Torquing ARP High-Strength Connecting Rod Bolts Like A Pro

When it comes to assembling an engine, we’ve always been warned to never torque a high-strength fastener with dry threads. According to many experts, torquing bolts with dry threads is an invitation for galled or stripped fasteners. We decided to check in with ARP, a leader in high-strength fasteners, to get the real scoop on torquing bolts. 

Decades ago, many enthusiasts’ first introduction to ARP bolts was in conjunction with connecting rods. Now ARP bolts are utilized everywhere you need a high-strength bolt – like suspensions, restraining belts, exhaust manifolds, etc. 

There are a lot of opinions on which fastener assembly lube to use and what the torque value should be. ARP provides torque specs for its fasteners based on using ARP Ultra-Torque fastener assembly lubricant. If you are not using ARP’s specs and the ARP Ultra-Torque lube, ARP’s engineer says you have no real idea what pre-load you’re getting.

The Dark Ages

Prior to the introduction of the ARP’s Ultra-Torque assembly lube, the torquing process took three or four steps. Many engine builders recommended torquing rod bolts for three fully-cycled times (tightened then loosened) to work harden (burnish) and polish the bolt and nut threads. This process also burnished the seating surface on the rod cap under the nut.

The old process involved cleaning the rust preventative from the new bolts, applying whatever lube was the choice of the month on the threads of the rod bolts, nuts, and seating area. The connecting rod nuts were installed finger tight in preparation for even tightening. Next, the connecting rod nuts could be fully torqued and loosened three times (without bearing in the rod cap or connecting rod). 

If this seems like a tedious process, at this point, the job was only halfway done. To finish the process, the engine builder would clean the threads on the bolts, nuts, and seating surfaces to remove the lube used in the burnishing process. This cleaning step was done to wipe away the microscopic metal (fuzz) that was created by the burnishing process. Finally, engine bearings were added to the connecting rods and caps, lube was applied and the final torque was applied to the fastener. If that seemed like a lot of work, it was. 

The Modern Method

With ARP’s Ultra-Torque lube, the process is less intensive and more exact. After years of testing various lubricants to produce a metal-free, repeatable, and consistent lubricant to achieve a target preload on a fastener, ARP developed its own lubricant. The most surprising result of the product was the ability to hit the target preload more accurately and consistently on the first torque cycle. 

When it comes to connecting rod bolts, there are so many variables present when applying torque to establish the proper load on the bolt, the best way to achieve the proper tension on the bolt is to use a stretch gauge. To learn more about that, we covered that specific topic in this article: Stretched to the limit.

Armed with laboratory tests, ARP published a detailed paper on its Ultra-Torque assembly lubricant that can be found here. Over a decade later, the product has proven to be everything ARP claims, making engine builder’s process easier with precise results. 

To find out more about ARP’s Ultra-Lube fastener assembly lubricant or other ARP products, visit them online at arp-bolts.com. 

Article Sources

About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.
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