Editing Valve train geometry

[[File:Miller diagram.jpg|thumb|370px|]]
==Overview==
Valve train geometry is an important aspect of building a performance engine. If done correctly, having the correct geometry will increase power and reliability.

==Measuring a pushrod==
Overall or "actual" length doesn't always equate to the pushrod length as specified by the manufacturer, odd as this sounds. The difference is in whether the pushrod was measured by overall or actual length (longest measurement of the three), "theoretical" length (about 0.020" longer than actual length) or "gauge" length (longest of the three). 

The theoretical length is longer than the overall length as measured w/calipers, etc. because it takes the oil holes out of the measurement and instead treats it as though the ends of the pushrods were round, w/o an oil hole. 

The gauge length is what Comp Cams (as an example) uses. This is measured as how the pushrod sits in the rocker arm and lifter seats. In the Comp Cams diagram below, 8" is just used as an example:

To accurately judge the geometry, the hydraulic lifter either needs to be pumped up, or made solid, or a solid lifter w/the same seat height substituted to judge the geometry.

[[File:P rod length.jpg]]

==Stock pushrod length==

===Chevy small block===
*Stock hydraulic flat tappet SBC- 7.8”
*SBC OEM hydraulic roller pushrods are 7.2” long. When using shorter V6 or L4 OEM roller lifters, start with 7.500" pushrods. 
*Crane retro hydraulic roller- 7.046" 
*Retrofit roller lifter pushrods are 7.300".

===Pontiac V8===
Stock pushrod length- 9.130". It has been published that if using Chevy lifters, the pushrod length is 9.290” as a starting point. However, caution must be used if Chevy lifters are going to be tried in a Pontiac engine because the oil band is in a different location. This could effect oiling. There are aftermarket lifters advertised for use in both Chevy and Pontiac engines. These should also be checked but the assumption is that the oil band is correctly located for both applications.

==Things that can effect pushrod length==
A large number of variables are involved in determining the correct length pushrod for your application. Pushrod length is affected by any of the following: 

*Block deck height 
*Head deck thickness changes from milling 
*Head stud boss height 
*Rocker arm brand/design 
*Cam base circle diameter 
*Lifter design/brand/pushrod seat height 
*Valve stem length
*Head gasket thickness

==Mid lift theory of valve train geometry==
;Info from Jim Miller<nowiki>:</nowiki>
*[[Media:MILLER ROCKER GEOMETRY.pdf|Rocker Geometry]] article from the AERA (Automotive Engine Rebuilders Association) quarterly magazine, '''''Engine Professional''''' by Jim Miller
*[[Media:MILLER ROCKER HISTORY MID LIFT.pdf|History of Mid-Lift]] By Jim Miller
  
;Hotrodders forum threads<nowiki>:</nowiki> 
*[http://www.hotrodders.com/forum/mid-lift-rockers-206836.html?highlight=MID+LIFT+MILLER Miller mid lift rocker theory]
*[http://www.hotrodders.com/forum/rocker-arm-199353.html Rocker arm geometry]

==Tools==
[[File:PUSH ROD LENGTH CHECKING TOOL 004.JPG|thumb|left|400px|Homemade adjustable pushrod length checking tool]]
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Miller uses a MID-LIFT G-TooL to measure for geometry. It is quite different from the other devices on the market, like from Comp Cams and others that are used to estimate pushrod length.

==Resources==
{{Note1}}Also see wiki page [http://www.crankshaftcoalition.com/wiki/Valve_train_points_to_check '''Valve train points to check''']
*Circle track article ''[http://www.circletrack.com/techarticles/ctrp_0611_rocker_arm_valvetrain_geometry/viewall.html Rocker Arm Geometry And Valvetrain Alignment], A Detailed Explanation Of Valvetrain Alignment Goals'' by Bob Bolles, from February, 2009 CT mag
*Lunati article [http://www.lunatipower.com/Tech/Valvetrain/HowToVerifyValvetrainGeometry.aspx How to verify valve train geometry] 
*Wiki page [http://www.crankshaftcoalition.com/wiki/Category:Adjust_valves Adjust valves]<br><br><br><br><br> 

[[Category:Engine]]
[[Category:Camshaft]]
[[Category:Cylinder head]]

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