Product Description
Arm-Matic Underrail 12-Foot Bent Arm Kit, Part 1125318
The Arm/Spring Kit – Underrail – 12' Arms with 2' Bend, DeBrovys SKU 20-1125318 and Shur-Co part 1125318, is an Arm-Matic replacement arm-kit configuration for qualified truck-tarp service. Shur-Co’s replacement-arm guide identifies 1125318 as a 30° bent, 12-foot, underbody Arm-Matic arm kit, while the Donovan catalog and this DeBrovys listing use the term underrail. These documented terms describe a distinct mounting/configuration path that must be matched to the installed system.
This part is not the same as the external four-spring 12-foot kit, part 1125317, and it should not be selected simply because the arm length and bend appear similar. Confirm the original part number, existing underrail or underbody mount arrangement, body rail and clearance conditions, arm geometry, rear crossover, tarp path, and all current hardware before ordering. A careful fitment review is the best way to avoid a replacement that cannot operate safely on the equipment in service.
Verified Product Configuration
| Specification | Verified detail |
|---|---|
| Product name | Arm/Spring Kit – Underrail – 12' Arms with 2' Bend |
| DeBrovys SKU | 20-1125318 |
| Manufacturer part number | 1125318 |
| Product family | Shur-Co / Donovan Arm-Matic replacement arm kit |
| Manufacturer guide description | 30° bent arm, 12-foot, underbody configuration |
| Donovan catalog description | Underrail, 12-foot arms with 2-foot bend |
| Store category | Truck Tarp Systems / Donovan Enterprises Inc. |
| Product role | Replacement arm-and-spring kit; fitment must be confirmed against the existing system |
The 1125318 identifier and its underrail/underbody configuration are more important than a broad phrase such as “12-foot tarp arm.” A straight arm, different bend, different length, external four-spring kit, or other underrail system can have a different mounting arrangement, arm path, crossover, and service requirement. Never assume similar-looking components share a safe interchange.
What the Underrail Arm-Matic Kit Is Designed For
Shur-Co describes the Arm-Matic family as a reliable, durable, and repairable tarp-system solution for demanding truck and trailer applications. Its end-dump Arm-Matic page presents an underrail system configuration with underrail torsion for complete end-dump systems, demonstrating that the underrail architecture is a specific system choice—not a generic replacement label. Part 1125318 must therefore be matched to the exact existing underrail/underbody assembly rather than to a complete-system specification or another part number.
A compatible replacement kit can be appropriate when a confirmed 1125318-style arm assembly has been damaged by wear, corrosion, collision, or a failed component. However, arm replacement is not a substitute for finding the root cause. Inspect mounts, rail-side structure, crossover, tarp roller, tarp tension, body clearances, and signs of interference before new components are installed. Correcting the condition that caused abnormal loading helps the repaired system perform as intended.
Replacement Kit Components and System Matching
Shur-Co’s replacement-arm guide states that replacement arm kits include pivot mounts, flat coil springs, a welded or clamshell lower arm section, pivot arms, an elbow when applicable, rear crossover, and hardware. The guide lists part 1125318 in its Arm-Matic arm-kit group. Verify the precise as-shipped contents against the current order, delivery paperwork, and manufacturer documentation before installation, especially where equipment has been modified or repaired with mixed components.
The catalog identifies 1125318 as an underrail kit and references a 12-foot return arm with a 2-foot bend in the associated parts group. This does not authorize mixing components from a different underrail, underbody, external, four-spring, or other arm system. Correct geometry requires the arms, mounts, crossover, roller, tarp, and body rail to work together through the entire travel path.
Underrail Fitment Checklist Before Ordering
| Confirm before ordering | Why it matters |
|---|---|
| Existing manufacturer part number | Part 1125318 is the strongest starting point for an exact underrail/underbody replacement decision. |
| Mounting arrangement | Confirm the current system is the compatible underrail or underbody layout; do not substitute an external four-spring configuration. |
| Arm length and bend | This listing is documented as a 12-foot, 30° bent configuration with a 2-foot bend description in the catalog. |
| Rail, body, and clearance conditions | Mounting and arm travel must clear body rails, tailgate hardware, lights, fenders, cab guards, exhaust, and accessories. |
| Rear crossover and tarp path | Arms, elbow, crossover, tarp, roller, and return path must operate without binding, contact, or excessive tension. |
| System condition | Worn pivots, loose hardware, bent structure, damaged tarp components, or roller misalignment can shorten replacement-kit life. |
| Qualified service plan | Spring-loaded components need trained personnel, current manufacturer instructions, appropriate tools, and a controlled work area. |
For a fitment review, provide DeBrovys the installed part number, photos of both sides of the arm assembly, underrail or underbody mounting details, rear crossover, finished body measurements, and all relevant repair history. These details help distinguish a direct 1125318 replacement from an application that requires a different arm, mount, crossover, or complete-system solution.
Underrail Compared with External Four-Spring Configurations
The underrail/underbody 1125318 kit is not interchangeable with the externally mounted four-spring 12-foot Arm-Matic kit. The external kit is a separate part, 1125317, with 12-foot, four-spring arms and a 2-foot bend. The two kits share a general Arm-Matic family relationship but have different documented configuration language and should be selected by the installed system rather than by arm length alone.
Do not convert an underrail/underbody setup to an external configuration—or the reverse—without a manufacturer-supported engineering and installation plan. Changes in mounting location, spring architecture, arm path, crossover geometry, body reinforcement, clearances, and electrical or tarp components may all be required. A replacement purchase should preserve the configuration that the equipment was designed to use unless a qualified system redesign is confirmed.
Spring-Tension Safety: Qualified Service Required
Arm assemblies are under spring tension while the springs are engaged. Shur-Co’s Arm-Matic manual directs users to use caution while assembling or disassembling arms. This is a safety-critical component group. Do not release, remove, index, disassemble, or modify spring-loaded arm components without current manufacturer instructions, suitable tools, trained service personnel, and a secured work area.
Keep people clear of arm movement and all pinch or release zones during inspection and service. If an arm is bent, a spring appears damaged, a pivot has excessive play, hardware is missing, or the tarp tracks incorrectly, remove the system from operation until it has been evaluated. Never substitute miscellaneous hardware for specified pivot, spring, mounting, or crossover components.
Pre-Installation Inspection and Commissioning
Before a replacement kit is fitted, inspect the mounting structure, underrail or underbody interface, pivot locations, lower arm sections, elbows, rear crossover, tarp roller, tarp attachment, motor or crank arrangement, wiring or controls where applicable, and body-side clearance. Confirm that lights, cab guards, toolboxes, fenders, tailgate hardware, exhaust, and accessories will not interfere with the designed arm path.
After qualified installation, check all hardware for complete assembly before operating—the Arm-Matic manual specifically emphasizes this first inspection. Complete the initial operating cycle slowly in a clear area while observing arm motion, tarp tracking, crossover position, tension, and clearance. Stop operation if an arm binds, the tarp pulls unevenly, the assembly contacts the body, or any component behaves unexpectedly.
Maintenance and Service Records
Replacement arms perform best when the tarp system is maintained as one working assembly. Include the mounting interface, pivots, springs, lower arms, elbows, rear crossover, roller, tarp attachment points, and tarp itself in the regular inspection routine. Look for abrasion, loose fasteners, corrosion, cracks, deformation, damage to coatings, changes in spring performance, and anything that alters normal tarp travel.
Record part number 1125318, the truck or trailer number, body style, installation date, service observations, and photos in the equipment file. Clear records help a fleet determine whether a later request matches the same underrail configuration or needs a different kit. They also help identify recurring failure patterns before they lead to another unscheduled repair.
Truck Tarp System Parts and Resources
Browse Truck Tarp Systems for complete systems, controls, replacement parts, and related components. For product-family background, read the DeBrovys guide to Donovan and Shur-Co tarp systems and visit the DeBrovys blog.
If the repair also needs a replacement tarp, send the finished tarp dimensions, material, roller and crossover details, and the confirmed arm configuration. DeBrovys can help identify related parts, replacement-tarp options, and custom fabrication or heat-sealed panels for compatible tarp applications. Tarp work does not replace the need to verify that the arm-and-spring kit is correct for the installed underrail system.
Minimum Order Fee Notice
Orders below $130.00 are subject to a $25.00 SHUR-CO minimum order fee. If an order total is $130 or below, DeBrovys will contact the buyer with an additional invoice for the fee before shipment. Before purchasing a replacement arm kit, confirm the exact part number, current price, shipping requirements, compatibility, installation plan, and all related components.
Frequently Asked Questions
What is Shur-Co part 1125318?
Part 1125318 is an Arm-Matic replacement arm kit identified by Shur-Co as a 30° bent, 12-foot, underbody configuration. The Donovan catalog and the DeBrovys listing describe it as an underrail kit with 12-foot arms and a 2-foot bend.
Is the underrail kit the same as the 12-foot four-spring kit?
No. The external four-spring 12-foot kit is part 1125317. Part 1125318 has a distinct underrail/underbody configuration. Match the original part number, mounting arrangement, arm path, crossover, and system geometry before ordering.
What should I provide for an underrail fitment review?
Provide the installed part number, photos of the arms and mounting interface, a rear-crossover photo, body measurements, and repair history. This helps confirm whether 1125318 matches the existing system or whether another component configuration is needed.
What is included with a replacement arm kit?
The manufacturer guide states that replacement arm kits include pivot mounts, flat coil springs, a welded or clamshell lower arm section, pivot arms, an elbow when applicable, rear crossover, and hardware. Confirm the exact as-shipped contents for part 1125318 against current order documentation before installation.
Can I install or service the spring-loaded arm components myself?
Spring-loaded arms are under tension while the springs are engaged. Shur-Co instructs users to use caution during assembly and disassembly. Installation and service should be completed by qualified personnel using current manufacturer instructions, suitable tools, and a controlled work area.
