Whether your next engine build is mild or wild, connecting rod bolts or studs at the crankshaft are among the most important fasteners within your engine. K1 Technologies recommends multiple ways to tighten your rod bolts properly, and the sole use of a torque wrench is low on the list.
The connecting rods must transfer a piston traveling “up and down” to the rotating motion to feed the drivetrain. Each rotation has high forces from all angles on the rod cap. The clamping load of the fasteners at top dead center (TDC) is the most demanding. This inertial stress occurs as the piston transfers from an upward motion to a downward one.
The Rod Bolt Stretch Gauge
First, you record each rod bolt length before tightening. You then monitor the stretch as you tighten and follow the recommended stretch length within your K1 rod instructions. Tightening your rod fasteners by monitoring stretch is the top recommendation by K1.
“The stretch method for tightening rod bolts is recommended regardless of the bolt’s material or fastener material,” says Michael Skeen, Technical Sales Representative at K1 Technologies. “If the bolt is not stretched enough, there will not be enough clamping force. Alternatively, it could fail if the bolt is overstretched beyond its yield strength.”
We looked at an example K1 big-block Chevy connecting rod with 7/16-inch rod bolts. K1 Technologies provides stretch specifications with each connecting rod set purchased. The instructions for this application noted a necessary stretch of .0060- to .0065-inch when tightening the bolts.
The Torque And Angle Method
Using the torque and angle method, you’ll then tighten the bolt to a low torque value, as noted on the individual K1 Technologies instruction sheets.
Following pre-torque, you use an angle gauge and turn the bolt a prescribed number of degrees to stretch the bolt properly. This method uses the highly accurate pitch of the bolt thread to control the amount of stretch.
The Torque Wrench
Torquing rod bolts does not measure the clamping forces actually applied to your rod bolts. The torque wrench only measures the friction on the treads that must be overcome to turn the bolt. Consider that measuring torque does not accurately inform you how much the fastener is stretched.
Some builders will install one rod via the stretch or torque angle method. Then they will use a quality torque wrench to duplicate that torque value on the following rods while measuring the final stretch for each bolt.
Lastly, keeping those rod bolt stretch records is a good practice for the next time you dismantle your engine. If the rod bolt length changes more than .001 inches from its first pre-installed length, the bolt has been stretched beyond design limits.
“The rod bolt is essentially an extremely stiff spring, and we rely on the material’s elasticity to stretch and rebound to maintain the correct clamping load,” Skeen finishes.
With the technology placed into each component such as the connecting rods from K1 Technologies, invest equally in following and learning why their assembly processes are recommended.