Author Topic: Rocker arm sweep across valve stem and resulting valve lift  (Read 1118 times)

Offline femtnmax

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Rocker arm sweep across valve stem and resulting valve lift
« on: June 03, 2009 - 01:06:24 PM »
I am determining pushrod length on my 360, but the following info should apply to any engine with pushrods and overhead valves.
First setup had the rocker arm roller contacting the center of the valve stem.  With valve closed, the contact pattern started on the intake side of the valve centerline.  Then the contact swept across the centerline and stopped on the exhaust side of the centerline with valve full open.   Contact pattern width was 0.13 inch, valve lift was 0.523 inch.
For second setup, the rocker arm roller contact started on the exhaust side of the valve centerline with valve closed, the contact then moved a little further toward the exhaust side at mid valve lift, then the contact moved back toward the centerline at valve full open.  So the point of contact at valve closed and full open was the same.
Now the contact pattern width was 0.08 inch  (a 38% decrease), and valve lift was 0.534 inch ( 0.011 inch increase, thats a 2.1% increase).  The rocker arm roller now rotates very little compared to the first setup, so side loading on the valve stem should be decreased an equal amount too.
Reading I've done said it's most important to have the rocker arm as perpendicular to the valve as possible (second setup), and not to worry if the contact pattern is not perfectly centered on the valve stem; although being centered and perpendicular would be ideal.   FYI
Phil




Offline moper

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Re: Rocker arm sweep across valve stem and resulting valve lift
« Reply #1 on: June 03, 2009 - 01:17:26 PM »
2nd setup is better. I use 1/2 lift and I want the rocker centered on the stem at that point. That should be the point where the rocker is exactly perpendicular to the valve centerline. That usually results in the shortest sweep. Mopar heads using the shaft system depend on the valve job to get the geometry perfect. If you shimmed the rockers to get the pattern better the valves might be a little long or the seat depth is too deep for the length. You will also want to make sure the valve spring installed heights are correct so the tension is right because a sunk valve seat or long valve will leave the spring taller and weaker.

Offline femtnmax

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Re: Rocker arm sweep across valve stem and resulting valve lift
« Reply #2 on: June 04, 2009 - 12:16:00 AM »
2nd setup is better. I use 1/2 lift and I want the rocker centered on the stem at that point. That should be the point where the rocker is exactly perpendicular to the valve centerline. That usually results in the shortest sweep.
Yes, thank you.  This is on a set of Edel magnum heads with the chevy rockers.  Looks to me like Crower offset trunnion rocker arms would correct the mid-lift "off center" error I measured.
Phil