Exclusive Tech: Inside ARP’s Custom Fasteners

Perhaps no engine builder in the world knows more about the historic Ford 427 SOHC motor than the “Old Master” himself, Ed Pink. And when he says custom fasteners are needed for the four “cammer” builds in his shop, you better believe that the local hardware store or even leading mail-order operation doesn’t offer what’s required to fulfill his level of craftsmanship.

Some of the Pro Mods are starting to run out on the edge where these fasteners are going to hold.–Chris Raschke, ARP

“That’s an engine Ford built in 1965,” explains Pink, who sold off Ed Pink Racing Engines a few years ago and now focuses on unique restorations and street-rod projects out of his Ed Pink’s Garage in Southern California. “Consequently there’s not a big selection of quality parts. The 427 FE is close to the cammer, but they’re not quite right. If you don’t get it custom made, you have to buy a generic bolt. These are all now state-of-the-art engines, and we’re going to use state-of-the-art fasteners.”

Whether it’s restoring a rare engine, building a 3,000-horsepower radial-tire bullet or spec’ing parts for a $200,000 Ridler hopeful, custom fasteners are often required.

“A lot of times we will have something off the shelf that will work,” explains Chris Raschke, director of sales and marketing at ARP Inc. “Also, we can look through the system to if something currently in stock could be easily modified. We already offer over 600 masters for head bolts alone, and many more for rod bolts.”

Ed Pink, shown talking to Tom McEwen in the '60s and working on a project from his shop these days, uses custom ARP fasteners to build vintage engines, such as the legendary Ford 427 SOHC used in the restoration of Don Prudhomme's Shelby Super Snake dragster.

Ordering custom fasteners isn’t much different than requesting bespoke pistons or other performance parts. You either have to know exactly what you want or be able to explain the problem that needs to be solved. Sometimes, doing both is the best approach.

“The customer may have the dimensions and knows the strength level needed,” says Raschke. “Or, say in the case of a vintage engine, they send us the part to examine and we’ll work backwards.”

ARP has more than 600 masters for head bolts alone, and even more for rod bolts. If an off-the-shelf part won’t fit or can’t be modified easily, then ARP will custom design a fastener.

Custom fasteners can help shorten pit-stop time and speed up field repairs by reducing the number of wrenches needed.

Fasteners can help reduce pit time

ARP does get a wide variety of requests from the vintage market. Rod bolts are especially in demand, and the applications are as diverse Jay Leno’s Dusenberg, a Rolls Royce aircraft engine used in a British tractor puller or an old Triumph motorcycle.

Personal Customizing

There are situations where you just have to custom make a fastener yourself.

“You have to make it fit,” says well-known engine builder Kenny Duttweiler.

He tells the story of how most head studs made specifically for a certain cylinder head manufacturer were designed for aluminum blocks.

“No one ran that head on an iron block, so I ended up having to cut a chunk off the stud so I could use it,” he says.

One service ARP offers is sometimes vital to an engine builder’s objective.

“You have the ability to order an undercut on the shank of a head stud,” says Duttweiler. “It gives you the ability to stretch the fastener without killing the block. For example, a half-inch stud might take 140 pounds of torque, but the block won’t hold 140. If you undercut the stud and lower the torque, you can still get the stretch and clamp load.”

Most of the custom requests at ARP are from race teams. Again, the needs vary across the spectrum of the race vehicle, not just the engine.

“It could be the team develops their own mounts and brackets, or a special bolt for a balancer,” adds Raschke. “Sometimes the new fastener allows for more convenient work in the pits. Instead of taking four different wrenches under the car, maybe we can get them down to two.”

When race teams design a completely new vehicle, they will try to use as many similar-sized fasteners throughout the vehicle. This strategy is especially important in off-road racing where the teams have to carry their own spares and tools to make repairs on the course.

“Fewer wrenches helps reduce weight,” says Raschke. “If you have to fix something in a hurry or change things, common wrenches will help.”

But the real test for a fastener is in the harsh environment of the engine.

“We had to custom order every fastener for the [Nissan] IRL engine because there was nothing made for it,” remembers Pink. “I was fortunate I had an engineer who knew metals, heat treating and finishes. All I had to give him were the lengths.”

Many race shops don’t have the luxury of a metallurgist on staff or an engineer with a strong background in fastener dynamics. But ARP does.

“Recently I built two billet engines,” says Kenny Duttweiller of Dutweiler Performance. “We needed studs and for one of the engines I just took the blocks down to the R&D department at ARP to measure and make sure we got everything just right.”

ARP has the engineering facilities to design fasteners for any automotive application.

Measure twice, thrice…!

Of course, not everyone’s engine shop is a quick drive away from ARP’s southern California location. But care must be taken when determining measurements for the ARP team.

“Interpretation of measuring can always vary,” says Duttweiler. “What you need to do is talk to ARP before you start measuring.”

Methods and equipment for measuring vary across the industry, so it’s a good idea to consult with ARP before submitting measurements.

In addition to the size and thread counts, material choice is crucial to ordering the proper fastener for a custom application. ARP has access to a dozen or so conventional and super alloys to meet almost any stress-load or strength requirement in a race engine.

“We will run calculations for reciprocating loads, such as a rod bolt. The problem isn’t on the compression but rather on the other side when dragging the piston back up,” explains Raschke. “We’ll use the stroke and weight with some math and come up with how much load the rod bolt sees trying to hold that piston to the crankshaft. Then, we want to double that. If the calculation comes to 18,000 pounds, then ARP wants a fastener capable of 36,000 pounds.”

Perhaps no part is more stressed in the engine than the rod bolt. ARP offers a wide variety of sizes and materials off the shelf or can custom design a bolt for limited space or lightweight applications.

Once ARP determines the material, the company develops a drawing of the part and a price quote for the customer to sign off before any manufacturing begins. Once the part is finished, ARP can help with determining the proper torque setting using a tension machine and run it through several cycles with a specific lubricant.

“You can’t measure head-stud stretch the way you can with a rod bolt,” says Duttweiler. “The end user doesn’t have much of an opportunity to determine those values.”

Some want smaller bolts, others need bigger

Strength is not the only priority for engine builders. Size is also a concern as engine dimensions change.

“Pro Stock builders were moving ports so far that they were breaking into the bolt hole,” says Raschke, also noting that dimensions on the block around the cylinder was also getting tight. “We stepped a 7/16ths bolt down to 3/8ths and they were able to leave the port closed. But you have to step up the material to the same clamp load.”

As more engine components are custom designed to reduce weight and improve strength at the same time, there’s pressure on fastener providers to develop new sizes.

Somewhat similar scenarios can also be found in Formula 1 where engineers fight and scrape for even the smallest reduction in weight, especially in the rotating assembly.

“They designed a package that had only so much real estate. When the fastener wasn’t adequate, we could take them up a half millimeter to a 7.5 bolt instead of a full 8,” says Raschke. “They were concerned about how much material they would be losing in the connecting rod itself.”

The intense cylinder pressures in boosted Pro Mod engines presents even more demands on engineers to provide strength within the confines of the connecting rod. Moving to a metric size may help.

“Some of the Pro Mods are starting to run out on the edge where these fasteners are going to hold,” continues Raschke. “We say the fastener did the best job it could. Even with the best material and highest strength, it wasn’t big enough. But if there’s not enough real estate for a half-inch, then we’ll go to 12mm, or from 9/16ths to 14 mm. You’re starting to play with this little area where it’s minute, but you’re picking up strength.”

Pro Mod engines are truly testing the strength limits of conventional fasteners.

ARP can analyze failures both in house and working with outside laboratories.

Analyzing failure

Another service ARP can offer is failure analysis. Let’s say a custom or off-the-shelf part breaks, ARP’s team can conduct a full investigation and offer suggestions for upgrades.

“If it’s a broken part we’ll analyze the failure,” says Raschke. “If we can’t determine the cause, we’ll send it to a lab for a full analysis. When we get a report we’ll move from there.”

ARP also offers options on coatings and finishes for special applications or dress-up needs. For example, NASCAR wheel studs usually get a dry lube. Most popular engine fasteners are offered either black oxide or polished stainless steel. There are also phosphate coatings for certain applications, and custom dyes are available for titanium fasteners. Other options include various head styles and services such as drilling for safety wiring.

Different finishes and services are available at ARP.

ARP’s catalog and webpage provides complete ordering information and calculation sheets to help with measurements.

“If we see a need or a lot of requests, then we will look at making it a stock part,” adds Raschke.

With all the services and custom options available, it’s just a good idea to stay in touch with ARP. Duttweiler tells a story of a casual conversation with engineers that led to a startling find in his shop:

“What we discovered was that the same torque number on a used nut will get you the same clamp load. Say you’re testing a stud at 100 ft-lb and getting 25,000 pounds of force with a new nut. Get a used nut, lube it up and even with the same number on the torque wrench you may be down 6,000 or 8,000 pounds of clamping force,” says Duttweiler. “We always put a torque plate on, and have nuts in a tray nearby the Sunnen hone machine. How I found out was that I grabbed one to check. It was 12,000 pounds compared to a new one at 19,000 pounds. I promptly threw all those nuts in the trash. Now, I’ll replace them after every 10 or 12 engines.”

One of the forms available for custom ordering at ARP.

Article Sources

About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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