How to Calculate Pushrod Length

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Modern automotive and lubricant technology has created engines that barely need to be touched, except for oil changes. A certain breed of automotive enthusiasts, however, lives and breathes to create high-performance engines. Those endeavors often include high-lift camshafts, high-compression heads, gasket changes, valve rockers and solid valve lifters. With each of these individual modifications comes the need to reconsider valve pushrod length. Calculating this length is a challenging trial-and-error process that requires careful measurements.

Things You'll Need

  • Calculator
  • Pushrod length measuring gauge
  • Feeler gauges
  • Define the new pushrod application. Suppose the engine builders have installed a new, high-lift cam with an extra 0.11-inch lift for both intake and exhaust valves in a V-8 engine with 8-inch-long stock pushrods. New solid lifters have also been installed that are 0.067 inches longer. At the same time, they have installed new cylinder heads where the valve rocker landing pads are 0.13 inches higher. Finally, the new head gasket is now reduced to 0.018 inches from 0.029 inches. You can estimate a new pushrod length with this information.

  • Calculate the net contributions of each of the changes. The 0.13-inch-higher landing pads increase the new pushrod length. Decreasing the new length are the 0.011-inch-thinner head gasket and the 0.067-inch-longer lifters, for a net decrease of 0.078 inches. The 0.11-inch higher lift camshaft dimension should be ignored because the point of the high-lift cam in the first place is to open all the valves more.

  • Calculate the new pushrod length by adding or subtracting the net changes from the original 8-inch stock pushrod length. This becomes 8 inches + 0.13 inches - 0.078 inches = 8.052 inches.

  • Validate the calculation with measurements. Set the pushrod measuring gauge to 8.052 inches. Check clearances throughout each rotation of the valve train for all intake and exhaust valves to assure there is no binding. Because the overall net pushrod change is plus 0.052 inches, you will require new pushrods to go with the other modifications.

Tips & Warnings

  • Always check valve-to-piston clearance whenever higher lift cams are installed.
  • Experienced, pragmatic engine builders will budget new pushrods into the project, knowing that many factors can affect this length.
  • Never gamble with pushrods that are slightly too long because they can quickly cause thousands of dollars of damage to an otherwise viable project.

References

  • Photo Credit hot rod engine image by itsallgood from Fotolia.com Large engine maintenance image by Alexey Stiop from Fotolia.com Midiendo image by andoni from Fotolia.com architects tools image by Chad McDermott from Fotolia.com
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