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How To: Measure Bolt Stretch For Proper Torque

When most people torque a bolt, they put little thought into how it works. Few realize a threaded fastener is like a spring that stretches to achieve optimum clamping tension.

The proper torque in any fastener is reached by stretching it within a specified range. A fastener’s “rebounding” property is what actually provides the clamping force, and will vary with different materials. Like a spring, under-stretching will not provide enough clamping tension, and over-stretching will exceed its yield strength. If an unloaded fastener is longer than it was when it was manufactured, by even as little as .001”, a partial failure condition has been reached and it must be replaced. It’s damaged and will not rebound enough for proper clamping – it will fail.

There are three common methods for torqueing fasteners. In order of accuracy, they are:

TORQUE ANGLE –  This involves calculating bolt stretch by thread pitch and has many variables that diminish accuracy. This generally works in non-critical preload applications that need to be “snug.” These are your “snug and then tighten another half-turn” instances.

TORQUE WRENCH – This is acceptable when measuring rod stretch isn’t possible. Though more precise than the torque angle method, there are many variables that can alter the friction factor. Lubricants, surface finish, and even inconsistencies within the torque wrenches themselves can alter the amount of actual stretch, despite a proper numeric reading on the wrench. The friction factor is at its highest value during the first tightening and lessens with each “tighten and loosen” cycle, as the surfaces plane against each other and level out. A required 50 ft-lbs of torque may only be a third of the proper stretch with the right combination of variables altering the friction factor.

BOLT STRETCH – Bolt stretch is the most accurate method. Regardless of the other factors, and no matter how much actual torque it takes to stretch the fastener to the correct length, it’s not properly torqued until it’s properly stretched.

In an engine, achieving proper torque on the rod bolts is the most critical element of the rotating assembly. Improper preload, or pre-stretch, will cause a separation of the connecting rod and rod cap with each crankshaft revolution. Centrifugal force will drive the cap outward until the bolts stretch enough to bear the load. They will then “unstretch” as the force recedes in the opposite direction during the upstroke. This repeated stretch and relax cycling will work-fatigue the bolt and cause disastrous failure.

A properly preloaded bolt won’t stretch with the cyclic loads. It needs to be adequately tightened to a load greater than the demand of the engine, and measuring bolt stretch is the most accurate way to make sure it’s reached. A bolt stretch gauge is a “must” even when building an engine, and it’s easy to use.

Follow these steps to make sure you do it right, do it once.

Our rod bolts were ARP2000 series, capable of achieving a clamp load at 220,000 psi. They are made from a hybrid alloy and a popular upgrade for both steel and aluminum rods in short track and drag racing. Dimpled hole in bolt head accommodates stretch gauge ends.

Our rod bolts were ARP2000 series, capable of achieving a clamp load at 220,000 psi. They are made from a hybrid alloy and a popular upgrade for both steel and aluminum rods in short track and drag racing. Dimpled hole in bolt head accommodates stretch gauge ends.

The first step is to make sure that all of the components are cleaned thoroughly.

The first step is to make sure all of the components are cleaned thoroughly.

Also be sure to lube the bearing halves.

Also be sure to lube the bearing halves.

Seat the rod and cap together snugly, and make sure there are no gaps or binding.

Seat the rod and cap together snugly, and make sure there are no gaps or binding.

Lubricate the threads and up under the bolt head with ARP’s moly lube, and be sure to try and use the same amount on each one. This will reduce metal-on-metal friction and offer a lower torque value on the wrench. It’s also worth noting that the moly needs to be worked evenly into the threads with a few rounds of tightening and loosening cycles. ARP recommends five torque cycles for proper disbursement and rod bolt stretch to achieve final torque.

Lubricate the threads and up under the bolt head with ARP’s moly lube, and be sure to try and use the same amount on each one. This will reduce metal-on-metal friction and offer a lower torque value on the wrench.

It’s also worth noting that the moly needs to be worked evenly into the threads with a few rounds of tightening and loosening cycles. ARP recommends five torque cycles for proper disbursement and rod bolt stretch to achieve final torque.

It’s also worth noting that the moly needs to be worked evenly into the threads with a few rounds of tightening and loosening cycles. ARP recommends five torque cycles for proper disbursement and rod bolt stretch to achieve final torque.

Run the bolts into the rod caps until they are snug a time or two. This evens out the moly lube in the threads and on the mating surfaces. Back them out slightly so there is no stretch for an accurate “at rest” measurement.

Run the bolts into the rod caps until they are snug a time or two. This evens out the moly lube in the threads and on the mating surfaces. Back them out slightly so there is no stretch for an accurate “at rest” measurement.

Place the rod stretch gauge on a rod bolt. Make sure the pointed ends are centered in the divots within both ends of the bolt.

Place the rod stretch gauge on a rod bolt. Make sure the pointed ends are centered in the divots within both ends of the bolt.

Rotate the indicator dial face until the needle is zeroed out and tighten the thumb screw to keep it from rotating.

Rotate the indicator dial face until the needle is zeroed out and tighten the thumb screw to keep it from rotating.

Tighten bolts to 25-30 ft-lbs to ensure a small amount of preload, then tighten one side to the manufacturer’s specs in one solid motion. Once torque has been reached, as indicated on the wrench, the stretch may be checked. More torque may be needed to get the bolt to stretch to the proper amount. Apply as necessary.

Tighten bolts to 25-30 ft-lbs to ensure a small amount of preload, then tighten one side to the manufacturer’s specs in one solid motion. Once torque has been reached, as indicated on the wrench, the stretch may be checked. More torque may be needed to get the bolt to stretch to the proper amount. Apply as necessary.

We had to go a little bit over our recommended specs to hit our target of .06”, but we gently tightened until it was reached. Loosen the opposing bolt and retighten carefully until proper stretch is achieved. Most engine builders make notes of their measurements in a simple chart to check for inconsistencies.

We had to go a little bit over our recommended specs to hit our target of .06”, but we gently tightened until it was reached. Loosen the opposing bolt and retighten carefully until proper stretch is achieved. Most engine builders make notes of their measurements in a simple chart to check for inconsistencies.