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Old 10-15-2009, 02:30 PM
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350 Build - What push rod length do I need??

Building a 350 with the following components:
1968 Block
Stock Crank
Stock Rod Length
summit flat top pistons
comp cams 268/280 .477/.480 Hydraulic Flat Tappet Cam
Vortec Heads


what size push rods will I need??

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Old 10-15-2009, 03:38 PM
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Quote:
Originally Posted by klutchmaster427
Building a 350 with the following components:
1968 Block
Stock Crank
Stock Rod Length
summit flat top pistons
comp cams 268/280 .477/.480 Hydraulic Flat Tappet Cam
Vortec Heads


what size push rods will I need??
It's been my experience with what you have you should be able to run a standard/stock push-rod. Unless you do some changes to the heads what rockers / springs are you going to run?.JMO....Cole
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Old 10-15-2009, 03:59 PM
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Quote:
Originally Posted by klutchmaster427
Building a 350 with the following components:
1968 Block
Stock Crank
Stock Rod Length
summit flat top pistons
comp cams 268/280 .477/.480 Hydraulic Flat Tappet Cam
Vortec Heads


what size push rods will I need??
The goal of pushrod length is to properly position the rocker's sweep across the valve stem. Many things as individual changes or in ganged will contribute to needing to correct the sweep by changing the length of the pushrod.

Pushrod length is dependent upon things done that could alter the factory production rod length selection. These are things that change the distance from the lifter's pushrod cup to that of the rocker arm such as head milling, block decking operations and changes to head gasket thickness. Other considerations are length of the valve stem tip which can effectively look longer to the rocker arm following a valve job because the head's seats become sunk into the head and a refaced valve becomes thinner, both of these position the valve's stem tip taller when assembled than it was originally. Also, if longer than OEM stem lengths are used this relationship is changed. Other things that contribute to a recalculation of the appropriate length pushrod are changes to the lift caused by the cam and or the use of different ration rocker arms or roller rocker arms replacing the standard ball and socket.

The determining factor of pushrod length is how the rocker tip sweeps the valve stem. The goal is to use a push rod length that keeps the rocker contacting as much as possible the area about the center of the width of the valve stem. Striking the valve stem off center increases wear on the stem and guide and at the extreme can run the rocker off the edge of the stem. It is also necessary when making these kind of changes to insure the rocker's body does not come into contact with the valve spring's retainer nor the spring.

Go here and read this for the details: http://www.compcams.com/Technical/In...20Geometry.pdf

Bogie
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Old 10-15-2009, 04:03 PM
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Z28 springs. .525" lift. and gm stamped steel 1.5 ratio rockers
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Old 10-15-2009, 05:55 PM
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you should be okay with stock, but you should buy one of those pushrod length checkers, there like $20 and you put that in there, and turn the screw untill the tip of the roller is in the middle of the stem... then you just measure the length with a tape measure and see wat it says, that the size you'll need. You can get one of those checkers from Summit or Jegs, or if you have a local machine shop, maybe the will let you borrow one or buy one off of them. Mark.
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Old 10-15-2009, 09:34 PM
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I'll pick up a length checker. thanks!
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Old 10-16-2009, 04:29 PM
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Sbc307, is correct you can buy a adjustable push-rod. Or length checker to know exactly what you need. ............Cole
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Old 10-16-2009, 04:47 PM
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Quote:
Originally Posted by sbc307
you should be okay with stock, but you should buy one of those pushrod length checkers, there like $20 and you put that in there, and turn the screw untill the tip of the roller is in the middle of the stem... then you just measure the length with a tape measure and see wat it says, that the size you'll need. You can get one of those checkers from Summit or Jegs, or if you have a local machine shop, maybe the will let you borrow one or buy one off of them. Mark.
When I'm doing it does that piston need to be at TDC or what...?
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Old 10-16-2009, 05:44 PM
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Thats what my machine shop said, bring number 1 piston to TDC (compression stroke) and put the length checker in and just set the rocker on top, dont tighten it down turn the push rod till the tip is in the middle or slightly towards the edge but not to much, that way when the valve is pushed open the roller is in the middle of the stem. Dont tighten the rocker down and turn it over the threads on the checker wont hold, they strip right off, i ruined the first one i bought but doing that, so i had to buy another one!! lol. i had to do that to my motor i built (307 chevy) i put world s/r head on and guide plates, studs and harland sharp roller rockers, i had to go from a stock chevy push rod (7.950) to a (8.000) push rod to put the roller in the right spot, i used the Comp Cams checker #7703 it measure pushrods from 7.800 to 8.800, its only $16 on Summit. let us know how you make out... thanks mark.
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Old 10-17-2009, 01:19 PM
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Thanks for the info. It will be a bit till I get results. still have to buy heads! and have no cash... but some day! lol
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