Maybe dumb question on push rod length
03-20-2012, 05:42 PM
#1
Thread Starter
Join Date: Mar 2012
Location: VA
Posts: 6
Maybe dumb question on push rod length
So I just installed new heads on my 88 and I need to size the push rods. As a base, I installed the stock push rods, and saw they were just a little off center of the valve stem. The mark on the valve stem was just a hair toward the exhaust side of the valve stem. At first I just assumed that meant the stock push rods were too long. I used a push rod length checker, set it a little shorter, marked the stem, and swore I saw it now centered. Here's where I got confused. I tried shopping for shorter push rods, and couldn't find any. I thought on it a bit, and then started to wonder if the answer was that the stock rods were too short. In any case, I'm at a loss. Any thoughts?
03-20-2012, 07:33 PM
#2
4th Gear Member
Join Date: Jul 2008
Location: alabama
Posts: 1,812
The stock length is a 6.25 If your heads were seriously and prolly wrongly milled or decked you may need shorter, what type roller rockers are you using? Sometimes the actual rocker style and or brand can change the length you need, with aftermarket heads you have to check the length, you may need shorter if the block was decked along with the heads and using a thinner head gasket.
You need to mark the top of the vlv with a felt tipped marker and then cycle the engine thru a few times and see where you roller travels get the wear mark closest to the center as you can and thats your length!
What heads are you using?
Once you get your length you may or may not get the exact length sold get the closest length you can if your length lands between 2 sizes then get the longer of the 2!
You need to mark the top of the vlv with a felt tipped marker and then cycle the engine thru a few times and see where you roller travels get the wear mark closest to the center as you can and thats your length!
What heads are you using?
Once you get your length you may or may not get the exact length sold get the closest length you can if your length lands between 2 sizes then get the longer of the 2!
Last edited by Venomantidote95; 03-20-2012 at 07:35 PM.
03-20-2012, 08:33 PM
#3
D.R. THE PATHETIC DORK
Join Date: Jun 2007
Location: West Virginia
Posts: 30,863
centering the witness mark on the stem is not and I repeat not an accurate method for getting the correct geometry, you want a length that gives you the narrowest witness mark, try going up one length then down one length and watch how the witness width either gets wider or more narrow, sometimes you'll end up a little off center toward the front or towards the back, as long as travel stays in the center 1/3 of the stem your fine, you want the narrowest witness mark you can get, don't believe me, go ask this question on the corral.net and wait for ED Curtis or Buddy Rawls to reply and confirm what I state 
03-20-2012, 08:49 PM
#4
D.R. THE PATHETIC DORK
Join Date: Jun 2007
Location: West Virginia
Posts: 30,863
Here is what Buddy Rawls has to say about it:
"Pushrod Length- Incorrect pushrod length can be detrimental to valve guide wear. Most sources say that centering the rocker contact patch on the valve stem centerline at mid valve lift is the correct method for determining the optimum pushrod length. This method is wrong and can actually cause more harm than good. The method only applies when the valvetrain geometry is correct. This means that the rocker arm lengths and stud placement and valve tip heights are all perfect. This is rarely the case. To illustrate this, think of the valve angle and the rocker stud angle. They are usually not the same. If a longer or shorter valve is installed, then the relationship of the valve tip to the rocker stud centerline has changed. Heads that have had multiple valve jobs can also see this relationship change. Note, the rocker length (pivot to tip) remains unchanged, so the rocker contact patch will have to move off the valve centerline some particular distance for optimum geometry to be maintained.
The optimum length, for component longevity, is the length that will give the least rocker arm contact area on the valve stem. In other words the narrowest wear pattern. This assures that the relationship is optimized and the rocker is positioned at the correct angle. This means that the optimum rocker tip contact point does not necessarily coincide with the valve stem centerline, and probably will not. What is the acceptable limit for being offset from the valve stem centerline? That will depend on the set-up. A safe margin to strive for is about +/-.080" of the centerline of an 11/32 diameter valve stem. This means that no part of the wear pattern should be outside of this .160" wide envelope. As the pushrod length is changed, the pattern will change noticeably. As the geometry becomes closer to optimum, the pattern will get narrowest. If the narrowest pattern is too far from the valvestem centerline, then the valve to rocker relationship has to be changed. In this case, valve stem length will need to change."
"Pushrod Length- Incorrect pushrod length can be detrimental to valve guide wear. Most sources say that centering the rocker contact patch on the valve stem centerline at mid valve lift is the correct method for determining the optimum pushrod length. This method is wrong and can actually cause more harm than good. The method only applies when the valvetrain geometry is correct. This means that the rocker arm lengths and stud placement and valve tip heights are all perfect. This is rarely the case. To illustrate this, think of the valve angle and the rocker stud angle. They are usually not the same. If a longer or shorter valve is installed, then the relationship of the valve tip to the rocker stud centerline has changed. Heads that have had multiple valve jobs can also see this relationship change. Note, the rocker length (pivot to tip) remains unchanged, so the rocker contact patch will have to move off the valve centerline some particular distance for optimum geometry to be maintained.
The optimum length, for component longevity, is the length that will give the least rocker arm contact area on the valve stem. In other words the narrowest wear pattern. This assures that the relationship is optimized and the rocker is positioned at the correct angle. This means that the optimum rocker tip contact point does not necessarily coincide with the valve stem centerline, and probably will not. What is the acceptable limit for being offset from the valve stem centerline? That will depend on the set-up. A safe margin to strive for is about +/-.080" of the centerline of an 11/32 diameter valve stem. This means that no part of the wear pattern should be outside of this .160" wide envelope. As the pushrod length is changed, the pattern will change noticeably. As the geometry becomes closer to optimum, the pattern will get narrowest. If the narrowest pattern is too far from the valvestem centerline, then the valve to rocker relationship has to be changed. In this case, valve stem length will need to change."
03-21-2012, 11:50 AM
#8
D.R. THE PATHETIC DORK
Join Date: Jun 2007
Location: West Virginia
Posts: 30,863
Last edited by mjr46; 03-21-2012 at 11:53 AM.
03-21-2012, 02:55 PM
#9
centering the witness mark on the stem is not and I repeat not an accurate method for getting the correct geometry, you want a length that gives you the narrowest witness mark, try going up one length then down one length and watch how the witness width either gets wider or more narrow, sometimes you'll end up a little off center toward the front or towards the back, as long as travel stays in the center 1/3 of the stem your fine, you want the narrowest witness mark you can get, don't believe me, go ask this question on the corral.net and wait for ED Curtis or Buddy Rawls to reply and confirm what I state
Teach me your ways
