style="font-size:15px;"> In simple terms, to measure bolt stretch, first measure the total rod bolt length (from the head surface to the tip of the shank) in the bolt’s relaxed state. Then measure the bolt again after the nut has been tightened to value.
The difference in length indicates the amount of stretch the bolt experiences in its installed state. For the majority of production rod bolts, stretch is likely in the .0045- to .006-inch range. If the stretch is less, the bolt is probably experiencing too much friction that is preventing the proper stretch (requiring lubricant on the threads). If stretch is excessive, the bolt may have been pulled beyond its yield point and is no longer serviceable.
Connecting rod bolt stretch gauges are preferred tools for many performance engine builders. This type of gauge allows you to precisely measure how far a rod bolt has stretched while under installed torque. Shown here are three manufacturers’ examples.
A rod bolt stretch gauge features a stationary anvil (seen here at the bottom of the tool frame) and a spring-loaded dial caliper spindle.
Although an outside micrometer may be used to measure the rod bolt length, the most accurate method is to use a specialty fixture that is outfitted with a dial indicator. Excellent examples of this gauge include units from GearHead Tools, ARP, and Goodson Shop Supplies. GearHead’s bolt stretch gauge features a heat-treated aluminum frame (with a very handy thumbhole) with a specially modified dial indicator with sufficient spring tension to hold the gauge firmly to the ends of the rod bolt. The indicator can be rotated for right- or left-hand operation, and the lower anvil is adjustable to accommodate various bolt lengths. Goodson Shop Supplies also offers a rod bolt stretch gauge, P/N RBG-4, featuring spherical points for consistent and repeatable readings, and can also be rotated for right- or left-hand operation. Also, ARP offers its own bolt stretch gauge, P/N 100-9941, designed with .0005-inch increments, with a heavy spring and ball tips.
Prior to installing a rod bolt, the bolt is installed to the tool, with a bit of preload on the dial gauge, followed by zeroing the dial gauge. This provides a reference length for the bolt in its relaxed state. The anvil and spindle engage into dimples in the rod bolt head and shank tip.
There is a debate among some engine builders regarding the validity of measuring rod bolt stretch, due to potential compression of the rod material as the rod cap is clamped to the rod. Although this can occur, the use of a stretch gauge remains the best practical method of accurately determining bolt load.
Connecting rod bolts can be viewed as high-tensile springs. The bolt must be stretched short of its yield point in order for accurate, and most important, repeatable, clamping of the rod cap to the rod. Improper or unequal bolt clamping force can easily result in a nonround rod bore.
Stock, or production, rod bolts typically offer a tensile strength of approximately 150,000 to 160,000 psi. However, due to variances in bolt production, tolerances can be quite extreme, with peak bolt stretch occurring anywhere from, say, .003 to .006 inch. If the installer uses only torque in the attempt to achieve bolt stretch, he runs the risk of unequal rod bolt clamping loads, due to the potential inconsistencies between bolts.
High-performance rod bolts are manufactured to much tighter tensile strength tolerances. ARP, for instance, calculates each and every rod bolt for stretch, and the bolt packages include reference data to that effect. The instructions actually recommend that a specific amount of bolt stretch should be achieved on each bolt (ARP cites 190,000 psi as its nominal or base tensile rating, with actual ratings much higher in some applications).
How can unequal/inadequate rod bolt tightness affect the connecting rod big end bore shape? Let me cite an example: If one technician reconditions the connecting rods using torque value alone to tighten the rod bolts, and another technician who is responsible for final assembly uses the bolt stretch method, the final result can be out-of-round bores. This is because of frictional variances. As a result, the assembler using the stretch method may achieve a higher clamping load on one or more bolts as compared to the loads imposed when the rod reconditioner torqued the nuts without regard to actual bolt stretch. When a bolt is tightened with dry threads, as much as 80 percent of the torque can be exerted because of friction, as opposed to bolt stretch.
In a high-volume production rebuilding facility, technicians may not have the time to measure for bolt stretch. However, a slower-paced operation that is attempting to obtain maximum accuracy (for a race engine, as an example) is far better off using the stretch method instead of relying only on the torque method.
A set of connecting rod bolts’ instructions may list both a torque value and a stretch range, effectively giving you a choice of methods. Yes, tightening only to a specified torque value is quicker, and measuring bolt stretch requires more time, but the best results are achieved by measuring bolt stretch. So, unless you’re in a rush, take the time to measure stretch, tightening each rod bolt to the recommended stretch range. It’s all about the quest for precision.
Connecting rod bolt tightening is an absolutely critical aspect of rod installation, to achieve the proper amount of rod bearing crush (contact force between the upper and lower bearing shells that serves to properly secure the bearings to the rod big end bore) and to obtain the correct level of rod bolt stretch and clamping force. Undertightened rod bolts don’t provide enough clamping force, and overtightened rod bolts can result in stretching the bolts beyond their elastic range. Either scenario can easily result in rod bolt failure, which in turn results in rod bearing failure and the very real potential for severe damage, including broken and/or twisted rods, damaged or broken crankshaft, broken camshaft, and rods busting through the block.
I cannot overstate the importance of connecting rod bolt tightening. Improper bolt installation results in a ticking time bomb, just waiting to destroy your engine and your wallet.
With regard to tightening rod bolts, there are three potential methods to consider: torque-plus-angle, torque alone, or tightening by monitoring bolt stretch. The only application for tightening rod bolts with torque followed by angle tightening applies to OEM rod bolts that are specified for this type of installation. Any serious builder of a performance engine most likely opts for high-performance aftermarket rod bolts. Generally speaking, if the engine is intended to produce about 450 hp or more, I strongly advise using high-performance rod bolts instead of the OEM bolts. In this book, I focus on the use of these superior quality bolts (as offered by such firms as ARP and others).
When using aftermarket performance rod bolts (or whenever you purchase a set of performance aftermarket connecting rods that include these bolts), bolt tightening instructions will be included, and it’s imperative that you follow the instructions.
Based on the rod bolt diameter, bolt grade, length, and application, the maker provides both a torque value and a target bolt stretch value. You can then decide which method to use. Some builders prefer to follow the torque spec; others prefer to measure bolt stretch. I always tighten by measuring stretch, because I feel that this provides a more accurate means of achieving the desired clamping load, as well as obtaining equal clamping loads on all of the rod bolts.
Many late-model OEM engines specify a torque/angle method for a variety of fasteners, including crank pulley bolts, rod bolts, main cap bolts, and cylinder head bolts. An initial torque value
