Bottom Line
Fixin’ up the frame
Frames and frame-mounted components need a periodic once-over

By Jeff Barker
contributing writer



I doubt any of us would like to look out at our mirrors at 70 mph and see our nice, shiny, diamond plate toolbox with mucho dinero worth of straps, chains, binders and tools torn up on the road after it worked its way loose from the frame.

The frame-mounted portion of an APU coming loose wouldn’t exactly be fun, either, now would it?

Regardless of how new or old your truck is, it’s all too easy not to pay close attention to its frame rails and everything attached to them.

In addition to factory-installed components, aftermarket items mounted to the frame need periodic examination to be sure they have not worked their way loose. Catching little problems early will keep them from becoming more serious problems later.

Cross members

With the amount of frame flex an average truck goes through in normal driving conditions, cross member bolts can easily work their way loose. On new trucks, they occasionally don’t get tightened down properly at the factory.

Mud flap brackets

These brackets see a lot of vibration and are usually among the most notorious items mounted to a truck frame to work their way loose. This is pretty evident when one notices how many mud flaps with brackets attached are out there on the shoulder of the highway.

Suspension components and fifth wheel platform brackets

While these particular parts are usually likely to get the most attention when a truck is assembled, they still require regular attention. Neglecting them can lead to very costly and dangerous consequences.

Drive tire quarter fenders

These are pretty important items to have, but all too often they tend to work their way loose. If you see your fenders doing this and can’t seem to keep them tight, try the following:

  • Remove the quarter fender from the “mounting horn,” which, on most quarter fender setups, is a round post mounted to the vertical portion of the truck’s frame.
  • Remove the mounting horn from the truck’s frame. In some cases, the bolt that holds this item to the frame may need to be cut off with an acetylene torch.
  • Clean the portion of the frame on either side of the mounting point, inner and outer, removing any rust that may have formed there. Paint the area and let it dry.
  • Install a new Grade 8 bolt of the proper length and diameter, put a drop of Loctite or a similar adhesive on the threads, and install a new nut. Tighten it down good, with an air impact gun if possible.
  • Measure the outer diameter of the quarter fender mounting sleeve that goes over the mounting horn and get an exhaust clamp, preferably one made of stainless steel. Slide the quarter fender back in place, line it up at the angle you want in relation to the drive tires, and then install the exhaust clamp on the mounting sleeve. Be careful not to over-tighten it. Do this for both sides.

Frame-mounted steps and toolboxes

These items see a lot of abuse during the normal life of a truck. Toolboxes are often heavily loaded, and the brackets and bolts that hold them in place have a brutal job.

Toolboxes mounted on truck frames are usually aftermarket items and the mounting hardware needs to be checked frequently. The brackets themselves can crack over time and if not caught and replaced soon enough, they can break apart at the worst time.

It also wouldn’t hurt to install a set of tamper-proof locking nuts – such as those often used for aftermarket wheels on cars – on the toolbox mounting bolts to keep your toolboxes from growing legs when your truck is parked. If you know the thread size of mounting bolts, you should be able to purchase locking nuts at an automotive tire dealer or an auto parts store.

Battery boxes, fuel tank mounts and tank fairing brackets

With the weight these items support, these brackets can form cracks over time that go unnoticed and then they can break apart when it’s too late. Worse yet, your truck can be placed out of service by a DOT inspector who finds these items to be defective. Besides, loose tank fairings flapping in the wind look pretty tacky.

Auxiliary power unit mounting brackets

The frame-mounted portion of these units weigh 300 to 400 pounds or more. With that in mind, the mounting hardware sees a lot of abuse and needs frequent attention – especially if your unit has aluminum brackets that are often prone to cracks.

On an APU I had on my truck, I replaced a few aluminum brackets that cracked before I decided to fabricate some new ones out of angle iron. After that, I never had any more problems with the brackets cracking.

This is another area where tamper-proof locking nuts would not be a bad idea.

Bumper mounting bolts

These often work their way loose in normal service and should be tightened periodically. It wouldn’t hurt to install new Grade 8 bolts and put a drop of Loctite thread adhesive on the bolt threads when installing them.

Rules of thumb

  • When initially tightened, they stretch. They will not re-torque properly if re-used and are prone to breaking.
  • If you find rust near and/or around a bolt head or nut, it’s usually an indication that it’s loose.
  • Always use the proper bolt for your particular application. Most any frame bolts are Grade 8, and they are usually identified by six marks that look like a wagon wheel hub on the head of each bolt.
  • Double-nutting is a common practice to prevent stuff from working loose. Many circle-track racers have been doing this on their racecars for years, and if it works for them, it will work on your truck as well.

Editor’s note: This article is for information purposes only. If you’re not sure about performing the work yourself, it’s advisable to seek the help of a competent professional.

Jeff Barker is an OOIDA member and a former certified diesel mechanic. He may be reached at