Choosing the Right Distributor Cap for Your 1958-1959 GMC FM340
Keep your classic GMC's V8 running strong with the correct distributor cap.
- A failing distributor cap on your GMC FM340 will cause misfires, rough running, hard starting, and stalling.
- Aftermarket brands like Standard Ignition and Walker Products are reliable choices for this part.
- Always replace the distributor rotor at the same time as the cap for a complete tune-up.
- Visually inspect the inside of the cap for cracks and corrosion; this is the best way to diagnose a failure.
Signs of a Failing Distributor Cap
On a vintage truck like the GMC FM340, the distributor cap is a regular maintenance item. Over time, the plastic can develop tiny cracks and the metal contacts inside can corrode or wear out. When this happens, you'll notice your engine isn't running right. Common symptoms include a rough idle, engine misfires (which feel like a stumble or hesitation), difficulty starting, and even stalling. You might also see moisture inside the cap or white, crusty corrosion on the metal terminals.
Visual Inspection is Key
The best way to check your distributor cap is to look at it. Remove the two clips holding it on and inspect the inside for cracks, carbon tracks (which look like pencil lines between the terminals), and worn or corroded metal points. Check the center button to make sure it isn't stuck or worn down.
Choosing the Right Aftermarket Cap
For a 1958-1959 vehicle, finding a New Old Stock (NOS) OEM part is nearly impossible and would be very expensive. Aftermarket parts are the most practical and widely available choice. The quality of aftermarket ignition parts for these older vehicles is generally very good, as the technology is simple and well-understood.
The two main brands you'll find are Standard Ignition (Standard Motor Products) and Walker Products.
- Standard Ignition: This is a well-respected brand in the aftermarket with a long history. They are known for making quality replacement parts that meet or exceed original specifications. Many professional mechanics trust and use their products. Their caps often feature brass terminals, which offer better conductivity and corrosion resistance than aluminum.
- Walker Products: Walker is another established manufacturer, known for fuel system and engine management parts. While some users on forums have mixed reviews about their components compared to OEM on modern cars, for a simple part on a vintage truck, their products are a reliable and affordable option.
Pro Tip: When you replace the distributor cap, you should always replace the distributor rotor at the same time. The rotor is an inexpensive part that wears out along with the cap. Replacing both ensures a complete and reliable repair.
Aftermarket Distributor Cap Cost
| Part Type | Price Range |
|---|---|
| New Aftermarket (Standard, Walker) | $24.44 - $34.65 |
| Shop Labor | $50 - $100 |
Frequently Asked Questions
How often should I replace the distributor cap?
For a classic truck that may not be driven daily, it's best to inspect the cap and rotor annually or every 10,000-15,000 miles. Replace them if you see any signs of wear, cracking, or corrosion. They are wear-and-tear items, just like spark plugs.
Is there a difference between brass and aluminum terminals?
Yes. Caps with brass terminals generally offer better electrical conductivity and are more resistant to corrosion and pitting over time compared to aluminum terminals. For long-term reliability, a cap with brass contacts is the preferred choice.
Can I just clean my old cap?
You can clean minor corrosion from the terminals as a temporary fix, but if the cap is cracked, has carbon tracking, or the terminals are heavily worn, it must be replaced. Given the low cost of a new cap, replacement is always the best option for reliability.
Are any special tools needed for the job?
No special tools are required. A simple flathead screwdriver is usually all that's needed to release the clips that hold the cap onto the distributor housing. You may also need a screwdriver to remove the two small screws holding the rotor in place.
Technical Specifications
The 1958-1959 GMC FM340 was equipped with a Pontiac-sourced 336 cubic inch V8 engine. The distributor cap must be compatible with the V8 distributor for these model years. Aftermarket caps are typically made from high-dielectric strength polyester or polypropylene.
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.