What’s Really Holding Your Car Parts Together?

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It's not just nuts, bolts and welds that keep car parts connected. You'll be surprised how many adhesives are involved!

Recently I needed to replace a bad power window motor on a particular vehicle. A few things about the job surprised me. When I went to remove the outer door skin to reach the motor, I discovered the skin was attached to the door frame with an adhesive, not hardware. I had to use a “hot knife” to release the adhesive, then glue the skin back into place.

Today, vehicle manufacturers are assembling vehicles with more adhesives than ever. And at the same time, conventional hardware has improved and evolved. Read on to gain a better understanding of how your automobile is held together.

Mechanical Fasteners

The humble nut and bolt and other standard fasteners have undergone a technical transformation. You no longer find many traditional fasteners like lock washers, cotter pins or loose flat washers in today’s vehicles. Here’s a look at some of the modern fasteners now used in cars.

Torque-to-yield (TTY) bolts

This specialty hardware replaces standard cylinder head bolts. Referred to as one-time use bolts, they stretch during installation. Stretching compromises the bolt’s integrity, so it should not be reused.

TTY bolts are found on aluminum cylinder heads that expand and contract faster than the cast iron blocks they’re bolted to. TTY bolts maintain their clamping load (strength) and resist shear stress (side-to-side movement between parts). These characteristics permit parts to move up and down without sliding on and ruining the cylinder head gasket.

Prevailing torque fasteners (PTF)

Recognized by a tapered (conical) end on one side of the nut, PTFs have replaced lock washers and cotter pins. The taper distorts the threads, leaving zero clearance between them. The generated friction “locks” the bolt and nut threads together. Like TTY bolts, PTL shouldn’t be reused.

Hollow fasteners

To help lower a vehicle’s weight, manufacturers use hollow fasteners in certain applications, like securing plastic and composite components. These course threaded screws are coated with a dry lubricant that allows assemblers to install them with precision torque drivers. However, over-tightening with a hand or power nut driver can be a problem.

Flange nuts and bolts

Flange nuts and bolts have a built-in flat washer, or “flange.” The flange surface spreads the clamping load over a wider area, eliminating the need for flat washers.

Some flange bottoms have bearing-like surfaces. This reduces friction, ensuring precise torquing and a durable connection that won’t weaken from vibrations.

Adhesives (More Than Just Glue)

According to the Center for Automotive Research, in 2001 cars used about 30 liner feet of adhesives. Today, it’s more like 400 liner feet — an increase of more than 1,200%. Adhesives create incredibly strong bonds that keep parts and components from rattling and vibrating loose.

Why are adhesives used?

Today’s vehicles feature more lighter plastic and composite parts than ever. Attaching them with mechanical fasteners isn’t practical. Adhesives are lighter than metal fasteners and improve a vehicle’s structural integrity by covering the full surface area. Bolts, spot welds or rivets must be spaced every two inches, creating uneven stress points.

Use of structural adhesives during the “body-in-white” assembly process (before it’s painted or joined to the chassis) has replaced traditional welds. According to SAE International, this method increases body rigidity, ride comfort and passenger safety.

Using adhesives decreases the likelihood of corrosion from nut and bolt-holes and spot welds, as well as oxidation when fastening unlike materials, like steel to aluminum. Adhesives also offer tremendous resistance to shear stress. This is critical for components operating in high-temperature environments, like drivetrain gaskets and brake pads.

Adhesives are used extensively in electric vehicle (EV) battery construction. It protects individual cells from vibration, seals out the elements and offers excellent heat conductivity.

Where are adhesives used?

Several high-strength adhesives hold your car together.

  • Hot melt plasticizers and electrically isolating adhesive tapes: Used in high stress areas, like bonding individual chassis components or attaching an EV’s composite battery tray.
  • Silicone: Adhesive/sealant for high-impact airbags, as well as form-in-place drivetrain gaskets. It protects sensitive electronics from vibration, dust and moisture.
  • Polyethylene and other injected molded plastic polymers: These are shaped into clips, retainers and rivets to secure trim, wire harnesses, tubing, hoses and glass. They’re also found in bumper covers and inserts that provide impact protection during a collision.
  • Nylon- and fiberglass-reinforced polymer composites: Fabricated into retaining brackets securing the engine and drivetrain.
  • Flexible non-hardening butyl adhesives: Adheres to and creates weather-tight seals between plastics, like the heater core housing and airflow duct system components.
  • Liquid and structural adhesives: Made from the same ingredients as super glue, these bond metal fenders and quarter-panels to composite frames.
  • High strength anaerobic adhesives: Drying in the absence of air, they bond transmission gears to shafts and bearings into housings.
  • Anti-vibration foams and films: Made from polyethylene, vinyl or acrylic. Foams conform to irregular shapes and fill gaps, perfect for sound dampening and stiffening hollow vehicle frames and chassis to absorb road shocks. They also seal and cushion electronic components and connectors under the hood.
  • Various varnished plastics and pressure adhesives: Used for emission, data, safety and information labels/stickers that must last the vehicle’s lifetime without peeling or fading.

Bob Lacivita
Bob Lacivita is an award-winning ASE and General Motors auto technician, educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics. His work has been featured in The Family Handyman, a Reader's Digest book and Classic Bike Rider magazine. He has been a career and technical educator for 25 years teaching automotive technology, as well as writing state, federal and organizational foundation grants. He also helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into career and technical education.