Choosing the Right Automatic Transmission Mount for Your 1977-1978 GMC K35
Stop the clunking and shaking in your classic GMC K35 by picking the perfect new transmission mount.
- Symptoms of failure are hard-to-miss: loud clunks when shifting and heavy vibration.
- Choose a standard rubber mount (Anchor, Westar) for a quiet, stock ride or a polyurethane mount (Prothane, Daystar) for durability at the cost of more cab noise.
- Replacement is a simple, DIY-friendly job that takes about an hour with basic tools.
- It is highly recommended to replace the two engine mounts at the same time as the transmission mount.
Is Your GMC K35's Transmission Mount Failing?
A transmission mount is a simple part, but when it fails, the symptoms are hard to ignore. Because your K35 is a heavy-duty truck, a bad mount can cause loud noises and vibrations. The original mounts were made of rubber, which can crack, collapse, or become soaked with oil and fail after decades of use. Look for these common signs of a worn-out mount.
Key Symptoms of a Bad Transmission Mount
- Loud Clunking or Banging: A distinct "clunk" or "bang" when you shift from Park to Drive or Reverse is the most common symptom. This is the sound of the transmission lifting and hitting the truck's frame because the mount isn't holding it securely.
- Excessive Vibration: You may feel a new, heavy vibration in the floor, seat, or steering wheel. This vibration often gets worse when you accelerate.
- Drivetrain Movement: In severe cases, you might feel the entire engine and transmission lurch when you accelerate or brake hard. You may even be able to see the engine and transmission assembly move excessively if an assistant shifts the truck between gears while you watch.
Choosing the Right Aftermarket Mount: Rubber vs. Polyurethane
Since genuine GM parts for a 1977-1978 K35 are generally no longer available, you will be choosing from aftermarket brands. For this truck, your choice comes down to two types of materials: standard rubber (like the original) or performance polyurethane.
Tier 1: Standard Rubber Mounts (OEM Style)
These are your basic, budget-friendly replacements designed to work just like the factory part. They are excellent at absorbing vibrations and providing a comfortable ride. If you use your K35 for regular driving and hauling and want to maintain the original feel, a rubber mount is a great choice.
- Brands: Anchor, DEA, Westar, Pioneer Cable.
- Pros: Low cost, quiet operation, absorbs vibration well.
- Cons: Not as durable as polyurethane, can fail sooner under heavy stress or from oil contamination. Forum discussions show mixed reviews on longevity for budget brands like Anchor and Westar, though some find them perfectly adequate for the price.
Tier 2: Performance Polyurethane Mounts
Polyurethane is a much stiffer and more durable material than rubber. These mounts are designed for performance and longevity, making them a popular choice for trucks that are used for heavy towing, off-roading, or have modified engines. They control drivetrain movement much better than rubber, which can improve throttle response.
Pro Tip: The biggest drawback of polyurethane is a significant increase in Noise, Vibration, and Harshness (NVH) inside the cab. The stiffer material transmits more of the engine and transmission's vibrations directly to the frame. If you use your truck as a daily driver, this added noise and vibration might be uncomfortable.
- Brands: Prothane, Daystar, Energy Suspension.
- Pros: Extremely durable, excellent at controlling torque and drivetrain movement, resistant to oil and fluids.
- Cons: Transmits significant vibration and noise into the cab, higher initial cost.
Cost of Aftermarket Transmission Mounts for a 1977-1978 GMC K35
| Part Type | Price Range | Notes |
|---|---|---|
| Standard Rubber Mounts (e.g., Anchor, Westar, DEA) | $5 - $30 | The most affordable option for a stock-like replacement. |
| Performance Polyurethane Mounts (e.g., Prothane, Daystar) | $35 - $136 | A higher price for increased durability and performance, with the trade-off of more vibration. |
| Shop Labor for Installation | $100 - $200 | This is a relatively quick job for a professional mechanic. |
Frequently Asked Questions (FAQ)
Is it hard to replace the transmission mount on a K35?
No, it is considered a relatively easy job for a DIY mechanic with basic tools. The process involves safely supporting the transmission with a floor jack and a block of wood, removing the bolts holding the old mount to the crossmember and transmission, and then installing the new one. It typically takes about an hour.
Should I use a rubber or polyurethane mount?
Choose rubber if you want a quiet, comfortable ride similar to the original. Brands like Anchor, Pioneer, and Westar offer this. Choose polyurethane from brands like Prothane or Daystar if you prioritize durability for heavy towing or off-roading and can tolerate a noticeable increase in cab vibration and noise.
Do I need to replace my engine mounts at the same time?
It is highly recommended. The engine and transmission mounts work together to support the drivetrain. If the transmission mount has failed, the engine mounts have been under extra stress and are likely worn as well. Replacing all three (two engine, one transmission) at once ensures the entire system is stable.
Are there any recalls for the 1977-1978 GMC K35 transmission mount?
No, searches of the National Highway Traffic Safety Administration (NHTSA) database show no recalls or technical service bulletins (TSBs) specifically for the transmission mount on this vehicle. This is common for a vehicle of this age.
Technical Specifications
The 1977-1978 GMC K35 with an automatic transmission typically uses a single, center-mounted transmission mount that attaches the rear of the transmission/transfer case assembly to the frame crossmember. The design is a simple metal bracket with a bonded rubber or polyurethane cushion.
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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.