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Valve Geometry...explained
Ive been following a few posts on valve geometry and pushrod length discussions and I thought Id chime in here and try and explain this mystery of engineering or lack thereof.
You engine builders already know all this stuff but for the novices I'll try and explain how all these angles get dangled. 1st .Valve Geometry is simply the position that the roller of the rocker arm rides on the valve tip, the roller should ride in the center of the valve tip through the cycle. But, because the rocker arm rotates on an axis and the valve cycles on a plain or straight line they move across the valve tip and are not stationary or pinpoint but rather they sweep across the valve tip. Good geometry will keep the roller centered as much as possible throughout the cycle. Bad geometry will allow the roller to ride either on the front edge or back edge of the valve tip and cause severe side stress on the valve and guide with eventual failure to one or both 2nd of all....pushrod length and valve geometry on a Mopar have no relationship...what! you ask?????... think about it...you basically have 2 things that remain constant...the height of the valve tip and the shaft. Now, if you think about a Chevy with a stud rocker system as you adjust the valve lash you are actually changing the elevation of the rocker arm with respect to the valve tip or. changing the valve geometry. Can you visualize the difference here? The only way to change the geometry on a shaft system is to raise or lower the shaft itself, which changes the relationship or angle that the rocker arm meets the valve tip or to lengthen or shorten the installed height of the valve by way of increasing or reducing the overall length of the valve. So if you cut .120 of the bottom of the head you still havent changed the geometry because the rocker shaft and the valve tip relationship have not changed what has changed is the pushrod length, why? you have moved the rocker shaft closer to the cam making any sense now? So after you get all the cutting done on your head (some straight and some angled...another topic) and the 4 angle or radius valve job done (still another topic) You need to get a checking spring (available from Powerhouse $7.60, 1-800-872-7223), assemble one cylinder, using an adjustable pushrod (Powerhouse $19.95), bolt down the shaft and start the geometry checking process. Using Prussian Blue available at most good parts houses, coat the top of the valve tip and adjust the push rod, the roller tip should contact the valve tip to the inboard slightly (about .010-.020 for straight 1.5:1 rocker arms) at this point the wear mark of the roller across the valve should be in the center of the valve tip through the cycle which will indicate the correct geometry. Now double check the lash, final adjust the length of the push rod until the desired lash is achieved, measure the pushrod and order yourself a set . youre done well almost Mopar heads have some inherent problems with valve geometry issues when relationships with big installed height springs are used. For example on a small block Mopar head the installed height on the valve spring is 1.670 if your valve spring and cam require an installed height of say 2.00 to prevent coil bind youve got problems as now you have drastically changed the fixed relationship of the shaft to the valve tip by lengthening the valve which will make the rocker pull way inboard on the valve tip. The easy theoretical cure would be to simply shim the shaft up .330 (2.00-1.67=. 330) and regain the geometry, this of course is simplified as the plane of the valve axis and the geometric plane of the rocker arm are not parallel and as the rocker arm goes through its radius this formula can be thrown out the window, its only an example to give you the idea. Does anyone know of a company that makes shaft shims that work anymore? Of course you can cut the spring seat down to maintain the valve tip elevation relationship to the shaft but your limited here as Too Much will hit water, the most you want to cut the pocket would be about .060 and even then your risking the casting and as you increase the spring diameter the risk gets even higher. The only way to accommodate a huge lift and big installed height springs is to scrap the entire system and replace it with a Jessel or W series head design or retro fit and go to pushrod oiling. This is our Christmas present to you all...have a good one and the very best in 2003 |
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Another Cool post to add to my never [too much racing ] info file!Can't wait for the valve job post . LOL New Year's present?
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all i got to say is,,,cuda66273,,you must be a mopar god,i'm one off those guys that are verry thankfull your here,,,speedfreak.
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There is also a very informative article here:
http://www.dvorakmachine.com/rocker_geometry1.shtml |
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Thanks Guy's I'm glad you found it informative and useful.
I'll work on the valve job post and see if I can get it up this month. I'm also working on a article that will remove all the mystery of crankshaft swapping in small blocks and how to get good rods into your Mopar at Chevy prices. Soon Don |
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geometry
Whoa...wait a minute. Pushrod length affects geometry on any pushrod engine. It doesnt matter if it is stud or shaft mounted. The pivot point of the rocker arm doesnt change-it can't! The fulcrum does not move up and down no matter how far the valve opens. Unless the overall valve length is changed to the point where the shaft position has to be raised then the best way to correct geometry is by changing the pushrod length. The location of the adjustment screw makes no difference in geometry. The only difference the location of the adjustment screw makes is how many turns it takes to set lifter preload on a hydraulic lifter.
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Yes, you're right, what was I thinking?
I'll remember not to overstep my limits in future. |
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cuda66273
It would be most informative to the readers (like me) if you could add/explain the benefits of the 4 angle vs 3 angle in your up and coming article.
thanks |
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I will..this one will have to have photos and drawings so it'll be awhile until I get the time to complete it.
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so whos right? Does push rod legnth change geometry?
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#12
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It kinda got confusing in the bantering...
The geometry is changed buy moving the valve tip up in relationship to the fulcrum or rocker arm shaft center, as in longer valves to allow for bigger installed heigths on the springs. To correct this you would need to raise the pivot center of the rocker arm to compensate for the longer valve, which can't be done unless you go to the flat pedastal mounted shafts like on a W2 head. If you could raise the rocker shaft up then a longer push rod would be needed to maintain geometry. |
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