The general mechanic would assume that bolts are to the tightened via torque alone, or torque plus angle. Its fairly common to find aftermarket (and some OE) bolts to require tightening via stretch, or give specs for both.
The most common bolts to specify tightening via stretch to be the conrod big end bolts in an engine. Many aftermarket anufacturers of Forged conrods will commonly supply them with ARP bolts, commonly ARP2000. ARP8740 and ARP L19.

What is tightening via stretch?

A gauge goes over the bolt whilst the bolt is in position. The gauge can be zeroed at the bolts rest length. When tightening the bolt, the gauge will measure how far the bolt has stretched. The stretch puts a compression force on the two parts on the big end. The bolt material is slightly elastic, and its this stretch that we are measuring.

Why would we tighten via stretch rather than torque?

Tightening via torque is highly dependant on the lubrication used and the cleanliness of the threads/head. A dirty or dry thread will require a higher torque on the bolt to achieve the same compressing force as a clean/properly lubricated thread. This is why ARP supply a specific lubrication to be used with their bolts and threads. However, tightening using a Stretch Gauge helps eliminate this inaccuracy, as the force applied by the bolt is consistent with how much it is stretched. Incorrect load from the bolts can effect roundness and size of the big end, and cause cyclic fatigue .

Tightening procedure for Conrod bolts.

  1. Clean all threads and mating surfaces.
  2. Assemble Bearing cap onto rod (assuming bearing fitted and correct orientation.
  3. Apply correct lubricant (ARP moly assembaly lubricant usually) to the threadsof the bolt, as well as under the head of the bolt.
  4. Finger tighten both bolts. If the cap has dowels that are tight, then tighten s gently as possible, alternating between bolts to keep the movement as even as possible.
  5. Once seated, torque bolts to 20nm.
  6. Loosen one bolt.
  7. Fit the stretch gauge over the bolt and zero gauge.
  8. Tighten the bolt to desired stretch.
  9. Loosen second bolt.
  10. Fit the stretch gauge over the bolt and zero gauge.
  11. Tighten the bolt to desired stretch,
  12. Repeat proceedures 6-11.

Big end ARP stretch settings/torque settings.

Firstly the bolt needs to be measured, as well as the text on the head of the bolt noted. Your Conrods should have been supplied with specific settings. The ones supplied below are to be used as a GUIDE ONLY, and it is ALWAYS recommended to use stretch settings rather than torque settings.

Bolt Material Thread Under Head Length Stretch Torque
ARP 625+ 5/16″ 1.260″ 0.0050″-0.0055″ 36lbf – 49Nm
ARP 625+ 5/16″ 1.500″ 0.0055″-0.0060″ 32lbf – 43Nm
ARP 625+ 3/8″ 1.500″ 0.0060″-0.0065″ 42lbf – 57Nm
ARP 625+ 3/8″ 1.600″ 0.0065″-0.0070″ 46lbf – 62Nm
ARP 625+ 7/16″ 1.750″ 0.0065″ – 0.0070″ 85lbf – 115Nm
ARP 8740 3/8″ 1.500″ 0.0046″-0.0050″ 40lbf – 54Nm
ARP 8740 7/16″ 1.600″ 0.0049″-0.0054″ 63lbf – 85Nm
ARP 8740 7/16″ 1.750″ 0.0055″-0.0060″ 63lbf – 85Nm
ARP 8740 7/16″ 1.800″ 0.0057″-0.0062″ 63lbf – 85Nm
ARP 2000 5/16″ 1.505″ 0.0050″ – 0.0055″ 30lbf – 41Nm
ARP 2000 3/8″ 1.500″ 0.0055″ – 0.0060″ 44lbf – 60Nm
ARP 2000 3/8″ 1.600″ 0.0055″ – 0.0060″ 48lbf – 65Nm
ARP 2000 3/8″ 1.700″ 0.0065″ – 0.0070″ 45lbf – 61Nm
ARP 2000 3/8″ 1.750″ 0.0065″ – 0.0070″ 42lbf – 57Nm
ARP 2000 7/16″ 1.550″ 0.0050″ – 0.0055″ 66lbf – 90Nm
ARP 2000 7/16″ 1.725″ 0.0065″ – 0.0070″ 65lbf – 88Nm

Forged Conrods and Billet conrods by Eagle, arrow precision, pauter, carillo, Williams, max speeding, manley turbo tuff, manly, cosworth, supertech, scat. Used with Forged rods, forge rods, billet rods, aluminium conrods, titanium conrods.

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