Ever been strolling through a hardware store or perhaps an auto parts establishment and wondered of the meaning behind the markings on the hundreds of thousands of bolt heads on the shelf? Whether you’re building a modified Corvette from scratch or performing a concours restoration, it is important to understand decisions crucial to your safety out there on the road. One of the greatest safety and aesthetics decisions is proper fastener selection. You want the right bolt or screw for the job and you want to be able to trust the source. Ordering fasteners online becomes even dicier if you don’t know what you’re ordering and you’re not familiar with the source.
Automotive Racing Products (ARP) is here to help you understand the very purpose and function of fasteners. In order to make the right decision when you’re ordering fasteners you have to understand metallurgy and how fasteners function. Fasteners provide a threaded clamping function the draws and holds components together. With that clamping power comes fastener tension and stretch. There’s always a certain amount of “give” in parts and fasteners when the torque is applied. Where bolts and screws become more complex is when we get into the materials they are made of, thread design and how strength is confirmed.
ARP explains fastener metallurgy begins with grain size in the material. Steel “freezes” from its liquid state during melting from many origins (called allotropic) and each one of these origins grows until it contacts another during the freezing (cooldown) process. Each of these segments is a grain. In iron and aluminum castings, these grains are fairly large. When grains become tighter (smaller) we get steel. Grains can be made tighter; therefore, even more of these grains can occupy the same space. This happens by first cold working the material and then recrystallizing at very high temperatures.
Alloys are special mixes of elements that become a stronger form of iron known as steel. And alloys, like chrome-moly steel, do not have to be cold-worked. The reheating process refines the grain size in steel. However, austenitic steels and aluminum have to be cold-worked before heat treating. Grain size in the metal is very important to mechanical properties, ARP tells Vette. High temperature “creep” is enhanced by large grains in the metal however; toughness and metal fatigue make it necessary to go with a finer (smaller) grain size. The finer the grain boundaries the better it is in terms of brute strength. High temperature “creep” happens at very high temperatures. This creep could be as much as .001 per inch/per hour, which is significant to strength.
What makes ARP bolts and studs optimum for extreme duty is very fine grain metallurgy usually ASTM 8 or finer, with 10 being the finest. ARP gives us insight into bolt strength with even more detailed information. With steel, as the strength goes up, we lose something known as toughness. When steel becomes stronger, it tends to become brittle and becomes more prone to breakage. Bolt, stud, and nut threads contribute to brittleness, ARP tells us.
Determining Bolt Strength
Over time, industry has learned how to create many different forms of steel and, as a result, has learned how to produce steel in a variety of strengths. The Rockwell Standard was originally developed to confirm bolt strength. A Rockwell Hardness Tester measures the depth of penetration that occurs in the metal when a pinpoint shock load is applied. For hard metals like steel, a diamond penetration tool is used. For softer materials, small balls, 1/16-inch or 1/8-inch diameter, are used. ARP uses the “C scale” for 120,000 psi strength level and higher. The Rockwell C scale uses the greatest load: 150 kg. The “A scale” uses only a 60 kg. load but can be correlated with C scale.
What makes ARP’s standards extraordinary are tough testing standards that measure something known as micro hardness. Some parts such as fasteners are too small to be Rockwell hardness tested. These parts are placed in hard plastic to begin the testing process. A microscope is used to place a small indenter into the steel. A microscope is used for close inspection and the length of the impression is measured.
Elasticity of all alloy steels is the same 30,000,000 psi according to ARP. ARP adds this is true whether or not steel is heat-treated and regardless of whether it is 100,000 psi strength or 300,000 psi. Steel and aluminum are like a spring, ARP tells Vette. Place these materials under a load and they will flex. Double the load and they will stretch at least twice as much. This is a very important issue when it comes to high stress hardware such as connecting rod bolts. By measuring bolt stretch we’re actually measuring load. ARP confirms load is what is important to understanding stretch. Measuring bolt stretch indicates how much load is on the bolt.
ARP manufactures its fasteners from a large assortment of materials ranging from popular stainless steel and 8740 chrome-moly to exotic alloys that have been developed to support space travel. This means these guys didn’t jump into the bolt business just yesterday. ARP is an aerospace fastener manufacturer with a lot of experience that also serves the racer and enthusiast.
As but one example, 8740 chrome-moly is available in four grades:
1. SDF (guaranteed seamless and defect free).
2. CHQ (cold head quality).
ARP uses only the first two (SDF and CHQ), even though they cost more than double “Aerospace” quality.
Stainless steel is suited to many automotive and marine applications because stainless is tolerant of heat and resistant to rust and corrosion. ARP “Stainless 300” is a special alloy produced for added durability. It is polished using a proprietary process to produce a perfect finish. Tensile strength is typically rated at 170,000 psi.
Let’s talk about 8740 chrome-moly steel. Until the development of today’s modern steel alloys, chrome-moly was considered the high-strength bolt material. Today, 8740 chrome-moly is viewed as only moderate strength with adequate fatigue properties for most racing applications, however, only if the threads are rolled after heat-treatment, as is the standard ARP production practice. Typically, chrome-moly is classified as a quench and temper steel that can be heat-treated to deliver tensile strengths between 180,000 and 210,000 psi.
ARP2000 is an steel alloy that can be safely heat treated to a higher level producing a greater strength steel than 8740 chrome-moly. While 8740 and ARP2000 employ similar characteristics in terms of strength, ARP2000 is capable of getting a clamp load at 220,000 psi. ARP2000 is used widely in short track and drag racing as an upgrade from 8740 chrome-moly in both steel and aluminum rods.
L19 from ARP is a premium steel processed to deliver superior strength and fatigue properties. Think of L19 as very high strength steel compared to 8740 and ARP2000 capable of delivering a whopping clamp load at 260,000 psi and is primarily used in short track and drag racing applications where inertia loads exceed the clamping capability of ARP2000. Like most high-strength, quench and temper steels, L19 requires special care during manufacturing to avoid hydrogen embrittlement and the risk of failure. L19 steel is easily contaminated and subject to stress corrosion. It must be kept well-lubricated and free from moisture.
ARP’s Aermet delivers a typical tensile strength of 290,000-310,000 psi and is a new martensitic super alloy that is stronger and less expensive than super alloy austenitic materials. Because Aermet is capable of achieving incredibly high clamping loads, it is ideal for short but extreme environments like Top Fuel, Funny Car and some short track applications. Like L19, Aermet must be kept well-oiled and not exposed to moisture.
Inconel 718 from ARP is a nickel-based material in the high temperature, super alloy class and is found to be equally suitable for low-temperature applications. It delivers tensile strengths in the 210,000-230,000 psi range and exhibits improved fatigue properties. Inconel 718 is completely immune to hydrogen embrittlement and corrosion.
ARP3.5 (AMS5844) is next on the list of ARP super fasteners. While ARP3.5 is similar to Inconel 718, this super alloys is found in many jet engine and aerospace applications where heat and stress attack the life of critical components. The high cobalt content of ARP3.5, though expensive, delivers a material with superior fatigue characteristics and tensile strength in the 260,000-280,000 psi range. ARP3.5’s immunity to hydrogen embrittlement and corrosion is significant. These materials, both APR3.5 and Inconel 718, are used primarily in connecting rods where extremely high loads, high rpm and endurance are important factors: Formula 1, NASCAR and IRL applications. We’re talking very high-end fasteners where failure is not an option.
Custom Age 625 Plus is a newly formulated super alloy demonstrating superior fatigue cycle life, tensile strength and toughness with total resistance to atmospheric corrosion and oxidation. ARP happens to be the first manufacturer to develop manufacturing and testing processes for fasteners with Custom Age 625 Plus. Custom Age 625 is less expensive and expected to ultimately replace MP-35 as the material of choice in high strength super alloys. Typical tensile strength is 260,000-280,000 psi.
ARP now offers special order titanium fasteners in an alloy (Ti6Al-4V) specially heat-treated and provides superior strength to other titanium alloys employed in racing and aerospace. The material has a nominal tensile strength of 180,000 psi and is very corrosion resistant. The main benefit of titanium is its light weight, which is about 40 percent lighter than a comparable fastener made of steel. Head studs and accessory bolts are ideal applications for this lightweight material.
Now About Bolts
The most common bolt is a “hex” head with six contact surfaces that penetrate a nut or threaded receiver. Fastener grade (US or Metric) is a bolt’s makeup physically and mechanically. There are five basic bolt hardnesses, also known as ASTM ratings: Grade 1 (Very Soft), Grade 2 (Soft), Grade 5 (Hard), Grade 7 (Harder) and Grade 8 (Very Hard). Across these grades there are variations in the alloy makeup. Metric size class is also a part of bolt identification.
A US Grade 1 or 2 fastener—your basic hardware store peanut butter bolt—should never be used on a motor vehicle in the interest of safety. US Grade 1 and 2 fasteners are low to medium carbon steel and are generally low quality. They break easily. Though the standard is 74,000 psi minimum tensile strength these are the bolt grades to avoid. They range in size from 1/4- to 1 1/2-inch in size.
Light-duty fasteners such as body bolts and interior fasteners are generally US Grade 5, sometimes less. US Grade 5 bolts are quenched and tempered medium carbon steel. They range in size from 1/4- to 1 1/2-inch. Minimum tensile strength for these is generally 120,000 psi, meaning minimum tension load is 120,000 psi.
Engine, driveline and chassis components should be fitted with US Grade 8 fasteners without exception. US Grade 8 fasteners are high strength quenched and tempered medium carbon alloy steel. They’re about as strong as it gets. They range in size from 1/4- to 1 1/2-inch in size. They can generally take 150,000 psi minimum tensile strength. This means they should be able to withstand a tension load of 150,000 psi minimum. We’ve also seen Grade 7 bolt heads with five hash marks, which fall somewhere in between Grades 5 and 8 in terms of strength.
There are four Metric classes: 4 (Grade 2), 8.8 (Grade 5), 10.9 (Grade 8) and 12.9 (higher than Grade 8). Metric Class 8.8 (M16-M30) has a proof load of 87,000 psi, yield strength of 95,700 psi, and minimum tensile strength of 120,350 psi. Class 9.8 (M6-M16) has a proof load of 94,250 psi and minimum tensile strength of 130,500 psi. Class 10.9 (M6-M30) has a proof load of 120,350 psi, yield strength of 130,300 psi, and a minimum tensile strength of 150,800 psi. Class 12.9 exceeds Grade 8 standards.
Bolt strength is measured three ways: tensile, yield and proof loads. Tensile strength is maximum tension load (tug of war) where we try to yank the bolt apart. Yield strength is the maximum load a bolt will take before it remains that length. Proof load is about bolt twist or the amount or torque a bolt will take before it stays that way.
Bolt coatings range from zinc to hot dip galvanizing to chrome plating. Zinc electroplating, which is silver in color, prevents corrosion. Hot dip galvanizing is also a corrosion preventative and tends to be denser than zinc plating alone. Chrome plating is also a good corrosion preventative.
Stainless bolts vary a lot in materials and construction. Stainless is a low carbon steel fastener that offers excellent corrosion resistance. Some believe stainless steel bolts are stronger than Grades 5 and 8 but this isn’t true. Because stainless is low carbon steel it cannot always be heat treated for strength. Stainless steel bolts are generally harder than Grade 2 but not as hard as Grade 5.
The downside to stainless fasteners is the potential for thread galling if you’re not careful. Although stainless is generally thought of as non-magnetic, not all of it is. Stainless steel fasteners are generally 18 percent chromium and 8 percent nickel, however, that can vary too. Stainless 316 exhibits excellent anti-corrosion properties. Stainless 410 is a harder steel, but not as corrosion resistant as 316. Which type you choose depends on the application and what you need most. It is important to remember harder isn’t always better. Harder can translate to more brittle depending upon the alloy. Be very mindful of the alloy when you’re shopping bolts and other fasteners.
There are many types of bolts out there; in fact, more than we could ever cover here. Expect to see regular hex-head, flanged or washer hex-head, carriage (button head with square shank), and on occasion a slotted hex-head for wrench or screwdriver. There’s also a socket cap style, which requires an Allen wrench.
Read This Thread
Next consideration with bolts is thread pitch and count. When you think bolt threads think “TPI,” or threads per inch. TPI is SAE (US) only. Metric bolts use thread pitch. If your Corvette is late-model, meaning from the 1980s forward, you’re bound to see a mix of SAE and Metric fasteners. With Metric fasteners, thread pitch is the distance between threads in millimeters. If we have a thread pitch of, for example, 1.5, that’s the distance between one thread and the next, meaning 1.5 mm. It is suggested you invest in a thread gauge, which is inexpensive and generally available from any hardware supply house.
Thread galling occurs when the threads bind when we’re screwing something together. The best advice we can offer you is to never force a bolt and nut that aren’t rolling together smoothly. When you force a binding bolt and nut, this galls the threads, which can cause the bolt and nut to cease. Generally, the softer the alloy that greater the risk of galling. Once a bolt and nut cease you will probably have to cut the bolt or split the nut to get them apart. In any case you will have to replace both the nut and bolt or chase the bolt threads.
Be kind and gentle with fasteners during installation. If there’s binding of any kind you have a problem requiring prompt attention. Examine the threads closely and discard flawed fasteners. Do not force it. It is strongly suggested you use a thread lubricant during fastener installation. Keep ARP’s Ultra-Torque thread lubricant close by to make fast work of an installation. If the fastener goes into a water jacket, opt for ARP’s Thread Sealer. For a more secure union, use a thread locker if you’re working with a high stress installation. And remember, some thread lockers will permanently secure the fasteners, meaning you will have a rough time loosening the fasteners during future service. Choose the right thread locker for the job.
Good housekeeping is everything to fastener installation. If you take a rusty or dirty bolt and expect easy installation forget it. Threads should be clean when you’re screwing things together. At the very least clean up the threads with a wire wheel, remembering to use eye and face protection. Give hardware a run-through with a lubricant before getting down to the final installation. And remember, dirty or rusty threads virtually guarantee damaged threads. Another name for galling could be cold welding or work hardening. And when you run into galling stop tightening and slowly back out. Do not force the fastener in either direction.
Bolt, washer and nut sizing should all be proportional. A 1/2-inch bolt shank should fit comfortably through a 1/2-inch inside diameter washer and screw right into the corresponding nut. The outside diameter of the washer will vary depending upon application. Fender washers, as one example, will have a larger outside diameter than a standard flat washer. There’s also washer thickness to consider. Never use a larger inside diameter washer than your bolt size.
While you’re shopping for bolts, be thinking about nuts. A Grade 1 or 2 peanut butter bolt is engineered to work with a corresponding low or no-grade nut. A Grade 5 bolt works with a Grade F nut while a Grade 8 or higher bolt is designed for a Grade G nut. Bolt properties include alloy steel, which is heat treated and typically sports a dull black finish. With steel alloy you get strength, however, you also get brittle. Brittle can break. If corrosion resistance is what you need, look to a silicon bronze alloy fastener, which consists of copper and tin and delivers extreme corrosion resistance. It just doesn’t rust.
Fastener compatibility is a huge factor in the success or failure of your project. We want metals that are happy together. Galvanic corrosion, also known as dissimilar metal corrosion comes from metal combinations that don’t work well together. Corrosion is nothing more than the flow of electric current from one metal to another. What that means for you is one metal corroding more quickly than the other. A good example is battery terminals where electrolysis and the resulting corrosion can occur rapidly. The metal that corrodes more rapidly is known as the anode while the metal that corrodes more slowly is the cathode.
Just Plain Nuts
Nuts are produced in virtually the same types of alloys as corresponding bolts. It is never a good idea to mix it up across material types. Grade 5 bolts must be used with Grade F nuts. Grade 8 bolts must be used with Grade G nuts.
You can expect to see standard hex nuts, jam nuts, flange nuts, star washer or K-lock-Kep, a locknut known as a prevailing torque lock nut, slotted or castle, and nylon or Nylock nuts. Some of these you’re not likely to find in an OEM application.
There’re but a handful of washer types you can expect to see on the average Corvette. Flat, fender or dock, finishing or countersunk, lock or split lock, external or internal star lock, and sealing washers. Sealing flat washers are rubber or urethane lined and designed to keep out moisture and isolate vibration.
When you’re tightening bolts, it’s vital to lubricate the threads and take it slow doing so. Bolts should always be seated, then slowly torqued in one-third values. Once you have accomplished all three torque values in proper order, torque must be checked again. Never jerk a torque wrench to hit breakaway torque (click!). Slowly pull into the value desired and be patient, wait for the click. When you’re finished always zero the torque wrench if you’re using a breakaway.
Late-model Corvettes employ torque-to-yield fasteners primarily in engines for greater consistency. Torque-to-yield is a calculated bolt stretch/tension process where the bolt is tightened to a pre-determined torque value, then, tightened further a specific number of degrees. When torque-to-yield fasteners are removed they may not be used again. You throw them away and buy new ones. If you opt for ARP fasteners they are not torque-to-yield and are tightened to a specific torque value.
A Word About Torquing Bolts
There’re a lot of misconceptions about the proper tightening of fasteners. One, bolt and nut torque is based on fastener size and properties. Check with ARP on proper torque values whenever you have doubts. Check your Chevrolet service manual as well.
When it’s time to torque fasteners, lubricate the threads and manually run fasteners through to check for binding. If there is any binding stop right there and examine threads for galling or contaminants. If you force a fastener you risk permanent damage. What you don’t want is a bolt broken off in a cylinder head or block. When there’s thread damage either chase the threads or replace the fastener.
When you torque fasteners, do it in one-third values. Once you have achieved total torque value, go back and double check. Make sure you have torqued every fastener and in proper order as recommended by Chevrolet. Never jerk a torque wrench seeking breakaway torque. Slowly and smoothly ease into the proper torque. Never over torque.
1. When you examine a bucket of bolts searching for the right fastener, keep bolt grade in mind before making a bad decision. Never use anything but the same grade fastener required by Chevrolet. It is suggested you opt for Grade 8/Grade G fasteners in all chassis, engine and driveline applications.
2. Here are two shank-style bolts. On top is a standard shank-style hex-head bolt with coarse threads. At the bottom is a flange or washer hex-head with a pilot coarse thread tip designed for quick and easy installation. These are both Grade 8 bolts.
3. From peanut butter to hard steel alloy from left to right. On the left is an SAE J429 Grade 1 or 2 bolt with no head markings. Center is a Grade 5 bolt with three hash marks. The triangle indicates a particular alloy or close tolerance. Right is a Grade 7 with five hash marks.
4. A trio of Grade 5 bolts of various lengths, all fine thread. Grade 5 bolts get three hash marks. The “JH” indicates Jinn Her Enterprise Ltd of Taiwan. You will find a lot of replacement hardware like this is offshore.
5. Grade 5 (left) and Grade 8 (right) bolts side by side. These are both bolts you would find at any hardware store. When you shop be aware of thread spacing. These are both fine-thread bolts with shanks.
6. This is a Grade 8 wheel stud. Note the six hash marks. Wheel studs are available in a variety of lengths and sizes. They are a press-fit design.
7. These are both Grade 8 bolts though the bolt on the right doesn’t sport six hash marks. Instead, it has the manufacturer’s logo. There’s also Grade 9 bolt strength with nine hash marks we don’t have here, which is greater in strength than Grade 8.
8. These are Grade 8 cad-plated bolts, coarse thread, with Grade G nuts available at any hardware store.
9. OEM Grade 8 fine-thread engine bolts with unique heads. Hash marks are forged into the bolt head.
10. Metric class bolts look like this. This is a close tolerance Class 10.9 metric bolt in coarse thread.
11. ARP connecting rod bolts are a must for stock and aftermarket connecting rods. Whether you’re reconditioning stock Chevrolet rods or are working with brute aftermarket rods spend the money and step up to ARP.
12. These are high-carbon steel Grade 5 or 8 OEM flange-head bolts for body, chassis and engine applications.
13. Cad-plated Grade 5 engine fasteners, which are corrosion resistant. Note some of these are equipped with permanently installed split lock washers and tapered tips.
14. These are Grade 5 or 8 high-carbon steel flange-head OEM bolts in both fine and coarse thread with tapered tips for fast, quick installation.
15. When you examine these ARP fasteners it is clear why ARP is the leading edge of fastener technology. Beyond Grade 8 toughness with a pristine finish.
16. ARP offers you the kit approach to fasteners, with complete kits for popular engine applications including Chevrolet. If you have an off-the-beaten-path engine like an old Chevy stove-bolt six, you will have to piece-meal fastener selection by providing ARP with fastener dimensions when you order.
17. OEM stud versus ARP. The ARP stud offers unequalled strength and is available in a wide variety of dimensions. Head and block studs offer greater clamping power and are virtually impossible to break. These are front accessory studs.
18. We like the way ARP packages its engine bolt kits. ARP isn’t just for engines and driveline, but chassis and body as well. All you have to do is provide dimensions.
19. Although this article is about fasteners, ARP also offers oil pump shafts for popular applications.
20. When torqueing fasteners, it’s always best to torque in one-third values to achieve gradual clamping and bolt stretch. Another good idea is to mark fasteners as they are torqued to full value. You would be surprised how many fasteners get missed during engine assembly with catastrophic consequences.
21. When you install and torque fasteners always lubricate threads and heads. You can use SAE 30 weight oil or ARP’s own Fastener Assembly Lubricant. Never use fastener assembly lube on engine parts such as bearings, journals and cylinder walls.
22. Thread lockers must be used with caution, meaning knowing the right one to use and when. Selection depends upon application, desired permanence, and heat range.
23. Flywheel bolts should get a thread locker, then, torqued in one-third values in crisscross fashion. We’re using ARP bolts on both the flywheel and the clutch.
24. Flat washers are available in more varieties than we could ever list. A washer is something of a bearing; a surface beneath the bolt head or nut designed to distribute load across a broader surface area. Inside diameter must fit the bolt shank.
25. These are split-style lock washers, which apply a certain amount of tension against a bolt head or nut to keep them from turning.
26. Star washers are produced in both internal and external locking. The star tabs keep bolts and nuts from turning. Think of these lock washers as light-duty fasteners.
27. Lock washers of any kind must fit the bolt shank. On the right is a split lock washer too large for the bolt. Though it is but one size larger it will not be effective.
28. Two conventional non-locking Grade F (Grade 5 bolt) nuts in fine (left) and coarse (right) threads.
29. Flange-style Grade F and G nuts. On the left is OEM style. On the right is aftermarket replacement.
30. Here’s a cross-section of locknuts from left to right — K-Lock, fiber nut or Nylock, flange nut, and a large-base flange nut.
31. Body bolts are produced in a wide variety of designs depending upon application. These are flange bolts suitable for fenders and other body panels. These are typically Grade 5 and of high-carbon steel. Cad-plated bolts (far right) are more corrosion resistant and aesthetically pleasing to the eye. Corvette body bolt kits are available from Corvette Central for the concours restorer.
Photography by Jim Smart