Correct push rods length
Thread Starter
Melting Slicks
Joined: Oct 2009
Posts: 2,560
Likes: 485
From: Moberly, Mo
Correct push rods length
Just wondering hows the best way to check for correct pushrod length if you don't have an adjustable push rod length tool? When I first bought the car it had 7.35 pushrods in it. I thought that was too short so I called thunder racing that ported the heads just to check what its supposed to have, they told me it should have 7.4's. I changed them since I had a set of 7.4. I'm still wondering why it had 7.3's. So, I want to check it myself. thanks
Racer
Joined: Aug 2005
Posts: 465
Likes: 16
Look around over on tech. Someone had a method that was close by counting the turns of the rocker bolt after zero lash.
Some aftermarket rockers require a shorter PR, and milling a bunch off the heads with a thinner head gasket might get you in the neighborhood for shorter PR's as may the Comp R lifters because they require so little preload.
Some aftermarket rockers require a shorter PR, and milling a bunch off the heads with a thinner head gasket might get you in the neighborhood for shorter PR's as may the Comp R lifters because they require so little preload.
Drifting
Joined: Oct 2004
Posts: 1,975
Likes: 9
From: "Philly" Pennsylvania
When installing new lifters, the 7.35's may give the more correct pre-load. This became known when people started putting in LS7 lifters and found out they were a little taller than LS1 replacement lifters (back then). The LS7 lifter is now the official replacement for LS1 engines. I had 7.4's in and had them measured and changed to 7.35, less noise from rockers also.
Pro
Joined: Aug 2007
Posts: 599
Likes: 39
From: citrus heights ca
Drifting
Joined: Oct 2004
Posts: 1,975
Likes: 9
From: "Philly" Pennsylvania
look at post #4 in this thread
http://www.ls1tech.com/forums/genera...od-length.html
http://www.ls1tech.com/forums/genera...od-length.html
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Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
Make sure you understand how that method works, it is not as straight forward as one would believe. Part of that turn counting is after the rocker has bottomed and the bolt is being tightened to 22 lb-ft.
Instructor
Joined: Dec 2008
Posts: 127
Likes: 0
From: Grand Rapids Mi.
Checking push rod length
To Vettenuts, I was reading Shane's article about using intake open/exhaust close method. Do you have to make sure that the rocker fulcrum is seated on the rocker head stand or just as it creates friction from contacting the pushrod and valve tip before counting turns to 22lb/ft. I hope i have explained this correctly. Thanks PS. i am having a problem getting a clean rocker print while centering valve to rocker contact
Last edited by hermancar; May 16, 2011 at 05:35 PM.
Burning Brakes
Joined: Oct 2004
Posts: 931
Likes: 35
From: Rowlett, Texas
Oops, I read the instructions from Shane at TR a couple of times; and it seems THEY MAY BE INCORRECT.
I did some study on this method a couple years back, but couldn't locate the thread just yet. But, working through it quickly again:
- If the rocker arm nut is a 1.25mm pitch, then yes, turning the nut will lower the CENTER of the rocker arm by 1.25mm, or approx 0.48", as he stated.
- However, the valve stem is acting as the pivot point, so when the screw in the middle of the rocker is turned, the other tip of the rocker - the pushrod end - will move farther.
Think of it as a seesaw - lock one end down (on the valve) and lower the center of the seesaw - the free end of the seesaw (on the pushrod) will lower more than the center.
If we are dealing with 1.7 ratio rockers, then I think the formula for movement at the pushrod end will be:
.048"/1.7 = X"/2.7 X= 0.076"
So, for every turn of the rocker arm nut, the lifter preload will increase by approx 0.076".
So, it seems TR has actually been setting lifter preload between 0.095" and 0.133".
Hmmm, gotta relook this if I can find it.
DG
I did some study on this method a couple years back, but couldn't locate the thread just yet. But, working through it quickly again:
- If the rocker arm nut is a 1.25mm pitch, then yes, turning the nut will lower the CENTER of the rocker arm by 1.25mm, or approx 0.48", as he stated.
- However, the valve stem is acting as the pivot point, so when the screw in the middle of the rocker is turned, the other tip of the rocker - the pushrod end - will move farther.
Think of it as a seesaw - lock one end down (on the valve) and lower the center of the seesaw - the free end of the seesaw (on the pushrod) will lower more than the center.
If we are dealing with 1.7 ratio rockers, then I think the formula for movement at the pushrod end will be:
.048"/1.7 = X"/2.7 X= 0.076"
So, for every turn of the rocker arm nut, the lifter preload will increase by approx 0.076".
So, it seems TR has actually been setting lifter preload between 0.095" and 0.133".
Hmmm, gotta relook this if I can find it.
DG
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
Oops, I read the instructions from Shane at TR a couple of times; and it seems THEY MAY BE INCORRECT.
I did some study on this method a couple years back, but couldn't locate the thread just yet. But, working through it quickly again:
- If the rocker arm nut is a 1.25mm pitch, then yes, turning the nut will lower the CENTER of the rocker arm by 1.25mm, or approx 0.48", as he stated.
- However, the valve stem is acting as the pivot point, so when the screw in the middle of the rocker is turned, the other tip of the rocker - the pushrod end - will move farther.
Think of it as a seesaw - lock one end down (on the valve) and lower the center of the seesaw - the free end of the seesaw (on the pushrod) will lower more than the center.
If we are dealing with 1.7 ratio rockers, then I think the formula for movement at the pushrod end will be:
.048"/1.7 = X"/2.7 X= 0.076"
So, for every turn of the rocker arm nut, the lifter preload will increase by approx 0.076".
So, it seems TR has actually been setting lifter preload between 0.095" and 0.133".
Hmmm, gotta relook this if I can find it.
DG
I did some study on this method a couple years back, but couldn't locate the thread just yet. But, working through it quickly again:
- If the rocker arm nut is a 1.25mm pitch, then yes, turning the nut will lower the CENTER of the rocker arm by 1.25mm, or approx 0.48", as he stated.
- However, the valve stem is acting as the pivot point, so when the screw in the middle of the rocker is turned, the other tip of the rocker - the pushrod end - will move farther.
Think of it as a seesaw - lock one end down (on the valve) and lower the center of the seesaw - the free end of the seesaw (on the pushrod) will lower more than the center.
If we are dealing with 1.7 ratio rockers, then I think the formula for movement at the pushrod end will be:
.048"/1.7 = X"/2.7 X= 0.076"
So, for every turn of the rocker arm nut, the lifter preload will increase by approx 0.076".
So, it seems TR has actually been setting lifter preload between 0.095" and 0.133".
Hmmm, gotta relook this if I can find it.
DG
Last edited by vettenuts; May 17, 2011 at 05:31 AM.
Burning Brakes
Joined: Oct 2004
Posts: 931
Likes: 35
From: Rowlett, Texas
"1.25 cm = ~ .5 inches ; 12.5mm = ~.5 inches".
Nag, Nag, Nag.
OK, I made a typo and left off a zero; that's shorthand for old demented guys.
Formula is still good though.
Yes, I agree the final torquing to 22 lb ft will not move the rocker arm down much more after it seats, it will just begin to stretch the bolt; so there is the inacuracy of the method.
But, of course, we are looking at a ballpark number for lifter preload, just trying to get it within 0.025" or so of optimal, using the available PR increments, correct?
DG
Nag, Nag, Nag.
OK, I made a typo and left off a zero; that's shorthand for old demented guys.
Formula is still good though.
Yes, I agree the final torquing to 22 lb ft will not move the rocker arm down much more after it seats, it will just begin to stretch the bolt; so there is the inacuracy of the method.
But, of course, we are looking at a ballpark number for lifter preload, just trying to get it within 0.025" or so of optimal, using the available PR increments, correct?
DG
Tech Contributor
Joined: Jan 2007
Posts: 19,421
Likes: 1,146
From: Dyer, IN
Whatever. You make a statement like this:
"Oops, I read the instructions from Shane at TR a couple of times; and it seems THEY MAY BE INCORRECT."
...and then make a typo. If don't use the correct units, how is one to know if the procedure you posted is wrong....or you just typed it wrong?
"Oops, I read the instructions from Shane at TR a couple of times; and it seems THEY MAY BE INCORRECT."
...and then make a typo. If don't use the correct units, how is one to know if the procedure you posted is wrong....or you just typed it wrong?
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
"1.25 cm = ~ .5 inches ; 12.5mm = ~.5 inches".
Nag, Nag, Nag.
OK, I made a typo and left off a zero; that's shorthand for old demented guys.
Formula is still good though.
Yes, I agree the final torquing to 22 lb ft will not move the rocker arm down much more after it seats, it will just begin to stretch the bolt; so there is the inacuracy of the method.
But, of course, we are looking at a ballpark number for lifter preload, just trying to get it within 0.025" or so of optimal, using the available PR increments, correct?
DG
Nag, Nag, Nag.
OK, I made a typo and left off a zero; that's shorthand for old demented guys.
Formula is still good though.
Yes, I agree the final torquing to 22 lb ft will not move the rocker arm down much more after it seats, it will just begin to stretch the bolt; so there is the inacuracy of the method.
But, of course, we are looking at a ballpark number for lifter preload, just trying to get it within 0.025" or so of optimal, using the available PR increments, correct?
DG
Its when you start to get away from one turn that things have to be corrected, and unless you realize what is happening with the bolt, preload, rocker ratio, etc. you can get a very inaccurate preload. That's why I still think for most using an adjustable pushrod may be the best approach.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
- If the rocker arm nut is a 1.25mm pitch, then yes, turning the nut will lower the CENTER of the rocker arm by 1.25mm, or approx 0.48", as he stated.
1.25 mm = exactly 0.0491". Even though I know you meant 0.048" ... the conversion is off by 0.001"
Last edited by ZeeOSix; May 17, 2011 at 03:52 PM.
