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Mopar Valve Geometry Let's discuss valve geometry 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 centre 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 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 haven't 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 centre of the valve tip through the cycle which will indicate the correct geometry. Now double check the lash, finally adjust the length of the push rod until the desired lash is achieved, measure the pushrod and order yourself a set…. you’re 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 you’ve 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 it’s radius this formula can be thrown out the window, it’s 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 deep with the cutter and you'll 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.
Spark Plug Reading
Carb Sizing
Hydraulic Cam Installation and Braking
Mopar Rear Ends
Remove/Install your Distributor |
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