Subscribe to Precision Engine
Sign up for our E-newslettercontinued from Part 1
CYCLIC FATIGUE PROTECTION
Whether measured by stretch or tightened by torque, rod bolts MUST be installed with the proper amount of preload (bolt stretch) in order to provide proper clamping force. If a rod bolt is installed without sufficient preload, every revolution of the crankshaft will cause a separation between the connecting rod and the rod cap. This movement will result in additional bolt stretch that then disappears when the load is removed during rod cycling. Eventually, this cycle of stretching and relaxing can cause the rod bolt to fail due to fatigue (similar to repeatedly bending a paper clip eventually it will fracture and break). In order to prevent this fatigue failure, the rod bolt’s preload must be “just slightly” greater than the load caused by engine operation. Either insufficient or excessive rod bolt preload can lead to bolt fatigue and failure.
A properly installed rod bolt remains stretched to an ideal degree by virtue of the preload and is not affected by loads imposed on the connecting rod.
MONITORING ROD BOLTS DURING ENGINE SERVICE LIFE
You can easily monitor rod bolt condition over the lifespan of the bolts with the use of a stretch gauge. Prior to installing the rod, measure the length of each rod bolt in a relaxed condition and record this length (be sure to keep all rod bolts in order for accurate identification relative to installed position). Each time you tear the engine down, again measure and record each rod bolt’s relaxed length. If any of the rod bolts have taken a permanent set and have stretched by 0.001″ or more, that rod bolt has passed its yield point and is no longer useable. Replace immediately.
Always view a bolt as a spring. The bolt stretch (as achieved when properly tightened to desired preload) acts as a spring during rebound, providing the needed clamping force. If a bolt is over-tightened and it stretches too far, it will exceed the bolt’s designed elastic limit and will remain permanently stretched, losing its ability to provide clamping force. Measuring a rod bolt’s stretch allows you to keep tabs on the bolt’s elastic state or, more accurately, its ability to provide the needed elasticity.
A NOTE REGARDING TORQUE/ANGLE METHOD
The torque/angle (or torque plus angle) method is primarily used on OEM applications that involve the use of OE connecting rods and OE rod bolts. Original equipment manufacturers have determined through extensive research that optimum clamping load and bolt preload can be achieved by initially tightening a bolt to a specified torque value and final-tightening by continuing to rotate the bolt head by a specified number of degrees. Since final tightening is based on bolt head rotation, this method reduces the variable of thread friction.
We need to consider that any OE torque/angle specification is based on using the OE rod (for that rod’s material makeup) and the OE rod bolt (based on that bolt’s material and design). When using aftermarket connecting rods and/or aftermarket rod bolts, these torque/angle specifications may not be applicable. Always follow the rod bolt maker’s procedure for rod bolt tightening, which will involve either stretch measurement or torque value.
ARP ROD BOLT STRETCH AND TORQUE VALUES
(Note: The following list refers to specific ARP rod bolt part numbers. As space does not permit us to list all engine applications here, we’ve limited this sample list to Chevy, Chrysler and Ford applications. Check with ARP for stretch data for other engine brands)
ARP P/N……….STRETCH (IN.)……….TORQUE (LBS./FT.)
CHEVY
131-6001……………0.0062……………40
132-6001……………0.0062……………40
132-6002……………0.0057……………25
133-6001……………0.0064……………50
133-6002……………0.0068……………40
134-6001……………0.0062……………40
134-6002……………0.0061……………50
134-6003……………0.0063……………50
134-6005……………0.0063……………50
134-6006……………0.0055……………45
134-6401……………0.0062……………40
134-6402……………0.0066……………50
134-6403……………0.0063……………50
135-6001……………0.0080……………75
135-6002……………0.0063……………50
135-6401……………0.0080……………75
135-6402……………0.0064……………50
234-6301……………0.0064……………40
234-6401……………0.0070……………40
234-6402……………0.0055……………45
234-6403……………0.0065……………50
235-6401……………0.0075……………60
235-6402……………0.0070……………45
235-6403……………0.0075……………60
CHRYSLER
141-6001……………0.0063……………50
141-6401……………0.0064……………50
142-6001……………0.0069……………50
142-6002……………0.0063……………50
144-6001……………0.0063……………50
144-6401……………0.0063……………50
145-6001……………0.0072……………75
145-6002……………0.0063……………50
145-6402……………0.0064……………50
244-6401……………0.0072……………55
245-6402……………0.0075……………50
FORD
150-6004……………0.0063……………50
150-6005……………0.006……………50
150-6404……………0.0064……………50
151-6001……………0.0065……………40
151-6002……………0.0065……………40
151-6003……………0.0050……………26
151-6004……………0.0055……………22
151-6005……………0.0049……………36
152-6001……………0.0071……………50
152-6002……………0.0063……………50
153-6001……………0.0069……………30
153-6002……………0.0063……………32
154-6001……………0.0063……………50
154-6002……………0.0069……………30
154-6003……………0.0063……………50
154-6004……………0.0055……………50
154-6005……………0.0063……………50
154-6402……………0.0069……………28
154-6403……………0.0064……………50
155-6001……………0.0063……………50
155-6002……………0.0063……………50
155-6003……………0.0063……………50
200-6001……………0.0045……………60
250-6404……………0.0063……………50
251-6201……………0.0047……………30
251-6301……………0.0061……………44
251-6402……………0.0065……………38
254-6402……………0.0070……………25
254-6403……………0.0065……………45
255-6402……………0.0062……………40
Tags: ARP, CYCLIC FATIGUE, FASTENERS, ROD BOLT, ROD BOLT STRETCH, STRETCH, STRETCH GAUGE, TORQUE, TORQUE VALUES, TORQUE/ANGLE
