A Guide to Spark Plugs for the 1950-1959 Buick Roadmaster
Keep your Nailhead V8 running smoothly with the right set of spark plugs.
- Always check and set the spark plug gap before installation; a range of .025"-.035" is typical for these years.
- Standard copper core spark plugs from brands like NGK, Autolite, or Champion are the best choice for the Nailhead V8.
- Torque new spark plugs to 22-28 ft. lbs. to ensure a proper seal without damaging the cylinder head.
- Symptoms like rough idle, hard starting, and poor acceleration are strong indicators that it's time to inspect your spark plugs.
Symptoms of Failing Spark Plugs
Your Roadmaster's engine will let you know when its spark plugs are tired. Because these vehicles do not have modern computer systems, you must rely on listening to and feeling how the car drives. Look out for these common signs:
- Hard Starting: The engine cranks for a long time before it finally starts, or it may not start at all.
- Rough Idle: When the car is stopped and in gear, the engine may shake or sound uneven.
- Engine Misfires: You may feel a hesitation or stumble from the engine, especially when accelerating. This happens when a cylinder fails to fire correctly. 🎬 Watch: How to diagnose a cylinder misfire on a vintage engine
- Poor Acceleration: The car feels sluggish and lacks its usual power when you press the gas pedal.
- Reduced Fuel Economy: You'll find yourself visiting the gas station more often because fouled plugs can't burn fuel efficiently.
Choosing the Right Aftermarket Spark Plug
The original spark plugs for your Buick were made by AC. While you can sometimes find New Old Stock (NOS) plugs, modern aftermarket brands offer excellent, reliable alternatives that are readily available. The key is choosing the right type for your vintage engine.
Key Spark Plug Specifications (1950-1959 Roadmaster)
| Original OEM Brand | AC Spark Plugs |
| Common OEM Part Numbers | 46-X (early 50s), 44-5, 44 (mid-to-late 50s) |
| Thread Size | 14mm |
| Hex Size | 13/16" |
| Recommended Gap | 0.025" - 0.035" (check spec for your year) |
| Tightening Torque | 22-28 ft. lbs. |
Brand Quality & Tiers
For a classic engine like the Nailhead V8, you don't need expensive, fancy spark plugs. A standard copper core plug is very similar to the original design and works exceptionally well.
- Good (Standard Replacement): Autolite and Champion are budget-friendly brands that offer reliable copper core spark plugs. They are a good choice for stock engines and normal driving. Some classic car owners have strong preferences for one over the other, but both are suitable replacements.
- Better (Premium Replacement): NGK and Denso are well-regarded for their quality control and consistent performance. Their standard copper plugs are an excellent choice and are often preferred by mechanics for their reliability right out of the box.
Pro Tip: Platinum and Iridium plugs are designed for modern engines with high-energy ignition systems and long replacement intervals. While they will work in your Roadmaster, they offer no significant performance benefit for the Nailhead engine and are not worth the extra cost. Stick with a quality copper core plug.
🎬 See this walkthrough on how to change spark plugs in a classic carNew Spark Plug Cost Comparison
| Part Type | Price Range (Set of 8) |
|---|---|
| New Aftermarket (Copper) | $26 - $50 |
| New Aftermarket (Platinum/Iridium) | $60 - $115 |
| New OEM (AC NOS, if found) | $80 - $200+ |
Frequently Asked Questions
Do I need to gap new spark plugs?
Yes. This is the most important step. While some plugs are marketed as "pre-gapped," you must always verify the gap with a feeler gauge before installation. 🎬 Watch: How to gap a spark plug properly using a feeler gauge The correct gap for your Roadmaster is between 0.025" and 0.035", depending on the year. Using the wrong gap can cause misfires and poor performance.
Which spark plug heat range is correct?
The original AC 44, 45, and 46 series plugs are in a "medium" heat range. When buying aftermarket plugs, use a cross-reference chart to find the equivalent heat range. Unless your engine is heavily modified or used for racing, the standard recommended heat range is always the best choice.
How often should I change the spark plugs on my Roadmaster?
For a classic car that is driven regularly, a good interval is every 10,000 to 15,000 miles or every 2-3 years. Because these cars often sit for periods, moisture can affect ignition components. Regular inspection and replacement is cheap insurance against ignition problems.
Are there any recalls or technical bulletins for my Roadmaster's spark plugs?
No. There are no active recalls or Technical Service Bulletins (TSBs) from the manufacturer regarding spark plugs for this vehicle. This is expected for a car from this era. Any known issues are related to general maintenance and tune-ups.
Helpful Videos
Technical Specifications
OEM Part Numbers: AC 46-X AC 44-5 AC 44
Thread: 14mm, Hex: 13/16", Gasket Seat, Recommended Gap: 0.025"-0.035", Torque: 22-28 ft. lbs.
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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.