calculating piston pin height/compression height



calculating piston pin height/compression height

Postby grumpyvette » July 4th, 2011, 10:08 am

"
HEY GRUMPYVETTE?
I picked up a tall deck,Chevy big block 427, block, at a yard sale and a friend donated a set of 454 .060 pistons
Ive got several intakes for big block engines ,both oval and rectangular port, styles, whats it going to take to build a 496 stroker from these components?"

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it should be rather obvious that youll need to know the exact distance the piston deck sits at TDC ,above or below the block deck surface and the valve notch recess or pop-up dome volume of the piston to do accurate quench or compression calculations
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notice the pin height in the pistons pictured above allow a longer or shorter connecting rod length
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heres a selection of commonly available big block chevy connecting rod lengths
lets look at a quick example, say you want pistons for your 400sbc, you can select either 5.7" or 6" connecting rods for use in a 400 sbc, either is an improvement over the stock 5.565" connecting rods, most guys feel the 5.7" is the best choice due to the need to place the wrist pin up into the oil ring groove on pistons with the higher 6" rods required pin location and shorter piston compression height.

http://scatcrankshafts.com/#6

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notice how the longer crank stroke effects the piston stroke distance in the bore, both at the lower and upper end of the cylinder
5.7" rod piston
https://www.uempistons.com/index.php?ma ... cts_id=149


6" rod piston
https://www.uempistons.com/index.php?ma ... cts_id=416
a small block has a 9" deck height, subtract 1/2 the stroke plus the rod length from that 9" to get the required pin height
most blocks actually measure between 9.00-9.023"
If you use a 5.7" rod in a 400 with its 3.75" stroke ,you need a 1.433 compression height .......because 1/2 of 3.75" stroke = 1.875" + 5.7"+1.433=9.008"



If you use a 6" rod in a 400 with its 3.75" stroke you need a 1.13 compression height.......because 1/2 of 3.75" stroke = 1.875" + 6"+1.13 =9.005"

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http://www.lunatipower.com/Tech/Pistons ... eight.aspx
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well the first thing youll need to do is get a machine shop to check the block for cracks, verify line bore straitness, look for the deck to be strait, and check for other problems, and sonic check the bore walls to see if it can be bored to .060 over bore to fit the pistons you have, have the pistons inspected and measured, before considering their re-use. now if that was a standard bore block with a standard deck the pistons would not work,but with the taller deck height of the tall deck block youve got a shot at using those components.

the standard compression height on a 454 piston is about 1.645" compression height,the stock rod is 6.135" and with a 4" stroke crank that adds up to a 9.78 total for the standard 9.8" deck block, but theres commonly about .023 extra deck thickness if its never been milled/decked
on your taller tall deck block with its 10.2"(plus .023") deck height you have a 454 piston is about 1.645" compression height, the stroke on a 496 is increased to 4.25" so you have 2.125"= 1.645" pin height and a 10.2" deck that leaves you with a 6.430-.6.453" potential rod length, the closest standard rod is a 6.385", leaving the piston deck at .010-.033 below the deck of the block before theres a head gasket added, the next longer rod is 6.535" you can find combos that can come close to fitting

be aware that the other common BBC piston pin heights are 1.270, 1.520 and 1.765",
bbc connecting rods come in 6.135", 6.385" ,6.535", 6.700" and 6.800"
obviously theres other combos if your willing to change connecting rods or pistons
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http://www.wallaceracing.com/Calc-Deck- ... length.php
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I,m amazed at how often this concept gets confused, basically a piston compression height or the distance from the center of the piston pin hole, to the pistons deck height , is supposed to be very close to the block deck height MINUS the connecting rod length and 1/2 the crank stroke.
theres three variables here, stroke rod length and piston pin height, the piston pin height must be low enough to allow the ring pack , but theres some latitude in measurements, a longer stroke will require either a shorter connecting rod or a shorter compression pin height , but you can for example use a longer rod and shorter piston pin height with the original stroke to gain some rod/stroke ratio or clear larger crank counter weights.

read thru these

viewtopic.php?f=53&t=3760&p=9968&hilit=clearances+skirt#p9968

viewtopic.php?f=50&t=1249


http://www.lunatipower.com/Tech/Pistons ... eight.aspx

http://www.kb-silvolite.com/calc.php?action=piston_comp

http://www.jepistons.com/PDFs/TechCorner/2006-je10.pdf
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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