Installing a tonneau cover on your truck bed can provide many benefits, like increased cargo security, improved gas mileage, and a sleek, stylish look. But in order to enjoy those benefits, your tonneau cover needs to fit properly on the bed rails of your truck. If there are gaps or misalignment, you may need to use tonneau cover shims during installation to get a tight and flush fit. Here’s what to look for to know if shims are needed for your tonneau cover installation.
What are Tonneau Cover Shims?
Tonneau cover shims are small wedge-shaped pieces of material that are inserted between the tonneau cover framework and the truck bed rail to take up space and create a snug, vibration-free fit. They act similarly to shims used in carpentry.
Shims for tonneau covers are commonly made of plastic, metal, or rubber. They come in different thicknesses to allow for small adjustments in the spacing. Using shims helps customize the installation for your particular truck bed and tonneau cover combination.
When Would I Need Tonneau Cover Shims?
There are a few telltale signs during tonneau cover installation that indicate you may need shims:
The tonneau cover doesn’t sit flush against the bed rails
Ideally, the tonneau cover should sit flush and tight with no gaps against your truck’s bed rails. If it sits too high or has spacing anywhere, shims can help take up that excess room.
The bed rail caps are uneven
Many tonneau covers mount to the bed rail caps. If your truck’s bed rail caps are uneven, you can use shims under the low areas to even it out so the tonneau sits flat.
There’s rattling or shaking of the tonneau cover
Any looseness or rattling as you drive likely means you need shims for a more secure fit. Vibrations from driving can worsen without a tight fit and lead to noise or damage.
Water or debris gets into the truck bed
One function of tonneau covers is keeping your truck bed and cargo protected from the elements. If you’re finding water or dirt making its way into the bed, a flush mount with shims can seal things up.
The installation instructions recommend it
Always check the user guide for your specific tonneau cover to see if the manufacturer recommends or includes shims as part of installation. The required steps can vary.
It’s an older truck model
The bed rails on newer trucks are often more standardized while older models tend to have more variation that requires shim adjustments. Using them helps customize and refine the tonneau mounting.
Where Should I Place the Shims?
To know exactly where to place shims, you’ll need to first test fit your tonneau cover. Note any areas where it doesn’t sit flush or rattles as you lightly press on it. Those are the spots that need shimming.
Some common shim placement areas include:
- Along the top edges of the bed rails if the cover sits too high
- Under rail cap mounting plates if they’re uneven
- Near latching hardware if the cover is loose when latched
- At rear tailgate openings if it slopes downward
- Under center brackets to prevent sagging in the middle
Use the thinnest shims possible to take up excess space gradually. You want a tight, zero-gap fit, not one that bows the cover by over-shimming.
How Many Shims Do I Need?
The number of shims required depends on the specific mounting design of your tonneau cover and how much adjustment is needed for optimal fit. Some covers may only need shims in one or two spots, while others could require them along most of the bed rail.
It’s smart to purchase an assortment of shim thicknesses, usually ranging from 1/16” to 1/4”. That way you can stack and combine them as needed to get the right fit. Having extras on hand allows you to replace any that become damaged too.
Some tonneau cover kits come with the required shim quantity included, but if yours doesn’t, plan on having at least a dozen shims available for test fitting. It’s easy to return unused ones after installation.
What Are Tonneau Cover Shims Made Of?
Tonneau cover shims come in a variety of materials, each with their own characteristics:
- Plastic – Lightweight but durable. Least likely to corrode. Often the most affordable option.
- Rubber – Provides vibration dampening. Won’t scratch truck bed. Can conform to uneven spaces.
- Aluminum – Very thin yet strong. Resists corrosion. Can slide into tight spots.
- Stainless Steel – Withstands high load without deforming. Resists moisture damage. Provides stability.
- Urethane – Oil and gas resistant. Good shock absorption. Stays flexible in weather extremes.
- Neoprene – Compresses easily to fill gaps. Waterproof and non-abrasive.
Choose shims that suit your particular needs and installation environment. Auto parts stores and tonneau cover retailers offer sets with an assortment of shim types and thicknesses.
How Do I Install Tonneau Cover Shims Correctly?
Installing shims takes a few simple steps:
- Clean the bed rail mounting areas so surfaces are free of dirt and debris.
- Do a test fit of the tonneau cover without shims to see where they’re needed.
- Place shims in the required locations flat against the bed rails. Tap gently with a mallet if needed to seat tightly.
- Re-install the tonneau cover mounting hardware and brackets over the shims. Tighten securely but not excessively.
- Inspect the fit and make any final shim adjustments, removing or adding as needed for optimal tightness.
- Fully tighten mounting bolts and latches once fitted properly.
Go slowly and check the fit often as you add shims. Too many can over-tighten the cover. Follow your specific tonneau’s mounting directions closely.
Maintaining Your Tonneau Cover Shims
Once installed, be sure to:
- Check shims occasionally for slippage or deterioration. Replace damaged ones promptly.
- Clean shim surfaces and bed rails periodically to prevent dirt buildup and corrosion.
- Re-tighten mounting hardware if bolts become loose over time.
- Adjust shims if tonneau fit becomes loose due to truck bed settling.
Proper shim maintenance will extend the life of your cover and keep it fitting like new.
Finding the Right Tonneau Cover Shims
Not all shims work universally for every cover, so use ones made specifically to fit your truck make and tonneau model. You can find compatible shims from:
- Your tonneau cover manufacturer – They’ll have shims tailor-made to work with their products.
- Auto parts stores – Many stock shim assortments for common truck brands.
- Online retailers – Large selections of shim types, sizes, and materials.
- Truck bed specialty shops – Experts in custom truck accessories and modifications.
- Hardware and home improvement stores – Offer standard shims for basic DIY shimming needs.
Be sure to precisely measure your truck bed size rather than relying on model year alone. Custom or aftermarket beds may not conform to factory dimensions. Bring these measurements with you when shopping to get the right shims.
Get a Perfect Tonneau Cover Fit with Shims
A properly shimmed tonneau cover not only looks great on your truck, it also performs better. Taking the time to install shims helps ensure your cover fits just right, seals out the elements, and remains securely attached on any terrain.
Don’t forget to check for shim recommendations in the installation guide and keep an assortment on hand. With the right shims in the right spots, you can enjoy the full benefits of your high-quality tonneau cover for miles to come.