Valve train geometry

by: Cobalt327
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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.

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". If using Chevy lifters- 9.290”.

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

Link to the [Miller Mid-Lift ] and rocker geometry article in the AERA (Automotive Engine Rebuilders Association) quarterly magazine Engine Professional.

Tools

Homemade adjustable pushrod length checking tool

Resources

• Lunati article How to verify valve train geometry
• Wiki page Valve train points to check
• Wiki page Adjust valves
  
  
  
  
  

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