A Guide to the Distributor Cap for the 1958-1959 GMC FM350
Keep your classic GMC's V8 engine firing on all cylinders with a reliable new aftermarket distributor cap.
- Aftermarket parts are the only practical option; choose a reputable brand like Standard Ignition.
- Always replace the distributor rotor at the same time as the cap for a complete repair.
- Opt for a cap with brass terminals over aluminum for better conductivity and longer life.
- Symptoms like misfiring, rough idle, and hard starting are common indicators of a failing cap.
Signs of a Failing Distributor Cap
A distributor cap is a wear-and-tear item. Over time, the high voltage and heat inside the distributor can cause the cap to crack, or the metal terminals inside to corrode or wear down. When this happens, the spark can't get to the spark plugs efficiently, leading to engine problems. Watch for these common symptoms:
- Engine Misfires or Rough Idle: You might feel the engine stumbling, shaking, or vibrating, especially when stopped or accelerating. This is often one of the first signs of a bad cap.
- Trouble Starting: A weak or inconsistent spark can make the engine hard to start, or it might crank but not fire up at all.
- Stalling or Backfiring: If the spark delivery is interrupted, the engine can stall unexpectedly. Backfiring can also occur due to incorrect spark timing caused by a faulty cap.
- Visible Damage: The best way to check your cap is to look at it. Remove it and inspect for cracks, moisture, corrosion on the metal terminals, or a worn-out center button.
Moisture is the Enemy
Cracks in the distributor cap can let moisture in, which can cause corrosion and prevent the engine from starting, especially in damp weather. Always check for a dry, clean interior when inspecting your cap.
Choosing the Right Aftermarket Distributor Cap
For a vintage truck like the 1958-1959 GMC FM350, finding a new original equipment (OEM) distributor cap is nearly impossible. The good news is that modern aftermarket parts from reputable brands are readily available and often built to a higher standard than the originals from the 1950s, using better materials.
The two main brands available for your truck are Standard Ignition and Walker Products. Both are established names in the aftermarket industry.
- Standard Ignition (SMP): Often considered a top-tier aftermarket brand, Standard has a long history of making quality ignition components. Their parts are generally well-regarded and are a reliable choice for a tune-up.
- Walker Products: While also a known brand, Walker is often more associated with fuel system components. However, their ignition parts are a viable alternative.
Brass vs. Aluminum Terminals
When choosing a cap, look for one with brass terminals instead of aluminum. Brass is a better conductor and is more resistant to corrosion, which ensures a stronger, more reliable spark over the life of the part. It's a small detail that makes a big difference in performance and longevity.
Distributor Cap Replacement Cost
Replacing the distributor cap is an affordable and straightforward job. Since you should always replace the distributor rotor at the same time as the cap, costs are often quoted for the pair.
| Part Type | Estimated Cost |
|---|---|
| New Aftermarket (Cap & Rotor) | $24.44 - $34.65 |
| Shop Labor | $50 - $100 |
Frequently Asked Questions (FAQ)
Do I need to replace the distributor rotor with the cap?
Yes, it is highly recommended. The rotor is the spinning part inside the cap that directs the spark. It wears out at the same rate as the cap, so you should always replace them as a pair to ensure a complete and effective tune-up.
How difficult is it to replace the distributor cap myself?
This is a very easy job, even for a beginner. The distributor cap is located on top of the engine and is typically held on by two clips or screws. You just need to unclip the old cap, move the spark plug wires over to the new cap one by one to keep the firing order correct, and clip the new cap in place.
Were there any recalls for the 1958-1959 GMC FM350 distributor cap?
No specific recalls or Technical Service Bulletins (TSBs) were found for the distributor cap on the 1958-1959 GMC FM350. The NHTSA's recall system was not formalized until later, and records for vehicles of this age are not comprehensive.
What is the difference between Standard Ignition and their "T-Series" line?
Standard Motor Products (SMP) offers different product lines. Their main line, often in a red or blue box, is considered their premium quality offering. They also have a budget-friendly "T-Series" line, which is generally not recommended as the quality can be inconsistent. For a critical part like this, it's best to stick with the premium line.
Technical Specifications
The 1958-1959 GMC FM350 was commonly equipped with a V8 engine, requiring a distributor cap with 8 spark plug wire terminals plus one central terminal for the ignition coil. Look for caps made from high-quality, crack-resistant plastic (often Bakelite-type material on older designs) with brass terminals.
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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.