Precision Measurement Supply - Fixtures, Tooling And Measuring Devices For Professional Engine Builders

Rod Bolt Stretch Gauge Instructions

1.     Adjust lower point to provide sufficient tension to hold gauge on rod bolt.  (Approximately .150 - .200 indicator pre-load).  NOTE:  When adjusting lower point, make certain upper and lower brackets (anodized clamps) are on a flat surface to assure proper point alignment.

2.   Place box, wrench, crowfoot, etc. on rod nut.

3.   Mount gauge on rod bolt.  Make certain gauge has sufficient pre-load to stay on bolt.  Also be sure upper and lower points seat in “dimples” in the rod bolt ends.  If bolts aren’t “dimpled”, check them in a lathe and use a #2 or #3 center drill.

4.   Loosen indicator thumbscrew and “zero” dial indicator.  Set limit points (black points on indicator face) to desired stretch.

5.    Tighten rod nut with box end wrench to manufacturer’s specs.  If no specs are available, use the guidelines below.  NOTE:  If specs for manufacturer differ from guidelines, use manufacturer’s specs.

All guidelines are based on use of 30-weight oil on bolt and nut

  • 7/16”- 20 tpi - .007 - .008 stretch (seven to eight thousandths)

  • 3/8” - 24 tpi - .005 - .007 stretch  (five to seven thousandths)

  • 11/32” - tpi - .006 - .007 stretch  (six to seven thousandths

  • 5/16” tpi - .0055 - .0065 stretch (five and ˝  to six and ˝ thousandths)

6.   It is not a good idea to use rod nuts after five or six times.  

7.    If you have problems getting repeat readings, check the following:

  • 1.  Bolts in upper and lower anodized clamps tight.

  • 2.  Upper and lower points correctly aligned.

  • 3.  Upper and lower points properly seated in bolt “dimples”.

  • 4.  Sufficient indicator pre-load (.150 - .200)


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