Valve train points to check

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by: Cobalt327, Crosley, Jon
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Selection of various valve train components


[edit] Overview

The valve train is one of the most important segments of any engine build. Obviously there can be no destructive contact/collision between any part of the reciprocating assembly or engine castings and the valve train. Besides avoiding physical contact, there are also minimum and maximum clearances that have to be established and checked to be sufficient.

[edit] Things that affect valve train clearances

Many things can affect valve train clearances. When an engine is rebuilt or modified, often the block and/or cylinder heads will be milled to provide a flat surface with the correct finish for the head gasket to seal. If a different lift camshaft is installed, or if a different design or ratio rocker arm is used, or different length valves, resurfacing the valve and seats, different compressed thickness head gasket, changes to the stud boss height and/or installing guide plates or different design studs, lifter having a different measurement between the foot or roller and the pushrod cup, changes to valve spring installed height or diameter, or spring retainer and/or lock type, and different pushrod length all can change the clearances and geometry of the valve train. So if any of these things have been altered, the valve train clearance points need to be checked and verified to be correct. Manufacturing and tolerance stacking and wear can all change the exact measurements, so these things should be physically measured rather than figured using published figures.

[edit] Valve train geometry

There needs to be a correct relationship between the components of the valve train to allow the efficient transfer of the cam lobe profile info into linear valve motion. This relationship is often referred to as the valve train geometry. On many OHV engines (like the Chevy V8) the geometry is adjusted by changing the length of the pushrod. 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 height
  • Head stud boss height
  • Rocker arm brand/design
  • Cam base circle size
  • Lifter design/brand/pushrod seat height
  • Valve stem length
  • Head gasket thickness

Warning Note: More at Valve train geometry.

Hotrodders forum threads:
Also see:

Warning Note: While opinions vary, many agree a narrow pattern is more important in many cases than a centered swept area, as long as the rocker tip is in no danger of riding off the edge of the valve tip. It is strongly suggested the subject of valve train geometry be researched until the builder is satisfied with their decisions.

To check for the correct length pushrod for the application, often an adjustable pushrod is used. They can be easily made or purchased from cam manufacturers like Comp. A brief run-down on making your own is here.

[edit] Areas of the valve train that need to be checked and verified

This list is not all-inclusive. Not all areas will apply to all engines and/or builds.
Warning Note: There is no substitute for physically measuring critical areas like piston-to-valve clearances or valve spring installed height and coil bind. For those measurements, do not rely solely on published figures.
Valve spring clearances. Seal to retainer may be less in some cases
  • Valve spring specs
    • Valve spring rate
    • Installed height
    • Coil bind
  • Correct geometry
  • Retainer to seal/valve guide boss at full lift
  • Retainer to rocker
  • Rocker trunnion (roller rockers) or pivot ball/rocker body (stamped steel rockers) to stud boss
  • Piston-to-valve clearance (both before and after TDC on overlap)
    • When advancing/retarding the cam, be sure to recheck piston to valve clearance. Each degree of change effects valve clearance approximately 0.010". Example: If you advance the camshaft 4 degrees, you will lose about 0.040" clearance between the intake valve and piston, and you will gain about 0.040" clearance between the exhaust valve and piston. It will be the opposite if the camshaft is retarded.
    • Also see Quench
  • Push rod to guide slot in head (see image below)
  • Use EITHER self-aligning rockers OR pushrod guide slots in head OR guide plates, but NEVER two or more of these at the same time
  • "Rails" of self aligning rockers to retainer and/or locks
  • Rocker to valve tip (if needed, adjust guide plates if used)
  • Rocker slot to stud (if using stamped rockers w/pivot balls)
  • Rocker arm/polylock to valve cover or oil baffle
  • Camshaft endplay
  • Distributor shaft endplay and distributor/cam gear mesh/alignment
  • Distributor gear material compatible with cam material
  • Mechanical fuel pump pushrod material compatible with cam material
  • Hydraulic lifter preload or solid lifter lash setting
  • Timing set phasing
  • Timing tabs and damper TDC lines SBC
  • Cam gear to crank gear alignment
  • Cam gear end play if roller cam
  • Cam gear to block casting/oil gallery plugs
  • Timing set to timing gear cover
  • If using the SBC OEM roller cam, lifters and retainers, the lobe lift must be kept below 0.354” so the lifter won’t lose contact with the retainers
  • Cam lobe to connecting rod bolt on stroker engines
Red arrows indicate the pushrod guide slot; slot needs elongating towards the rocker stud if 1.6 ratio rockers are used

[edit] Spring cups


When using a spring cup or locator it's important that it closely fit either the ID of the spring seat or the OD of the guide boss. This keeps the cup or locator from moving around- which is one reason for using it in the first place.

Aluminum heads need to have something that will protect the aluminum from wear. At the very least a hardened shim at least 0.030" thick should be used.

[edit] References

Crankshaft Coalition wiki articles
Hotrodders forum threads

[edit] Resources

[edit] Calculators

Static compression ratio
Dynamic compression ratio

Jessel mohawk AFR ford windsor.jpgComp roller 374 ford sherman.jpg

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