Valve train geometry

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by: Cobalt327
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Miller diagram.jpg


[edit] 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.

[edit] 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.

P rod length.jpg

[edit] Stock pushrod length

[edit] Chevy small block

  • Stock hydraulic flat tappet SBC- 7.8”
  • SBC OEM hydraulic roller pushrods are 7.2” long. When using shorter GM OEM roller lifters from the 3.1L or 3.4L V6, or the 2.2L inline 4 cylinder engines, start with 7.500" pushrods. These lifters are mostly only used when converting a flat tappet block to use an OEM roller cam, although they might be an advantage in a roller block due to their lighter weight.
  • Crane retro hydraulic roller- 7.046"
  • Retrofit roller lifter pushrods are 7.300".

[edit] 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.

[edit] Things that can affect 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

[edit] Mid lift theory of valve train geometry

Info from Jim Miller:
Hotrodders forum threads:
Example of instructions for Miller Mid-Lift PA Series rockers

[edit] Tools

Homemade adjustable pushrod length checking tool

Instructions for making an adjustable pushrod length checker.

Other homemade pushrod length checking tools at

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.

[edit] Resources

Warning Note: Also see wiki page Valve train points to check

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