How to Use a Rod-Bolt Stretch Gauge

There are several methods for setting the clamping force of a bolt. Torque, torque angle and stretch. The torque method uses a torque wrench to tighten the bolt to a specification measured in foot-pounds. Torque angle uses a torque setting plus an additional tightening measured in degrees of rotation. These are the most common methods because most bolts are fitted into blind holes where the end of the bolt is not accessible. The stretch method measures how far a bolt is stretched as it is tightened. The stretch method is primarily used on automotive connecting rod bolts because the head and end of the bolt can be accessed.

Instructions

1

Seat the crankshaft journal in the big end of the connecting rod. Install the rod cap. Place a very thin film of assembly lube on the exposed threads of the bolt and the flange on the bottom of the nut. Hand-start the nut on the bolt. Fix a box-end wrench on the nut. Do not turn the wrench yet.

2

Mount the stretch gauge on the rod bolt. You can adjust the gauge for your particular rod bolt by using an Allen key to loosen the fixture opposite the dial indicator. The dial indicator and fixture both fit on a rod. You can slide the fixture up and down on the rod. When the fixture is in position, tighten the Allen key. The fixture also has a special screw with a pin in it that fits into a dimple on the head of the rod bolt. Turn the screw in or out as needed for the final mounting adjustment. The end of the gauge with the dial indicator fits in a dimple on the end of the rod bolt where the nut is.

3

Zero the dial indicator by loosening the hold-down screw on the rim of the dial. This will allow you to rotate the dial so you can line up the "0" on the dial with the indicator needle. When the dial is zeroed, tighten the hold-down screw.

4

Tighten the nut with the box-end wrench. As you tighten the nut, watch the needle on the dial indicator. Continue to tighten the nut until the stretch specification -- in thousandths of an inch -- is reached. You will have to know the specification for your particular engine and size of bolt. For example, the stretch specification for a standard, small-block Chevrolet 11/32-inch ARP bolt is .0063 inch. Repeat for the other rod bolt. Repeat for each connecting rod.