I'm no small block Chevy expert but if you think about it where the roller tip is riding is probably exactly where it should ride. As the rocker rotates the roller tip will push the valve down and as it does so will move toward the center of the valve stem simply because the center of (rocker) rotation makes it move that way.

Take a close look at one by turning the crank with a ratchet slowly and watch how the roller moves on the valve stem as it goes through it's entire movement just to make sure.

 

COMPETITION CAMS has an excellent TECH ARTICLE on how to obtain proper valvetrain geometry. You really need to read it.

 

Manley make a simple pushrod checker length checker - another cheap tool.
When converting stdheads to screw in studs it pays to enlarge the 'as cast' pushrod guide holes because 2 guide points can side load the pushrods.
I know it's old news and dismissed by many, but I still aim for the roller centred on the valve at 1/2 gross lift.

 

The roller center point should be in the center of the valve stem and the contact pattern should be equal distance either side of the center point. If you have replaced OEM parts or machined the heads or deck, then you need to check the valve geometry when your rebuild. There have been several posts on how to do this and how to make your own push rod length checker to do the job. I use black magic marker to cover the end of the valve stems and install the lifter and push rod checker, adjust to proper lash and turn the motor to operate the valve through one complete cycle and inspect the pattern which is well defined in the magic markered valve stem. I then adjust the push rod checker tool and move to the other valve on that cylinder and check it. I re marker the first valve if I need to make another check. Once I have the pattern in the center of the valve stem, I remove the Checker and mic the length and call Summitt and tell them the length of push rods I need. I have seen as high as 1/4" difference from OEM on some builds and have not changed the valve geometry but very little.

Trees

 

That's the Manley tool, I have 2 in the shed - for 3/8 and 7/16 studs.

 

Ian, my bulb is not burning very bright today so please explain how this tool works. Ref Jmark's picture. I understand measuring the gap, but where are you placing the crank/cam to get this gap and what do you do if the gap is between the end of the push rod and the tool?

Trees

 

Yup, Jmark is on the money. Set the cam on base circle (360 CRANK degrees off peak lift) and measure as described.
Don't feel bad about being a little dim, you are about 14 hours ($26000 Canadian) behind us so it must be late at night when I correspond.
See ya in 14, Ian.

 

With the part of the tool over the push rod being angled, how do you get an accurate measurement or is this an illusion created by the camera angle? Also, do you make an adjustment for the lifter piston not being centered (hydraulic)? Maybe I am looking for a more precise measurement than what I think this tool will give me.

Trees

 

Draw a theoretical centreline through the rocker tip (valve contact point) and the pivot point on a stock sbc rocker, then one from pivot to the inside top of the pushrod seat. You'll find they align with the 2 angles on the Manley tool. To allow for lifter preload add 20 thou if the rod is too short and subtract 20 if it's too long. A 5/16 unf thread gives approximately 40 thou per turn so 20 though equates to a half turn preload. Obviously this ONLY applies to hydraulic lifters. Solids you just measure and order.
At first I used to recheck with an adjustable pushrod, but this cheap plastic tool is so good I don't even bother anymore. See ya, Ian.

 

Push Rods..

The best way to get the correct push rod lenght is to.

1. clean the rocker and the valve stem with brakecleen to remove any and all oil.

2. Use a black sharpie and mark the valve stem tip.

3. Install the rocker and set the correct lash

4. Rotate the engine by hand 2 or 3 revolutions

5. Remove the rocker and inspect the width of the mark left by the rocker on the valve stem...

6. The correct pushrod length will produce the smallest contact patch...and also that results in the most valve lift...


Trying to get the contact patch centered is not the correct way to do it..

 

One thing to remember about the Manley tool is it's designed around a camshaft with a lift of .500, if the cam is more or less than .5 then the accuracy lessens but it's still a good tool for a quick check.

Like K-Star I use an adjustable pushrod and shoot for the narrowest patch as close to center as you can get.

 

Ian, thanks for clearing this up for me. Kstar, we do it the same way, except my goal is to get the pattern center to be the center of the valve end. I had never read or been told the length of the push rod would change the width of the pattern. I thought the rocker arm ratio and cam lobe shape effected the pattern width. I have also been told by very experienced performance engine builders that pattern centering is more important than many think.

Trees

 

push rods

I can tell you i have worked with this issue for many hours in my shop... I have also confirmed it with other pro builders and the correct way to find pushrod length is by the contact pattern, or using a dial indicator and finding the push rod that gives you the greatest valve lift...

If you center the pattern it will reduce the amount of valve lift, this also scrubbs the top of the valve causing guide ware..

Look at the picture in this thread and draw a line through the valve stem center line and the stud center line. You will see that at a point they intersect. So if you add a longer valve or sink the seat, or what ever to change the height you move the tip of the valve closer to the stud. Many people will try to correct this with a shorter push rod to lower the rocker and get the tip of the rocker back to the center of the valve, but that wrecks the geometry.. If you get the narrow pattern and the center line of the pattern is off the center line of the valve the only correct way to fix it is with a rocker arm that has a shorter distance from the fulcrum to the tip....Good luck fiinding one of them....

here is an example of a race engine i just did.

pushrod length......total lift..........contact width........ amount off center

7.850 .622 na

7.900 .626 .120 .010

7.950 .627 .115 .025

8.000 .629 .105 .050

8.100 .621 na


The first and last numbers were just to see the start and falling off points, so i did not do patch and center... If you notice the most lift .629 matches the narrow pattern of .105 but had the contact patch off of center .050.... That is the push rod i ran.. These heads had .100" longer valves in them.


Keith

 

Kstar, thanks for the detailed explanation with measurement data. Jmark's link verifys your post as well. The is thread is the very reason I sift through a lot of fluff to learn something new or debunk something I thought to be true.

Trees

 

build

Trees,,, The deeper/more detail i get into engines, The more i find that very few people including the so-called pro's really understand why they do things...or the results of what they do...

I have been blessed to have a engine building mentor that is about the best in the usa to give me reasons to think through these deals....

Keith