBC. They refuse to even consider it, even though the piston selection is huge compared to 427 pistons. And they say this even though they are using an aftermarket rod. No different than a 496. Thousands of kits sold with 6.135 aftermarket rods. I don't try to understand it, or explain it.

Scott Shafiroff sells a 4.5 stroke low deck with a 6.385 rod(1.418 R/S ratio)
Sonny Leonard says he only needs 2" rod than the stroke and he's fine.

Even with a .043 ring, the pack gets tight, and the top ring gets close to the valve notch after 1.27.



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Discussion Starter #17
with a 1.120 pin height and the 535 rod, that would put the piston 0.06 out of the hole? how much out is ok to too much?
Although switching to a 385 rod would defeat the cost of the existing crank package deal, just for curiosity, wouldnt that be a bad rod ratio. Hell i was getting beat up for doing the 4.25/ 6.135 combo. If I rebuild with just replacing the broken parts, I am still at 4.25/6.135.
I guess I need to just keep looking for a 4.25 crank.
 

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Discussion Starter #18 (Edited)
:oops: I guess i am one of those dummies!:bangmyhead:

Well off to the spam section to place a WTB 4.250 crank, add!
 

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:oops: I guess i am one of those dummies!:bangmyhead:

Well off to the spam section to place a WTB 4.250 crank, add!
I only say that because I have never been able to determine why it is some one uses a stock length rod in a 4.25 stroke low deck. Don't get me wrong, I have similar thoughts about people that build 5.535 rod 4.25 stroke tall decks. The SHORTEST rod I have ever used in a tall deck 4.25 stroke engine was a 6.635 because I had them, Longest was a 6.8, and a couple inbetween. But never a 6.535.

If you are using aftermarket rods, and the rods and pistons are sitting on the shelf, for the same price, why not use them. One added bonus of using the longer rod is the crank if bought for longer rods, has better shaped and placed counterweights and can be easily internally balanced.

There are occasions when a short rod 4.25 engine may be beneficial, but I doubt the vast majority of the guys building them are thinking about when and why a short rod might be better,

When all is said and done, I bet by the time you buy 2 rods, a couple pistons, and a new crank, a whole rotator assembly with better pistons will be close to the same price.



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