My Pushrod length & valve geometry inspection/checking techniques

Pro_usa1776

Registered User
I am new to the forum but I wanted to share the techniques I used to check for the correct valve geometry and pushrod length.

1989 SC 3.8 Liter - zero miles since complete rebuild (nothing fancy; OEM pistons/Connecting rods)
Stage II Comp Cam part # 44-701-9
Scorpion 1.73 roller rockers
Scorpion .720" pedestals
OEM roller lifters

* I used a COMP Cam (7702) adjustable pushrod and a 12" set of precision calipers to verify the pushrod length.
* I used the Exhaust Open Intake Closed (EOIC) technique to make sure I was on the base circle of the camshaft when measuring pushrod length.
* I torqued the roller rocker retaining bolts to 20ft lbs.

Step 1: Mark all the valve stem tips with a sharpie marker, this will result in witness marks being left on the valve stem when turning the motor over.

Step 2: Place the adjustable pushrod in the head to ensure it is in the exhaust valve #1 cylinder roller lifter and roller rocker correctly.

Step 3: Turn the engine over and look at the intake roller lifter and as it just moves down all the way in the engine block you are on the base circle of the Camshaft for the exhaust valve (exhaust valve is closed.) Then you are ready to adjust for zero lash.

Step 4: Adjust zero lash by lifting up and down on the adjustable pushrod while simultaneously tightening the roller rocker retaining bolt on the exhaust valve. Once there is zero up and down movement you have successful adjusted for zero lash.

Step 5: According to Scorpion's instructions, in order to set the correct preload on the roller lifter , one must torque the roller rocker retaining bolt from 1/2 (equals approx. .025" preload) to a full turn (equals approx. .050" preload) until your torque wrench clicks at 20ft lbs. Less than 1/2 a turn, the engine requires a longer pushrod, more than 1 full turn, the engine requires a shorter pushrod.

Step 6: Turn the motor over completely 4 times (360 degrees x 4 - hence 4 stroke engine) then you can loosen the roller rocker retaining bolt and take the roller rocker off. The witness mark on the valve stem should be in the middle. If it is not the pushrod is too short if the witness mark was towards the inside of the head (nearest to the intake manifold), or the pushrod was too long if the witness mark was towards the outside of the head (nearest to the exhaust manifold). If necessary reassemble and follow steps 3 - 5 and adjust the pushrod length to move the witness mark to the center of the valve stem.

Step 7: Do the same for the intake, except turn the engine over until the exhaust roller lifter just starts to push up, then you are on the base circle of the Camshaft for the Intake valve (Intake Valve is closed.)

Step 8: Use the same procedures listed above and follow the firing order when checking the rest of the valves. (SC firing order: 1-4-2-5-3-6). Some recommend only doing the outside cylinders on both heads. I chose to do them all for practice and there are only 12 valves …


My pushrods all measured to 7.425"

**** The only issue I have, is I have seen other instruction on line to add some length to your pushrod to allow for lifter preload. I'm calling BS on that, because I did that by turning the roller rocker bolt a full turn. According to the specs on a 8mm x 1.25 bolt. One complete turn should equal .050". Am I wrong?
 

Attachments

  • 1-2-3 Passenger's  side cylinder head-1.jpeg
    1-2-3 Passenger's side cylinder head-1.jpeg
    475.7 KB · Views: 119
  • 4-5-6 Driver's side cylinder head-1.jpg
    4-5-6 Driver's side cylinder head-1.jpg
    574.1 KB · Views: 123
Last edited:
The only problem you are running into is that while you are setting everything up correctly, once you start to rotate the engine to check witness, you will be experiencing anywhere from zero bleed down of the lifter during the rotation to full compression of the plunger. Of course it comes right back up as the lobe goes down, so you won't really SEE this happening, but it is. The result is that your witness will only reflect a portion of the full lift that will be seen when there is oil pressure present.

I have simulated this by taking a spare lifter apart and locking it up solid with the prescribed .050" preload built in. When doing this, set the lifter up just at zero lash and then you'll get the full effect of normal hydraulic preload as well as max lift.

That being said your pattern looks great so far. I have found though that I often have to run the contact patch a bit inward from center due to lack of pedestal engagement to keep the rockers going straight. To run a 7.4xx pushrod generally requires a lot of shim to get proper preload. I use 7.400 quite a bit though so it's not unusual.
 
Roller Lifters

The only problem you are running into is that while you are setting everything up correctly, once you start to rotate the engine to check witness, you will be experiencing anywhere from zero bleed down of the lifter during the rotation to full compression of the plunger. Of course it comes right back up as the lobe goes down, so you won't really SEE this happening, but it is. The result is that your witness will only reflect a portion of the full lift that will be seen when there is oil pressure present.

I have simulated this by taking a spare lifter apart and locking it up solid with the prescribed .050" preload built in. When doing this, set the lifter up just at zero lash and then you'll get the full effect of normal hydraulic preload as well as max lift.

That being said your pattern looks great so far. I have found though that I often have to run the contact patch a bit inward from center due to lack of pedestal engagement to keep the rockers going straight. To run a 7.4xx pushrod generally requires a lot of shim to get proper preload. I use 7.400 quite a bit though so it's not unusual.


I might try a different technique that I saw another engine builder use.
 
Last edited:
Back
Top