The hows and whys of measuring rod bolt stretch

From Chris Jensen, ASE Certified Master Machinist

Most engine rod failures aren’t from faulty parts, they’re from improper installation. When you’re installing the rods, you need to check the rod bolt stretch by performing a series of torquing and loosening and recording the readings. It sounds like a lot of work, but in the end, it will save you time and the possibility of a failed engine rod along with all of the damage that could do.

Here’s what you need to do:

  1. Torque the bolts to 15 foot pounds and measure the stretch. Record the reading, then loosen the bolts.
  2. Repeat at 15 foot pounds and record the reading
  3. Repeat, increasing to 30 foot pounds, record the reading
  4. Repeat, increasing to 60 foot pounds, record the reading
  5. Repeat at 60 foot pounds and record the reading

Each time you loosen the bolts, measure to be sure that the rod bolts have returned to within .001″ of the starting size. If you have a greater variation, get rid of the rod bolts and start with new ones.

When you’re conducting final assembly, be sure you use the stretch specified by your rod manufacturer. Attach a rod bolt stretch gauge to the connecting rod bolt and zero it. Use an extra-long box wrench to tighten until the proper stretch is achieved.

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