Nuts and Bolts - Even the little items count in collision repair

Collision repairers determined to truly return a vehicle to its pre-loss condition would be wise to take cues from the aviation industry when it comes to choosing and installing fasteners: An airplane's threaded fasteners are so critical to its performance and safety that each one has an assigned torque specification, and in the event of a crash, investigators must determine whether they were properly tightened.

Fasteners are also essential to the safety and performance of an automobile--just to a lesser extent. Using the wrong type, size or strength of fastener can lead to premature failure, unwanted squeaks and rattles and dissatisfied customers. Missing or mismatched fasteners also affect a vehicle's perceived value--especially if a potential buyer notices two types of fasteners or spies a few empty holes where bolts should be.

Automotive engineers select fasteners for very specific reasons, and technicians are responsible for respecting those choices. This includes meeting torque requirements and following installation directions, which increasingly affect the look and performance of a component. For example, I-CAR reports that some composite fenders and quarter panels are mounted using special fasteners and slotted mounting holes, which are designed to accommodate thermal expansion. Shoulder bolts and spring washers allow the panel to expand and slide along the slotted holes. Failing to follow the installation "instructions" can prevent the panels from expanding and sliding as designed--a problem that can cause the doors to bind.

Clamping Down

One of the most important characteristics of a fastener is its grade, or tensile strength--the amount of stretch it is able to withstand. The type of material as well as the diameter of the fastener (such as a bolt) determine this.

A bolt's grade in the English system is indicated by radial lines on the head. The greater a bolt's strength, the greater the number of lines. In the metric system, the bolt is marked with a property class number--4.6, 4.8, 5.8, 8.8, 9.8 or 10.9. Like the English system, the higher the number, the stronger the bolt. All bolts in the same application must be of the same grade, or they will not perform equally.

Nuts are also labeled according to their tensile strength. The English system marks one side of the nut with dots, while the metric system uses numbers. Nuts must be matched with the correct grade of bolt, or the pair will not provide the intended "connection." If a grade 5 nut is used with a grade 8 bolt, the pair will only have a grade 5 level of strength.

Important Variables

In order for fasteners to properly secure parts, they must squeeze the pieces together with enough force to prevent them from moving. This clamping force must exceed the force trying to separate the parts. A number of factors can affect this, including the thread pitch, the grade of the metal, the applied torque, the type of nut or washer required, the type of corrosion protection, the reusability of the fastener and the level of performance in high-stress or high-temperature applications.

Use a pitch gauge to identify the thread pitch of a bolt or stud. In the English system, pitch is determined by counting the number of threads per inch of threaded bolt length. Pitch in the metric system is determined by measuring the distance in millimeters between two adjacent threads.

Talking Torque

Once a fastener is selected, torque specifications come into play. Manufacturers recognize the importance of torquing a bolt, and they are increasingly assigning torque values to body panel fasteners, says Steve Schmidt, associate research administrator for State Farm. All metals can be stretched and compressed to a certain degree. This is how threaded fasteners work--threading and tightening a bolt into a hole stretches the metal just a few thousandths of an inch. Its desire to return to its original shape fights this expansion and provides the holding force. If a bolt is stretched too much (overtorqued), it cannot return to its original shape and will not support the load. Even worse, the bolt will continue to stretch each time it is used, growing weaker and eventually snapping.

Torque-to-Yield Bolts

One-time-use, or torque-to-yield (TTY), fasteners take advantage of the torque yield principle. Most torque specifications build in a 25 percent safety margin--they can be overtorqued by 25 percent before damage occurs. TTY bolts do not have this safety zone. They are designed to be tightened just into the yield point but not to the extent that the metal is distorted. The advantage is they deliver 100 percent of their intended strength, versus the 75 percent strength provided by regular bolts. The drawback is that they can only be used one time. Once they are removed from an application, they can no longer support the load they previously did, and they must be replaced with TTY bolts. Installing a conventional fastener in their place will result in a 25 percent weaker clamping force. Because TTY fasteners are used in a variety of applications, such as subframe mountings, it's important to identify these during the estimating process--not just for the additional cost involved but also to alert the technician.

Tightening Sequence

In addition to torquing specs, manufacturers can also specify a tightening sequence. Just as a criss-cross tightening pattern is recommended to evenly install wheels, some manufacturers suggest installation patterns for body panel fasteners. Composite panels, for example, expand and contract with temperature changes. The fasteners attaching these panels must allow for expansion while securing the panel to the vehicle, according to I-CAR. For example, Saturn models use composite doors, fenders and quarter panels, which are all designed to expand.

Shop Tips

The installation and removal procedures for most fasteners are relatively straightforward. But here are some tips for organizing, selecting and working with fasteners:

* Replace every fastener with one that is identical, unless the repair manual specifies a different part or procedure. The engineers called for a specific fastener for a reason.

* To keep track of parts when you're removing fasteners, Schmidt recommends a system of baggies and markers. Throwing all of the fasteners into a former antifreeze container doesn't work for him, so he switched to his current system--securing the fasteners inside a plastic bag and then labeling the outside.

* If you need a manual for a vehicle but don't have access to it, ask a dealership to see the appropriate service manual. This is time consuming but can prove useful in important situations, and Schmidt says many dealers will oblige the request. Manufacturers' customer assistance numbers can be an additional resource of service information.

* If a replacement part comes with replacement hardware, use it.

* If a fastener is not labeled, compare its height, diameter and thread pitch to new hardware to help to select the correct replacement fastener.

* To prevent galvanic corrosion, ensure that like materials are mated or that fasteners are coated. Installing an aluminum panel with a non-coated steel bolt will lead to corrosion of the panel. If coated fasteners are not available, use a plastic or fiber gasket, a washer or a layer of paint to separate the materials.

Future Techniques

Unlike today's automobiles, fasteners aren't exactly changing at a breakneck pace. But car manufacturers are researching and implementing some new ideas that affect the repair process.

One change that Schmidt notes is the increased use of coated fasteners in the bumper mounting area, especially on European vehicles. While this means that it's only appropriate to install coated fasteners--which cost more--on replacement panels, it should also reduce the number of bolts corroded in their threads. Corrosion isn't a widespread problem today, but the carmakers' extensive corrosion warranties have led to preventative measures such as these.

There is also a trend to increase the number of plastic fasteners attaching fascia. The advantage here is that they will "let go" when they need to and won't corrode. If the carmakers continue to explore the use of alternative materials for body panels, their efforts could drive a need for different types of fasteners. Aluminum fasteners, for example, are preferred for attaching aluminum panels. This helps to avoid corrosion.

In addition to using new types of fasteners, the manufacturers want to reduce the number of fasteners required to construct a vehicle. This speeds up assembly and reduces costs.

Paul Gustafson, president of Emhart Fastening Teknologies, supports the manufacturers' goals. During an April press conference at the American Welding Society exposition, he said too many fasteners add useless weight, hamper quality and slow assembly.

His company hopes to reduce the number of fasteners used during assembly by convincing the OEMs to use drawn arc welding systems that attach studs and brackets to the surface of the sheet metal. If the OEMs agree, future technicians may need to brush up on their spot-welding talents instead of their bolt-torquing techniques.

Common Fastener Applications

* Sheet metal body panels (including the trunk lid and hood)--These are typically attached using some sort of threaded fastener, often Grade 2 screws or rivets, says Rex Schroeder, product manager for R&B Inc., a Colmar, Pa.-based company that sells automotive aftermarket parts. When using screws and bolts, remember that although they may look alike, they are often different in length, pitch and strength.

* Bumper assemblies--Threaded fasteners are also the most common devices used to attach bumper beams. But because the bumper is critical to the occupants' safety, these bolts are typically high strength.

* Headlight, grille and fascia--Depending on the manufacturer, fasteners can be steel- or plastic-threaded types or plastic clips. In many cases, plastic rivets are the primary fasteners for grille and fascia components, Schroeder says.

* Interior components (dash padding, door panels, headliners)--Dashboards are usually bolted in place with steel threaded fasteners or screws, and upper dash pads are held in place with a combination of plastic clips, steel clips and screws. Although clips are primarily used for door trim panels, some carmakers use bolts. When working with plastic clips, exercise caution because many of these are one-time-use clips. Removing them from their holes ruins them. This includes "Christmas tree" fasteners, which are named for their shape.

* Fiberglass components--Steel bolts, screws and plastic clips are used for these applications. The latest rage on vehicles like the Corvette and Prowler seems to be mechanical fasteners for outer panels, Schmidt says, adding that this is for ease of service. Adhesives are used to bond inner structure panels.

* Steering and suspension parts--Because of the crucial role these components play in the safety of a moving automobile, high-strength bolts are responsible for holding everything together. They may be the same strength as or slightly weaker than bumper bolts.

Increase Revenues by
Managing Your Fasteners

Shop managers looking to increase revenues by $50 per repair order should note the answer to the following word problem: A manager looking to improve the shop's bottom line discovers that the facility never charges for supplemental materials, including fasteners, chemicals and supplies such as masking tape. After conducting some research, the manager learns that if the shop properly itemizes and charges for these additional items, it can collect an average of $50 more per vehicle. If the facility implements such a plan, how much will annual revenues rise if the shop repairs 200 cars per month?

The answer is $120,000, and Wurth Group of North America classifies that sum as profit for shops that go from losing money on supplemental items to making money. The fastener, chemical and tool supply company also purports that implementing a fastener and supplemental materials management system increases technician productivity, ensures that fasteners are always in stock and measures technicians' use of supplies.

Computers are making this management task easier. Repairers have traditionally classified fasteners as an operating expense: They paid for them but were rarely reimbursed because insurers resisted or refused to cover these items unless they were itemized, and technicians were reluctant to spend valuable time handwriting parts lists and looking up prices for each repair order, reports Au-ve-co Products, a fastener supplier. As a result, body shops lost hundreds to thousands of dollars per month. But by using a computerized fastener or supplemental materials management system, repairers should be able to easily recoup the cost of the fasteners and materials they once gave away. Implementing a management system of this type also prevents a $2,000 repair job from being delayed while the technician waits for a few little fasteners, says Joe Hohe, executive coordinator for Au-ve-co.

John Ford, manager of Collex Collision Experts in Livonia, Mich., says properly charging for fasteners alone increases monthly revenues by $400 to $500 in his shop. Quentin Olmstead, who works in Collex' parts department, says computerizing fastener ordering, distribution and charging also saves time. "If you need rivets, you have to order them from an auto parts store," he says. "That takes half an hour to an hour to get to you. It's time-consuming, and you have to wait. When you have something like this, [the technicians] can come back here, get the part and do the job. It's a lot more efficient."

Determining a Bolt's Strength in the English System

Coarse Categories

The Unified National Series (UNS) is used to describe different types of threads. It places threaded fasteners into four categories, which are:

* Unified National Coarse (UNC or NC);

* Unified National Fine (UNF or NF);

* Unified National Extra-Fine (CH) (UNEF or NEF);

* Unified National Pipe Thread (UNPT or NPT).

Coarse bolts have the widest thread spacing, and extra-fine pitches describe fasteners with the narrowest thread spacing.