Why your 604 has broken spring shims

(This is pertaining to 'OLD' style spring shims.)

Questions or Problems contact us at: Ph.: 330-540-7223 or email brad@race-1.com

(all information below is a recommendation from Race-1 not necessarily from GM)


The factory spring shims in the 604 that commonly break are actually called locating shims and serve multiple purposes. —Some racers refer to these as retainers which is an incorrect term. The retainer is at the top of the valve spring and is an entirely different subject. The locating shim commonly breaks for several reasons.

  • 1st: Harmonics in the valve train (most commonly on cylinders 5&7)
  • 2nd: A stamping burr on the bottom of the shim preventing shim float
  • 3rd: Uneven seating surfacemost commonly caused by a previously broken shim
  • 4th: Weak valve spring pressurewhich permits float, bounce and harmonics

There is what we are calling a “new style” locating shim coming in the latest orders from GM. They are a little bit thicker than the ones we have been using in the past—not sure if this was produced by accident or possibly to help in the failure rate. These newer, thicker shims have raised another concern with us at Race-1.

Since it is now permitted by most major sanctions to install as many aftermarket shims as needed to achieve the minimum installed height of 1.780”—if a competitor has set the minimum installed height at its absolute minimum with the older style (thin) locator and then during regular maintenance discovers a broken shim and replaces it with newer style (thick) the engine could possibly be found illegal.

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