Distributor Cap Replacement Guide for 1955-1959 GMC PM250 Trucks
Keep your classic GMC's ignition system firing correctly with the right new distributor cap.
- The distributor cap is a critical maintenance item; a failing one causes misfires, hard starts, or a no-start condition.
- Always replace the distributor rotor at the same time as the cap for a complete repair.
- Verify your specific engine (inline-6 or V8) before ordering, as the caps are different.
- Choosing a cap with brass terminals over aluminum can offer better longevity and corrosion resistance.
Is Your Distributor Cap Failing?
On a classic truck like the 1955-1959 GMC PM250, the distributor cap is a regular maintenance item that wears out over time. It lives in a tough environment with high voltage and heat. Eventually, the plastic can crack, or the metal contacts inside can corrode or get covered in carbon, leading to engine problems.
Common Symptoms of a Bad Distributor Cap:
- Engine Misfires: The truck may jerk, stumble, or run rough, especially when you accelerate.
- Hard Starting: The engine might crank for a long time before it starts, or it might not start at all.
- Rough Idle: You might feel shaking or vibration while the truck is stopped and the engine is running.
- Stalling or Backfiring: The engine could unexpectedly shut off or you might hear popping sounds from the exhaust.
Choosing the Right Aftermarket Cap
For a vehicle from this era, finding a "new OEM" part from GMC is nearly impossible. Your reliable choices are new aftermarket parts. The key difference often comes down to the material used for the electrical contacts inside the cap.
- Aluminum Contacts: These are common in standard replacement caps. They work fine but tend to corrode faster.
- Brass Contacts: Often found in premium or "heavy-duty" caps, brass contacts resist corrosion better and can provide a longer service life. For a work truck like the PM250, a cap with brass contacts is a worthwhile upgrade.
Available Brands: Standard Ignition vs. Walker Products
Both Standard Ignition and Walker Products are established aftermarket brands. Standard Motor Products (SMP), the parent company of Standard Ignition, is generally well-regarded by mechanics for producing quality ignition components. Their "Blue Streak" line, if available for your engine, often features higher-quality materials like brass. Walker Products also provides reliable replacement parts. When choosing, look for a cap with brass terminals for better durability.
🎬 Watch this comparison of different distributor cap materials and brands.Pro Tip: Always replace the distributor rotor at the same time as the cap. The rotor is the spinning part inside that makes contact with the cap's terminals. Replacing both together ensures a complete and reliable ignition tune-up.
Distributor Cap Cost for 1955-1959 GMC PM250
| Part Type | Estimated Cost |
|---|---|
| New Aftermarket | $24.44 - $34.65 |
| Shop Labor | $40 - $80 |
Frequently Asked Questions
Can I replace the distributor cap myself?
Yes, this is a very straightforward job for a DIY mechanic. It typically involves unclipping or unscrewing the old cap, carefully transferring the spark plug wires one by one to the new cap to maintain the correct firing order, and securing the new cap. No special tools are needed beyond a screwdriver.
How do I make sure I get the right cap for my truck?
The 1955-1959 GMC PM250 could be equipped with different engines, such as a GMC inline-six (like the 248, 270, or 302) or a Pontiac-sourced V8. The distributor cap is specific to the engine. Be sure to confirm your truck's engine type before ordering to ensure a perfect fit.
What is a carbon track and why is it bad?
A carbon track is a thin line of black dust that can form inside the cap, usually between terminals or from a crack. This line can create a short circuit, causing the spark to go to the wrong cylinder or to ground, which leads to misfires. If you see one, the cap must be replaced.
Are there any recalls for this part?
There are no specific recalls for the distributor cap on 1955-1959 GMC trucks. The NHTSA database generally does not list safety recalls that are more than 15 years old unless a manufacturer extends coverage.
Helpful Videos
Technical Specifications
Varies by engine. The 1955-1959 PM250 could have an inline-6 or a V8 engine, which use different distributor caps. The cap style is typically a point-style ignition with an external coil.
We Have This Part in Stock
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.