Guide to the 1979-1985 GMC K3500 Automatic Transmission Mount
Eliminate drivetrain clunks and vibrations in your classic GMC truck by choosing the right replacement transmission mount.
- Clunking when shifting into gear is the most common sign of a failed transmission mount.
- Replacement is a straightforward DIY job that takes 1-2 hours with basic tools.
- Choose a polyurethane mount (Prothane, Daystar) for heavy-duty use, but expect more cabin vibration.
- Always inspect your engine mounts when replacing the transmission mount, as they likely need replacement too.
Is Your K3500's Transmission Mount Failing?
After 40+ years of use, the original rubber on your truck's transmission mount is likely worn out. Heat from the exhaust, exposure to leaking fluids, and age cause the rubber to crack, get soft, or completely separate from the metal plates. When this happens, the transmission is no longer held securely, leading to a number of obvious problems. Ignoring these signs can put extra stress on your engine mounts, U-joints, and driveshaft, leading to more expensive repairs.
Choosing the Right Aftermarket Mount: Rubber vs. Polyurethane
For the 1979-1985 K3500, you have two main choices for aftermarket mounts: standard rubber or heavy-duty polyurethane.
Standard Rubber Mounts (Good for Most Owners)
These are direct replacements for the original factory mount and are designed to provide a smooth, quiet ride by effectively absorbing vibrations. They are the most affordable option.
- Brands like Anchor, DEA, and Westar are widely available economy options. However, owner feedback is very mixed. While some have no issues, many report that the rubber is too hard (increasing vibration) or fails prematurely. Consider these if budget is the primary concern.
- Brands like Pioneer are sometimes regarded as a step up from the economy brands, offering a better balance of quality and price.
Polyurethane Mounts (Heavy-Duty & Performance)
Polyurethane is a much stiffer and more durable material than rubber. It's an excellent upgrade for trucks that are used for heavy towing, hauling, or off-roading.
Pro Tip: Polyurethane is resistant to oil and other fluids that can destroy a standard rubber mount.
- Brands like Prothane and Daystar specialize in these performance mounts. They will hold the transmission much more securely, improving throttle response and durability.
- The major downside is a significant increase in noise and vibration (NVH) felt inside the cab. The stiff material transmits more of the drivetrain's vibrations directly to the frame. This may be uncomfortable for a daily-driven truck.
Check Your Engine Mounts! The transmission mount and two engine mounts work together. If the transmission mount has failed, the engine mounts have been under extra stress and are likely worn out too. It is highly recommended to inspect and replace all three at the same time.
Cost of Replacing a Transmission Mount on a 1979-1985 GMC K3500
| Item | Estimated Cost |
|---|---|
| New Aftermarket Mount (Rubber) | $5 - $30 |
| New Aftermarket Mount (Polyurethane) | $30 - $136 |
| Estimated Shop Labor | $100 - $200 |
Frequently Asked Questions (FAQ)
Can I replace the transmission mount myself?
Yes, this is a very DIY-friendly job for a K3500. The process involves safely supporting the transmission with a floor jack and a block of wood, removing a few bolts, and swapping the part. For most home mechanics with basic tools, it can be done in under two hours.
What tools are needed for the job?
You'll need a basic socket set (a 15mm socket is common for these trucks), a ratchet or breaker bar, a floor jack, and a piece of wood to place between the jack and the transmission oil pan to spread the load. Soaking the old bolts in penetrating oil the night before can make them much easier to remove.
Are there any recalls for this part?
No, a search of the National Highway Traffic Safety Administration (NHTSA) database shows no specific recalls or Technical Service Bulletins (TSBs) for the transmission mount on the 1979-1985 GMC K3500. Any failures are due to normal age and wear.
How do I know if the mount is bad for sure?
Besides the noises and vibrations, you can often see a failed mount. With the truck safely parked, look under the vehicle at the crossmember where the transmission sits. If the rubber is cracked, separated from the metal, or looks compressed and smashed, it needs to be replaced. You can also have a helper shift the truck from Drive to Reverse (with their foot firmly on the brake) while you watch the transmission; excessive lifting or movement indicates a bad mount.
Technical Specifications
OEM Part Numbers: 2638
The most common design for the 1979-1985 K3500 is a single or dual-stud mount that bolts to the transmission tail housing and the frame crossmember. It consists of a steel bracket with a bonded rubber or polyurethane insulator.
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
- 🛍️ Shop This Part
- Is Your K3500's Transmission Mount Failing?
- Choosing the Right Aftermarket Mount: Rubber vs. Polyurethane
- Standard Rubber Mounts (Good for Most Owners)
- Polyurethane Mounts (Heavy-Duty & Performance)
- Cost of Replacing a Transmission Mount on a 1979-1985 GMC K3500
- Frequently Asked Questions (FAQ)
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