SL-C Manual: Fasteners

The topic of fasteners could fill a book. Actually, it has already filled several books, in particular the classic tome on the subject, Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook . This book has a wealth of information about when to use what grades (no, you don't always use grade 8 everywhere), how to use them, and their failure modes.

Some of that information is duplicated here, but every builder should have this book (and possibly the others he has written) on his bookshelf.

Bolt Grades

I.D. Marks No markings 3 lines 5 lines 6 lines Allen head
Material Low carbon Medium-carbon, tempered Medium-carbon,quenched & tempered Medium-carbon, quenched & tempered High-carbon, quenched & tempered
Tensile strength (Minimum) 74,000 psi 120,000 psi 133,000 psi 150,000 psi 160,000 psi

Bolt Torque Values

All fasteners need to be torqued correctly in order for them to function as engineered. Use the chart below as a guide, and take into consideration local conditions (e.g., coatings, dry vs lubricated, etc.).

Bolt Thread Treatments

Use nylon lock nuts on all bolts except where they may be exposed to extreme heat (as near exhaust components) to reduce the risk of self-disassembly due to vibration and movement. Nylon lock nuts are not suitable in high temperature applications; use jet nuts, or safety-wired nuts in that case. 

In cases where a nylon lock nut cannot be fitted, another form of thread lock, such as Loctite, shake proof washers etc. should be used. There are several suppliers of locking nuts and washers including Stage 8, and Nord Lock.

For certain critical fasteners, use safety wire. This is covered later under Safety Wiring
To prevent bolts from galling and seizing use an appropriate anti-seize compound on all threads into aluminum and stainless. Stainless fasteners in aluminum without anti-seize will invariably gall, usually with a bad result. Where appropriate, use rivnuts (available at McMaster Carr and other places) to permit a steel thread in the chassis. Aluminum rivets will also work, where they are a fit.

Fraud & Counterfeit Fasteners

Good quality fasteners are expensive, and unsurprisingly, there are counterfeit fasteners in the supply stream. These represent a real safety problem, and are thought to have been responsible for aircraft crashes like Partnair Flight 394

Using counterfeit fasteners in the SL-C is clearly not desirable, so how can we tell which ones are real, and which are the despicable fakes? You can't be sure unless you are buying direct from a known, reputable supplier, but that isn't possible for most of us -- we don't have that kind of volume, so we get our parts from resellers. Even they are fooled sometimes, but there are some things we can do to minimize the risk. There is a federal government document on the issue, including suspect bolt head markings: SCI Awareness Training Manul l062007 published by the Department of Energy.

The SL-C is a very fast car indeed, and the consequences of fastener failure at speed can be severe. Exercise caution when installing any fastener, and analyze it to be sure it is the optimum solution for the task, that it is genuine, and that it is installed in accordance with good practices.


If you are unsure as to how to evaluate fasteners, including proper torque settings and application of torque-checking tools, consult a professional before you proceed.

Fastener Errors

Sometimes a manufacturer will make mistakes, and ship fasteners that are incorrectly heat treated, or even have the wrong mix of nut-and-bolt threads. For example, the Ford GT had an early problem with axle retaining bolts that had an incorrect heat treatment, and there have been reports of other parts from vendors that were shipped with mismatched nuts and bolts. Occasionally assembly errors in other parts of the supply chain introduce problems into parts or assemblies.

It is your responsibility to be sure that your car has the correct fasteners for the task. You should approach every fastener on every critical use with skepticism, and validate that they don’t appear on known fraudulent lists, that they have matched threads on the bolt and nuts, and that they are the correct length for the task. Don’t assume that because it came on the car, that it is right. 

It is also important to verify correct shank length. For example, do not assemble anything with all-thread bolts in places where the threads can cut moving parts, as in a suspension bushing. If necessary, trim bolts with a hacksaw to obtain the correct overall length and shank length.

Also, verify adequate thread penetration in all bolts and nuts. A good rule of thumb is that the minimum thread engagement is equal to the diameter of the bolt. So, for example, a 1/4” bolt needs around 1/4” of thread engagement. More doesn't hurt.

Do not re-use ‘one-time’ fasteners, as are used in some engine assemblies like rod bolts, or other critical areas. These are designed to stretch on proper installation, and cannot be safely reused. When removing these, it is best to destroy their threads by hammering, and immediately dispose of them to reduce the risk of accidental re-use. If a fastener won’t hold torque (and every fastener should be torqued to spec, not just tightened by hand), there is always a reason, and it always means you should remove the fastener for inspection, and likely replacement.

Every fastener should be verified for the correct nut and bolt thread match. An easy way to do this is to acquire a nut-and-bolt checking tool like the Rockler Standard Thread Detective Screw Gauge. These are available in standard and metric and are invaluable for checking fasteners. Just because a nut seems to screw on to a bolt does not mean the threads are matched. Certain metric and SAE threads are very close (3/8”-24 and M10-1.0 are one example), but will not reliably hold a torque setting and will quickly fail. Check with a known tool like the one referenced above.

With very high performance comes and obligation for great rigor in fasteners, and failures can have a greater effect than they might in a slower car. As a builder, you have an obligation to check every fastener as if your life, and the lives of your loved ones, depends on it.

Every fastener, especially suspension and brake-related ones should be validated and verified before you drive the car.


Every critical fastener on the car should be safety wired. This process is well-understood in aircraft maintenance, and in the race car fabrication trade. If you are new to the idea, check Wikipedia, then begin with more detailed texts on the topic.

Safety wiring is easy, and requires only an inexpensive safety wiring tool (safety wiring twisting pliers), the appropriate wire, and a lot of drill bits (unless you can find pre-drilled fasteners for your application).

There are many ways to wire a fastener so it will resist loosening, and the chart below shows some recommended ways to accomplish that.Every fastener, especially suspension and brake-related ones should be validated and verified before you drive the car. 

You can find safety wire, and the appropriate twisting pliers at most aircraft supply stores, and most racing equipment vendors.

The following chart gives a guide for selecting the correct hole size for the selected wire diameter. Pre-drilled holes in fasteners will normally dictate the desired wire size.

Wire Diameter Twists per Inch Recommended Hole Diameter
0.020” 9-12 0.037-0.057”
0.025” 9-12 0.060-0.080”
0.032” 7-10 0.060-0.080”

Washers and their use

Always use washers under the heads of all bolts and nuts, especially when the bolts or nuts contact the aluminum parts of the chassis. The factory may not have assembled the car for shipping purposes with these washers, but it is your responsibility to use washers to spread the load at the end of the fastener, and to prevent gouging, galling, loosening and other problems that can lead to fastener failure.

Grade 8 hardware is available at some farm supply stores by the pound, and in boxes from popular suppliers like McMaster Carr. Get lots of washers and use them liberally, as they are an inexpensive way to improve safety, as well as adding a more professional appearance to your car. In some cases, you may elect to make your own washers from sheet steel of the right thickness, especially when you have a specific shape to accommodate, and where typical round washers wouldn’t fit or be as functional.

Sometimes builders have used a stack of washers to raise the nut to a desired height, as in the case of Castle nuts. This is acceptable in some cases but a better solution is to make spacers of the desired height instead of washers because it is easy to lose one or more from a stack of washers and thus replace them with the wrong height. Use steel tubing of the correct diameter to fabricate these spacers.

Some pictures in this manual may have been taken before the required washers were installed. Your car should always use washers properly.

Marking Fasteners

After a fastener has been torqued to final specs, it is good practice to mark the end with a paint slash over the end of the bolt head or nut, and crossing to the adjacent part. This serves two purposes: one, to show that the final torque has been applied (only apply the mark when final torque has been set), and two, to show when a fastener has begun to loosen due to vibration or other cause.

This is standard race car practice, and is a great idea for the SL-C builder. Use a paint stick or a bottle of fingernail polish for this (a nice bold, red works well).

Look closely and note the use of yellow marker in this picture to indicate the bolt position on the tire rod, and in the nuts near the lower shock mount.