ARP Connecting Rod Bolts Explained
Connecting rod bolts are among the most critical fasteners in any performance engine. They hold the connecting rod and cap together while the piston and rod assembly changes direction at high RPM. This guide explains ARP rod bolt materials, aftermarket rod bolts, OEM replacement rod bolts, Wave-Loc designs, and why correct stretch or torque procedure is essential.
Why Rod Bolts Matter
In a high-performance engine, the connecting rod bolt is responsible for keeping the rod cap securely clamped to the rod body. If that joint loses clamp load, the result can be bearing movement, cap walk, fatigue failure, or complete engine damage.
Rod bolts are exposed to severe cyclic loads. Every revolution of the crankshaft loads and unloads the bolt as the piston accelerates, stops, and changes direction. That is why rod bolt material, fatigue strength, correct preload and installation procedure matter so much.
ARP rod bolts are commonly used by aftermarket connecting rod manufacturers and engine builders because they offer controlled material quality, high tensile strength, consistent manufacturing, and application-specific designs.
Aftermarket Rod Bolts vs OEM Replacement Rod Bolts
ARP rod bolts generally fall into two broad categories: aftermarket connecting rod bolts and OEM replacement rod bolts.
1 Aftermarket Connecting Rod Bolts
Aftermarket rod bolts are commonly used in performance connecting rods from brands that specify ARP fasteners as original equipment. These bolts are usually chosen by dimensions, material, head style and rod manufacturer specifications.
They are available in SAE and metric sizing, with different thread diameters, under-head lengths, grip lengths, head heights, wrench sizes and collar diameters.
2 OEM Replacement Rod Bolts
OEM replacement rod bolts are designed for factory connecting rods used in rebuilt stock engines, modified street engines, circle track, marine, drag racing and other performance applications.
ARP offers multiple levels of replacement rod bolts depending on application, including standard high performance bolts, Wave-Loc bolts and Pro Series Wave-Loc bolts.
Browse OEM replacement rod bolts, then confirm the exact engine, rod design and kit instructions.
Engine builder note: Replacing rod bolts can change the big-end housing bore of the connecting rod. Rods should be checked, and resized if required, after new rod bolts are installed and torqued or stretched to specification.
ARP Connecting Rod Bolt Material Overview
ARP rod bolts are available in several materials. The best material depends on the engine, RPM, reciprocating weight, power level, duty cycle and maintenance expectations.
| Material | Typical Strength | Common Use | Practical Notes |
|---|---|---|---|
| 8740 Chrome Moly | Approx. 180,000-200,000 psi tensile | Standard high-performance rod bolts | A tough and reliable steel when processed correctly. Common in many performance and replacement rod bolt applications. |
| ARP2000® | Approx. 220,000 psi tensile | High-performance and race applications | A common upgrade from 8740 where higher clamp load and increased strength are required. |
| L19® | Approx. 260,000 psi tensile | High-load drag racing and short-track applications | Very high strength, but requires careful handling. L19 should be kept well-oiled and protected from moisture. |
| ARP 3.5® / AMS5844 | Approx. 260,000-280,000 psi tensile | Extreme high-RPM and endurance rod bolt applications | A premium super-alloy used where fatigue life, high load and durability are major priorities. |
| Custom Age 625 Plus® | Approx. 260,000-280,000 psi tensile | Severe racing, high-strength and corrosion-resistant applications | Offers excellent fatigue life, toughness and resistance to atmospheric corrosion and oxidation. |
Do not choose rod bolts by tensile strength alone. The strongest material on paper is not automatically the correct material for every engine. Correct fitment, proper stretch, fatigue performance, corrosion resistance and installation procedure are just as important.
ARP OEM Replacement Rod Bolt Options
ARP offers different replacement rod bolt designs depending on the application and performance requirement.
Good: Standard High Performance Bolts
ARP high performance replacement rod bolts are commonly manufactured from premium 8740 chrome moly steel. They are designed as a stronger, more consistent alternative to many factory rod bolts.
Better: Wave-Loc High Performance Bolts
Wave-Loc rod bolts use a shank design with symmetrical waves that help locate the rod cap accurately without relying on sharp knurling. This helps avoid stress risers and supports better cap alignment.
Browse ARP Wave-Loc rod bolts and compare the application and specification of each kit.
Best: Pro Series Wave-Loc Bolts
Pro Series Wave-Loc bolts are used for more severe applications and are commonly made from ARP2000 material. They are intended for engines where higher clamp load and improved fatigue resistance are required.
What Is ARP Wave-Loc?
ARP Wave-Loc rod bolts use a waved shank design instead of a sharp knurl. Traditional knurling can create stress risers and may cut into the connecting rod. Wave-Loc is designed to locate the rod cap while reducing the risk of sharp stress concentrations.
1 Better Rod Cap Alignment
The Wave-Loc surface contacts the rod and cap to help position the cap accurately. Correct cap location is critical for maintaining the big-end bore and bearing alignment.
2 Reduced Stress Risers
Because the Wave-Loc design avoids aggressive sharp knurling, it helps reduce stress risers that can contribute to fatigue problems in highly loaded fasteners.
3 Consistent Factory Rod Fitment
Wave-Loc bolts are designed to suit factory rod tolerances while providing positive cap location. This makes them a practical upgrade for many OEM rod applications.
4 No Aggressive Digging
Unlike sharp knurled designs, Wave-Loc is intended to provide smooth contact rather than cutting into the rod material.
Understanding Rod Bolt Dimensions
Aftermarket rod bolts are often selected by dimensions. This is especially important when matching ARP bolts to aftermarket connecting rods.
| Measurement | Meaning | Why It Matters |
|---|---|---|
| Thread Size | The nominal thread diameter and pitch, such as 3/8", 7/16", M8, M9 or M10. | Thread size must match the connecting rod and nut or threaded rod design. |
| UHL | Under Head Length. The distance from the underside of the bolt head to the end of the fastener. | Incorrect UHL can affect thread engagement and whether the bolt seats correctly. |
| Thread Length | The length of the threaded portion of the bolt. | Thread length affects nut engagement and available clamping range. |
| Grip Length | The non-threaded shank area that passes through the rod and cap. | Correct grip length helps locate and support the rod cap correctly. |
| Head Height | The thickness of the bolt head. | Head height affects clearance around the rod and crankshaft. |
| Wrench Size | The tool size required to tighten the bolt or nut. | Important for assembly access and correct tool selection. |
| Collar Diameter | The diameter of the locating shoulder or collar near the head. | Must suit the rod’s cap location and bolt bore design. |
For ARP aftermarket connecting rod bolts, the grip length is commonly measured from the underside of the head to the start of the undercut. Do not assume total shank length and grip length are the same thing.
The Importance Of Rod Bolt Stretch And Torque
Correct rod bolt preload is essential. If a rod bolt is installed with enough preload, the operating loads of the engine should not add additional cyclic stretch to the bolt. If preload is too low, every revolution can stretch the bolt beyond its installed stretch, creating fatigue and increasing the risk of failure.
Use stretch where ARP specifies stretch
Measuring rod bolt stretch is usually the most accurate way to confirm preload. This is especially important for serious race engines and high-load connecting rod applications.
Compare rod bolt stretch gauges and choose one suitable for your rod and measurement procedure.
Use the correct lubricant
ARP Ultra-Torque fastener assembly lubricant is designed to reduce installation friction variation. Changing lubricant can change the clamp load produced at the same torque.
Browse fastener assembly lubricants. Use the exact brand and formulation specified in the instructions for your fasteners.
Measure free length before installation
Recording each rod bolt’s free length before installation allows it to be checked during teardown. This is important for determining whether the bolt has permanently stretched.
Replace bolts that have permanently stretched
If a rod bolt measures more than 0.001" longer than its original free length after use, it has likely been stressed beyond its safe range and should be replaced.
Rod bolts are not the place to guess. Follow the ARP instruction sheet for the exact part number. If the rod manufacturer provides a specific stretch or torque recommendation for the ARP bolt supplied with their rods, follow that instruction.
Common Rod Bolt Mistakes
Choosing by thread size only
Thread size is only one part of the specification. UHL, grip length, head style, collar diameter and material also need to match the rod.
Ignoring stretch specifications
For critical rod bolts, stretch is often a better indicator of preload than torque. If ARP gives a stretch specification, use it.
Using the wrong lubricant
Rod bolt torque values are lubricant-sensitive. Dry threads, engine oil and ARP Ultra-Torque can all produce different clamp loads at the same torque.
Not checking rod big-end sizing
New rod bolts can alter the big-end bore. The rods should be checked by an engine builder after the bolts are installed.
Reusing yielded bolts
A rod bolt that has taken a permanent stretch should not be reused. It may no longer provide correct clamp load.
Mixing bolts between rods
Rod bolts should be kept with their matching rod and cap location during inspection, measurement and assembly.
How To Choose The Correct ARP Rod Bolt
The safest way to choose the correct rod bolt is to identify the engine, connecting rod, rod manufacturer and existing bolt dimensions.
Identify the engine and connecting rod
Confirm whether you are using factory rods, aftermarket rods, steel rods, aluminium rods or rods from a specific aftermarket manufacturer.
Check the existing bolt head style
OEM replacement rod bolts often use different head styles. Match the existing bolt carefully, especially when replacing factory bolts.
Measure the bolt dimensions
Confirm thread size, UHL, thread length, grip length, head height, wrench size and collar diameter where required.
Select the correct material level
Choose 8740, ARP2000, L19, ARP 3.5 or Custom Age 625 Plus based on application severity, RPM, load, duty cycle and maintenance expectations.
Follow the correct installation procedure
Install using the correct lubricant, torque value or stretch specification. Always follow the ARP or rod manufacturer instructions for the specific fastener.
ARP Rod Bolt FAQ
Are ARP rod bolts reusable?
ARP rod bolts may be reusable if they have not been permanently stretched, damaged, corroded or otherwise compromised. The correct method is to record each bolt’s original free length and check it during teardown. If the bolt has taken a permanent stretch, it should be replaced.
Should ARP rod bolts be torqued or stretched?
Where ARP provides a stretch specification, measuring stretch is usually the preferred method because it more directly confirms fastener preload. If a stretch specification is not provided, follow the torque procedure supplied with the rod bolts or connecting rod manufacturer.
What is the difference between ARP 8740 and ARP2000 rod bolts?
8740 chrome moly is a strong and reliable high-performance rod bolt material. ARP2000 is a higher-strength alloy commonly used as an upgrade where increased clamp load and fatigue resistance are required.
Are L19 rod bolts better than ARP2000?
L19 is stronger than ARP2000, but it is not automatically better for every application. L19 requires more careful handling and protection from moisture. It is best suited to demanding racing applications where the extra strength is needed.
What does Wave-Loc mean?
Wave-Loc refers to ARP’s waved shank design used on selected rod bolts. It helps locate the rod cap while reducing the stress riser problems associated with sharp knurling.
Can I replace factory rod bolts without resizing the rods?
The rods should be checked after new bolts are installed. Changing rod bolts can affect big-end bore size and roundness, so an engine builder should inspect and resize the rods if required.
Can PFA identify the correct ARP rod bolt for my rods?
Yes. Provide the engine, rod brand, current bolt markings, thread size, under-head length, grip length, head style and any existing ARP part number. PFA can help cross-reference the correct ARP rod bolt option.
Need Help Finding The Correct ARP Rod Bolt?
Performance Fasteners Australia supplies ARP connecting rod bolts for OEM replacement rods, aftermarket rods, street engines, drag engines, circuit engines and high-RPM race applications. If you are unsure which ARP rod bolt suits your rods, contact PFA with your engine and bolt measurements.
Contact PFA