A Guide to Universal Joints for the 1975-1978 GMC P35
Understanding and replacing the U-joints on your heavy-duty P35 chassis is critical to avoid a driveline failure.
- A failing U-joint is dangerous; address symptoms like clunking, vibrations, or squeaking immediately to prevent a driveshaft separation.
- For a heavy-duty P35, choose a high-quality, non-greasable U-joint from a brand like Dana Spicer for maximum strength and reliability.
- Your P35 may have a one-piece or two-piece driveshaft; inspect your vehicle to confirm how many U-joints you need before ordering.
- Replacing U-joints is possible for a skilled DIYer, but rust and the need for a press or large vise can make it a difficult job.
Symptoms of a Failing Universal Joint
A bad U-joint on your GMC P35 will give you clear warning signs. Because the P35 is a heavy vehicle, often carrying significant loads as a motorhome or work truck, these symptoms can appear suddenly and should be addressed immediately.
- Loud Clunking or Ringing Noise: The most common sign is a sharp "clunk" or metallic ringing sound when you shift from drive to reverse or vice-versa. This is the sound of excessive slack in the worn-out joint.
- Vibrations While Driving: A vibration that you can feel through the floor or seat is a serious symptom. This vibration often starts at a certain speed and gets worse as you accelerate. It feels different from a tire imbalance because it's a higher frequency, as the driveshaft spins much faster than the wheels.
- Squeaking or Chirping Sounds: A rhythmic squeaking that changes with your speed, usually heard at lower speeds (under 10 mph), means the U-joint is dry and running without lubrication. This indicates metal-on-metal contact, and failure is imminent.
Warning: Do Not Ignore These Symptoms
A U-joint that fails completely can cause the driveshaft to detach from the vehicle. This can lead to a loss of control, severe damage to the undercarriage, transmission, and exhaust, or cause the driveshaft to dig into the pavement, potentially flipping the vehicle.
Choosing the Right Aftermarket U-Joint
For a vintage heavy-duty truck like the 1975-1978 P35, aftermarket U-joints are the most common and practical choice. The quality can vary significantly between brands. Original equipment (OEM) parts from GM for a vehicle this old are likely unavailable or are simply rebranded aftermarket parts.
Brand Quality Tiers
- Tier 1 (Recommended): Dana Spicer
Dana/Spicer is widely considered the gold standard and was likely the original equipment manufacturer for your truck's driveline components. Forum users and mechanics consistently recommend Spicer parts for their strength and durability, which is essential for a heavy P35 chassis. Their non-greasable "Life Series" joints are particularly well-regarded for being stronger and better sealed than other options. - Tier 2 (Good Value): GMB
GMB is a major manufacturer that supplies parts to many car companies and the aftermarket. They are known for producing reliable, good-quality parts that offer a balance between performance and price. While not always having the top-tier reputation of Spicer, GMB is a solid and dependable choice for a replacement U-joint. - Tier 3 (Use with Caution): Moog
Moog was once a top-tier aftermarket brand, but many professional mechanics and experienced owners report a significant decline in quality in recent years. While some of their products are still reliable, their U-joints are frequently cited as being lower quality and prone to premature failure. Given the heavy-duty nature of the P35, it may be best to choose a more consistently reliable brand like Spicer.
Pro Tip: Greasable vs. Non-Greasable U-Joints
You will see options for both greasable (with a zerk fitting) and non-greasable (sealed) U-joints. For the GMC P35, a non-greasable U-joint from a quality brand like Spicer is the recommended choice. Non-greasable joints are physically stronger because the main body isn't drilled with a channel for grease, which can be a weak point. They also use superior seals to keep lubrication in and contaminants out, making them a fit-and-forget solution that often outlasts poorly maintained greasable joints.
Typical Replacement Costs
The cost to replace U-joints on a 1975-1978 GMC P35 can vary based on how many joints need replacing and local labor rates. Your truck may have two, three, or more U-joints depending on whether it has a single or two-piece driveshaft.
| Part or Service | Estimated Cost |
|---|---|
| New Aftermarket U-Joint (per joint) | $17.93 - $237.75 |
| Shop Labor | $150 - $350 |
| Total (for two joints) | $185 - $825+ |
Frequently Asked Questions
Can I replace the U-joints on my P35 myself?
Yes, if you are mechanically experienced. The process is straightforward but requires heavy-duty tools. You will need to safely lift the vehicle, remove the driveshaft, and use a bench vise, a ball joint press, or a C-clamp and sockets to press the old joints out and the new ones in. The biggest challenge on a vehicle of this age will be dealing with rust and seized components.
How many U-joints does my 1975-1978 GMC P35 have?
It depends on the specific configuration of your P35. Most will have at least two U-joints on the main driveshaft (one at the transmission, one at the axle). However, models with a longer wheelbase may have a two-piece driveshaft, which includes a third U-joint and a center support bearing. It is essential to inspect your vehicle to confirm the exact number before ordering parts.
Do I need to get my driveshaft balanced after replacing the U-joints?
In most cases, if you are only replacing the U-joints and are careful to mark the driveshaft's orientation before removal, balancing is not required. However, if you still experience vibration after the replacement, the driveshaft may be bent or have lost a balance weight, and you should take it to a driveline shop for inspection and balancing.
What is the most common mistake when ordering U-joints?
The most common mistake is ordering the wrong size. While many GM trucks of this era used common parts, there can be variations. It is critical to check the specifications for your vehicle, such as noting whether the joints use inside or outside snap rings and measuring the yoke and bearing cap diameter if possible. Some part listings specify yoke outside diameters like 3.875 inches, which is a key detail.
Technical Specifications
Check for U-joints with outside snap rings. Common yoke outside diameters are 3.875 inches and bearing cap diameters are around 1.188 inches, but always measure or confirm with the supplier before ordering.
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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.