Choosing the Best Spark Plugs for Your 1967-1974 GMC P35/P3500 Van
Get your classic GMC workhorse running smoothly with the right set of new spark plugs.
- Confirm your engine type (I-6, V6, or V8) before purchasing plugs.
- For stock engines, standard copper plugs from ACDelco or NGK are excellent choices.
- Always check and set the spark plug gap to 0.035 inches before installation.
- Consider replacing old, brittle spark plug wires at the same time for best performance.
Symptoms of Bad Spark Plugs in a Classic Van
Engines from the 1960s and 70s don't have computer systems to tell you what's wrong. You have to listen to the engine. If your spark plugs are worn, dirty, or fouled, your van will show several signs.
- Rough Idle: The engine may shake or sound uneven when you're stopped.
- Hard Starting: It might take longer than usual for the engine to crank and fire up.
- Engine Misfires: You may feel the van hesitate, stumble, or lose power for a moment when you accelerate. An engine misfire means the fuel and air in a cylinder didn't ignite properly.
- Poor Fuel Economy: If you're stopping for gas more often, it could be because worn plugs are not burning the fuel completely.
- Sluggish Acceleration: A lack of power or a feeling that the van has lost its 'get-up-and-go' can be caused by ineffective sparks.
Choosing the Right Aftermarket Spark Plug
For a vintage vehicle like the P35/P3500, you don't need to hunt down a part from a GMC dealer. The original plugs were typically made by ACDelco, and today's high-quality aftermarket brands are just as good, if not better. The most important thing is to choose a reputable brand that fits your engine. Your van could have a Chevy-built inline-6 or V8, or a less common GMC-specific V6 engine; always confirm your engine type before buying.
Brand Quality Tiers
| Tier | Brands | Best For |
|---|---|---|
| Premium Aftermarket | NGK, Denso | Owners seeking the best reliability, smoothest idle, and longest life. Forum users and mechanics frequently recommend NGK as a top choice for these older GM engines. |
| Standard / OE-Style | ACDelco, Autolite, Champion | A great balance of quality and value for a stock, daily-driven van. ACDelco is the original brand for GM vehicles, and Autolite and Champion are trusted American brands that have been around for decades. |
| Performance | MSD | Best for modified engines with upgraded ignition systems. Likely unnecessary for a standard work van. |
| Budget | Skyward | An entry-level option that will work but may not offer the same longevity or quality control as the more established brands. |
Pro Tip: Always Check The Gap!
Even if the box says the plugs are "pre-gapped," you must check the gap yourself before installation. For most standard V8 engines of this era (like the Chevy 350), the gap should be set to 0.035 inches. Use a simple feeler gauge or gapping tool to verify. An incorrect gap can cause misfires and poor performance.
Frequently Asked Questions (FAQ)
Which spark plug material is best: copper, platinum, or iridium?
The original plugs were copper. Copper plugs provide excellent performance but have the shortest lifespan (typically 20,000-30,000 miles). Platinum and iridium plugs cost more but can last much longer, up to 100,000 miles in some cases. For a stock P35/P3500, standard copper plugs from a good brand like NGK or ACDelco are affordable and work perfectly. If you want to change them less often, iridium is a great upgrade.
Is ACDelco the best brand for my GMC van?
ACDelco was the original equipment supplier for General Motors, so their parts are a reliable choice that will perform like the originals. However, many mechanics and enthusiasts now prefer NGK for its consistent quality and performance in older vehicles.
How often should I change the spark plugs?
For standard copper spark plugs, a good interval is every 30,000 miles or every few years. If you use your van for heavy-duty work, you might consider changing them more frequently. If you upgrade to long-life iridium plugs, you may not need to change them for many years.
Do I need to replace the spark plug wires at the same time?
It's a good idea to inspect the spark plug wires for cracks, brittleness, or corrosion whenever you change the plugs. Old wires can leak voltage, leading to a weak spark and the same symptoms as a bad plug. If they look worn out, it's best to replace them as a set.
Technical Specifications
OEM Part Numbers: R45TS
Common Spark Plug: ACDelco R45TS or equivalent. Typical Spark Plug Gap: 0.035 inches. Socket Size: Typically 13/16" or 5/8".
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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.