6 Best Rigid Plastic Shims For Custom Mount Fitting

A loose mount on a roof rack or a vibrating electronics display can turn a smooth gravel road drive into a nerve-wracking ordeal. Precision hardware is only as effective as the surface it rests on, and finding that perfect fit often comes down to a few millimeters of high-grade plastic. Mastering the art of the shim saves expensive gear from stress cracks and ensures your setup remains rock-solid through every technical climb or bumpy forest approach.

Precision Brand Color-Coded Shims: Best Overall

Precision Brand is the industry standard for a reason: their color-coded system removes all guesswork from the leveling process. Each thickness is assigned a specific hue, allowing for rapid identification when dialing in a camera mount or leveling an equipment base in the field. They are manufactured to incredibly tight tolerances, ensuring that your adjustments are uniform and predictable.

For those managing a variety of mounting hardware, this variety pack is the ultimate problem solver. The material is rigid enough to hold its shape under heavy bolt tension, yet flexible enough to be cut or notched if the mounting footprint is irregular. Choose these if your gear kit requires high-frequency adjustments and absolute consistency.

RAM Mounts Track Shims: Best for Electronics

When securing high-end fish finders or GPS units, vibration management is essential for preventing screen flickering and connector fatigue. RAM Mounts track shims are engineered specifically to lock into T-slot rails, creating a custom tension point that keeps your electronics locked in place. These shims provide a high-friction interface that prevents the “creep” common with heavy display units.

Because electronics are sensitive to even minor mechanical movement, these shims are non-negotiable for anyone mounting gear on powerboats or off-road vehicles. They are precision-molded to fit standardized track systems, eliminating the frustration of loose-fitting DIY solutions. If your priority is protecting sensitive internal circuitry from persistent trail vibration, these are the only choice.

Broadfix U-Shims: Best for Heavy-Duty Racks

Broadfix U-Shims are the heavy lifters of the shim world, designed to bear significant compressive loads without splitting or deforming. Their horseshoe design allows them to be slid around an existing bolt or stud, meaning gear doesn’t need to be completely disassembled for a quick adjustment. This makes them ideal for structural roof rack mounts where load distribution is paramount.

The durability of these shims ensures that once a heavy rack or rooftop box is tightened down, the spacing remains constant regardless of temperature changes. They perform exceptionally well under high-torque scenarios, maintaining their structural integrity where cheaper alternatives would crumble. Select these for critical, weight-bearing mounts that require a permanent, high-strength solution.

Thule Fit Kit Spacers: Best for Roof Mounts

Thule engineering operates on a proprietary level, and their specific fit kit spacers are designed to match the exact curvature of modern vehicle roofs. These are not general-purpose hardware store items; they are aerodynamic interfaces meant to distribute pressure evenly across a car’s roof line. Without these specific spacers, high-tension rack clamps can cause permanent denting or surface damage.

If you are transitioning a rack system between vehicles or mounting a cargo box, never substitute these with generic shims. They are shaped to account for the unique geometry of automotive exteriors, providing both stability and paint protection. For anyone prioritizing vehicle resale value and rack security, these spacers are a non-negotiable component of the fit kit.

Marker Alpine Binding Shims: Best for Skiing

Precision is a matter of safety in alpine environments, where binding release settings must be exact to prevent injury. Marker alpine binding shims are designed to bridge the gap between varying boot sole lug heights and ski interface plates. They ensure that the boot-to-binding power transfer remains direct and efficient, which is vital for control on steep, icy slopes.

These are specialized components that require careful installation to ensure consistent release values. They are manufactured from high-impact, low-friction polymers that won’t become brittle in sub-zero temperatures. Use these if you are customizing your own ski setup to optimize performance and responsiveness on challenging terrain.

Grove Plastic Shims: Best Budget Assortment

For the adventurer who wants a “just in case” solution for the gear bin, Grove plastic shims offer the most versatility for the price. This assortment provides various thicknesses that can be stacked, snapped, or trimmed to fit virtually any mounting scenario. While they may not offer the extreme precision of professional machining, they are more than adequate for camp lighting mounts or general equipment organization.

These are best kept in the repair kit for field-expedient fixes on long trips. They serve as an excellent bridge for gaps in aftermarket mounts or to stop rattles in camping storage drawers. For the DIY enthusiast who values a broad range of options over highly specialized engineering, these are an essential addition to the garage.

How to Choose the Correct Shim Thickness for Gear

Start by identifying the exact gap measurement between your mounting bracket and the mounting surface using a set of calipers. Never eyeball the measurement, as even a one-millimeter error can lead to uneven torque distribution. Aim for a total stack height that is slightly less than the measured gap to ensure the bolt provides the final pull-down tension required for a secure fit.

Consider the surface composition of the gear being mounted. Rigid, metal-on-metal mounts require high-density shims that won’t compress over time, while mounts involving plastic housings benefit from slightly softer, impact-absorbing shims. Always test the setup by applying pressure to the gear; if it shifts even slightly, increase the shim thickness rather than over-tightening the mounting hardware.

Material Matters: ABS Versus Polycarbonate Plastics

ABS (Acrylonitrile Butadiene Styrene) plastic is favored for its impact resistance and relative ease of cutting or shaping. It remains stable in moderate temperatures but can become brittle under extreme, prolonged UV exposure. Use ABS for internal gear mounting or applications where the shim will be largely shielded from the elements.

Polycarbonate, by contrast, is significantly stronger and maintains its properties in much wider temperature ranges. It is the preferred choice for external mounts, roof racks, or anything subjected to high wind and heavy vibration. While it is more difficult to trim to size, its resistance to environmental stress makes it a superior investment for permanent outdoor installations.

Proper Installation Tips for Outdoor Custom Mounts

Always clean the mounting surfaces with isopropyl alcohol before placing your shim to remove dust, grit, or oily residues. A clean interface ensures that the shim stays in place while you align the bolt holes. If you are using multiple shims to achieve a specific thickness, stack them with the largest surface area against the mounting surface to maximize load distribution.

Apply a small amount of thread-locking compound to your mounting bolts after you have reached the desired shim height. Vibration is the enemy of all custom mounts, and a chemical thread locker prevents the nuts from backing off over time. Periodically check these connections after significant trips, especially after driving on rough, washboarded roads.

Weatherproofing Your Shim Setup for Winter Months

Winter conditions introduce the risk of ice expansion, which can work into micro-gaps between your gear and the mounting surface. Before the first freeze, inspect your shimmed mounts and ensure that all gaps are tightly compressed so water cannot seep behind them. If necessary, use a small bead of marine-grade silicone sealant around the perimeter of the shim to create a moisture-proof seal.

Avoid using metal shims in sub-zero conditions, as they can cause rapid temperature-induced contraction that leads to loose fittings. High-quality plastic shims are better suited for cold-weather adventures because they possess a thermal expansion coefficient closer to that of many common mounting materials. Prioritize flexible, high-impact polymers that remain compliant rather than turning brittle when the mercury drops.

A well-shimmed mount is the difference between a seamless adventure and a day spent fighting loose, vibrating gear. By choosing the right material and taking the time to match the thickness to your equipment, you ensure that your focus stays on the trail ahead rather than the hardware behind you. Pack your tools, verify your fit, and get outside with confidence.