GMRCK25S Timbren Suspension Kit Parts List: What’s Included and How It Works

If you own a GMC truck equipped with the GMRCK25S Timbren suspension kit, understanding its parts and function helps with maintenance, upgrades, or troubleshooting. This guide breaks down the complete parts list, explains each component's role, and offers practical advice for installation and care.

Overview of the GMRCK25S Timbren Suspension Kit

The GMRCK25S is a rear suspension enhancement kit designed for GMC 2500 and 3500 series trucks. Timbren’s unique design uses rubber springs (also called bump stops) to improve load support, reduce sag, and maintain ride quality. Unlike air bags or add-on leaf springs, this kit is a passive system that engages only when needed, providing progressive resistance.

Complete Parts List for the GMRCK25S Kit

The kit typically includes the following items. Note that some variations may exist depending on the exact model year and cab configuration, but the core components remain consistent.

Main Components

• Two (2) Timbren SES (Suspension Enhancement System) bump stop assemblies – These are the active rubber springs that replace the factory bump stops. They feature a hollow rubber cone that compresses progressively.
• Two (2) metal brackets – Heavy-duty steel brackets mount the Timbren units to the axle. They are powder-coated for corrosion resistance.
• Two (2) jounce bumper pads – Durable rubber pads that sit between the frame and the Timbren unit to prevent metal-to-metal contact during full compression.
• Mounting hardware kit – Includes grade 8 bolts, washers, lock nuts, and spacers as needed for secure attachment.
• Installation instructions – Detailed manual with torque specifications and step-by-step guidance.

Optional or Additional Parts (may be sold separately)

• Extended brake line brackets – Some applications require relocating the brake line bracket to avoid interference.
• Spacer plates – For trucks with lifted suspensions or additional block height.

Detailed Component Descriptions

Timbren SES Bump Stop Assemblies

Each assembly consists of a cast metal housing with a hollow rubber cone inside. The cone is designed to compress under load, providing a progressive spring rate. Initially soft, it firms up as it compresses, preventing harsh bottoming. The rubber is formulated for durability and resistance to weather and road chemicals.

Mounting Brackets

The brackets are vehicle-specific. For the GMRCK25S kit, they bolt to the axle housing using existing U-bolt mounting points. The bracket positions the Timbren unit directly under the frame’s bump stop pad. Precision welding and thick material ensure long life under heavy loads.

Jounce Bumper Pads

These flat rubber pads fit between the frame pad and the top of the steel housing. They absorb micro-vibrations and prevent noise. Over time, they may compress or wear, but replacement is simple.

Hardware

Included bolts are typically M14 grade 8 with a locking nylon insert. Washers and spacers adjust the gap between the frame and the Timbren top. Proper torque is critical to avoid loosening.

Installation Considerations

Before starting, verify compatibility with your truck’s year, model, and suspension setup. The GMRCK25S is for leaf spring rear suspension, not coil or air. Installation requires lifting the vehicle, removing the factory bump stops, and attaching the bracket to the axle.

Tools Needed

• Floor jack and jack stands
• Socket set and torque wrench
• Penetrating oil for rusted hardware
• Angle grinder (if trimming excess hardware)

Step-by-Step Outline

1. Chock front wheels and lift the rear of the truck.
2. Support the axle with jack stands so the suspension is slightly extended.
3. Remove the factory bump stops (usually two bolts per side).
4. Test-fit the Timbren bracket over the U-bolts on the axle. Use the provided spacers if needed.
5. Tighten all hardware to the torque specified in the instructions (commonly 75–85 ft-lbs for bracket bolts and 100–120 ft-lbs for U-bolt nuts).
6. Reinstall the jounce pad on top of the Timbren unit.
7. Lower the vehicle and check clearance. The Timbren should be about 1–2 inches above the frame pad at ride height.

Tips

• If the truck has a lift block, you may need longer U-bolts or a spacer plate.
• Do not overtighten the bracket bolts; refer to torque specs.
• Test drive and re-torque after 100 miles.

Maintenance and Longevity

Timbren components are nearly maintenance-free. Periodically check for:

• Cuts or cracking in the rubber cone.
• Loose bracket bolts.
• Excessive rust on hardware. Replacement parts are available individually if needed. The rubber typically lasts 5–7 years under normal use, but heavy towing may shorten lifespan.

Final Recommendation

The GMRCK25S Timbren suspension kit is a solid choice for GMC truck owners who want to improve load capacity and reduce sag without compromising ride quality. Its passive design means you won't feel it when unloaded, yet it provides robust support when hauling heavy loads. For most users, the included parts are sufficient for installation. If your truck has modified suspension or larger tires, confirm that the bracket and brake line relocation parts are included or available separately. Follow the instructions carefully, and this kit will serve well for years.