When selecting a fastener to use for any critical function, there is much more to consider than simply diameter and length — especially if it’s to be threaded into the block or cylinder heads.
In the past, we’ve discussed using the right specialty tools for accurately assembling engines, but for input on getting the right fasteners and maximizing them, we turned to the tech team at ARP, which deals with a plethora of OEM and aftermarket engine block and cylinder head combinations on a daily basis. ARP’s drag racing field rep, Rick Feaster, also contributed.
The dynamics of a fastener are fairly straightforward. It must be stretched a specific amount in order for its inherent elasticity to provide the desired clamping force — like stretching a spring. By way of example, a ⅜-inch diameter rod bolt made of 8740 chrome moly must be stretched from 0.0055- to 0.0060-inch to obtain 10,000 lbs. clamping force. Additional clamping force can be obtained through employing a better alloy or increasing the diameter of the bolt.
It’s More Than Just Nuts & Bolts
ARP offers the three most crucial engine fasteners (head and main studs or bolts and connecting rod bolts) in five alloys that range from 190,000 psi tensile strength to 280,000 psi. According to the factory tech reps, ARP’s 8740 chrome moly fasteners are more than sufficient for many normally aspirated engines.
When cylinder pressures increase, so does the need for a stronger alloy. For higher compression unblown engines and many supercharged or turbocharged applications the proprietary ARP2000 material, which is rated at 220,000 psi tensile strength, works great. Above that, Custom Age 625+ is recommended.
Once the proper alloy has been determined, the critical issue is the design of the fastener itself. There are no less than nine factors involved:
• Counterbore depth
• Counterbore/Spot Face Width
• Stud/Bolt Hole Depth
• Stud/Bolt Hole Diameter
• Thread Start Depth
• Gasket Thickness
• Thread Stop Depth
• Overall Thread Length
• Thread Size and Pitch
Optimizing these factors assures that the stud is properly anchored in the block and the clamping force can be precisely applied to seal the combustion chamber or secure the crankshaft. Bear in mind that ARP’s engineering team has, over the years, developed over 100 head stud kits for small block, big block and LS and LT Chevy applications alone — given the huge number of OEM and aftermarket engine block and cylinder head combinations available today. There is no compromising and there are no “universal” kits.
Threads, of course, play a major role in a fastener’s effectiveness. But there’s more to it than simply having the correct diameter and pitch. The Society of Automotive Engineers (SAE) has developed standards for threads, with AS8879D being the most stringent in terms of tolerances.
ARP has a major focus on thread quality, having invested in digital scanning equipment that is accurate to 0.0001mm (that’s 0.000003937-inch). It doesn’t get any better than that.
You can actually feel the improved snugness (engagement) compared to fasteners made to lesser standards.
What’s more, ARP rolls the threads instead of cutting them on screw machines. This is far superior because the material is formed, providing compressive strength, as opposed to cutting, which affects the grain structure. In a sense, rolled threads are “work hardened.” Additionally, ARP rolls the threads after heat treating, when the material is harder. This improves the fatigue strength significantly (up to 20 times better).
Modern Tech Improves The Breed
ARP has a major focus on thread quality, having invested in digital scanning equipment that is accurate to 0.0001mm (that’s 0.000003937-inch). It doesn’t get any better than that.
The connecting rod bolts are the most crucial fasteners in an engine as a failure can lead to catastrophic damage. The recommended method of keeping tabs on a rod bolt’s integrity is to measure its “relaxed” length prior to assembly and make note of it. Preload is applied by stretching the bolt. And if it’s stretched too much the bolt can be yielded.
Increasing the clamping force can be accomplished by increasing the size of the faster or using a stronger material. ARP manufactures rod bolts from five different alloys.
When the engine is disassembled for servicing, the rod bolts should be measured again. If any bolt has stretched by .001-inch or more it should be replaced. There’s a handy chart in the ARP catalog that can be employed to keep track of things.
Properly preloading a fastener is essential. To that extent, ARP has offered free torque wrench testing at NHRA National events for many years. ARP’s drag racing field rep, Rick Feaster, who has conducted hundreds of tests commented, ”You’d be surprised at the number of racers who are using inaccurate torque wrenches. I’ve seen them off by 30-40 percent, which can have grave consequences. I highly recommend having your torque wrench calibrated.”
This article merely scratches the surface when it comes to fastener technology. There are about three dozen pages of valuable tech information in ARP’s catalog, which can be accessed online at www.ARPcatalog.com. A free printed copy is available upon request.
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