John Deere A Spark Plug
Selecting, gapping, and maintaining the correct spark plug for a vintage John Deere Model A two-cylinder tractor is essential for ensuring easy hand-crank starting, smooth low-RPM idle, and complete fuel combustion. Manufactured by Deere & Company from 1934 through 1952, the iconic Model A features a massive horizontal two-cylinder engine displacing 309 to 321 cubic inches that runs at a leisurely 975 RPM. Because the Model A transitioned through multiple cylinder head revisions, compression ratios, and fuel configurations (distillate, all-fuel, and gasoline), selecting the correct 18mm or 14mm thread size and heat range prevents spark plug fouling.
Thread Sizes and Cylinder Head Generations (18mm vs 14mm)
The first step in selecting the correct spark plug for a John Deere Model A is determining the thread size of your cylinder head. Early unstyled and early styled Model A tractors (serial numbers below 584000) feature cylinder heads threaded for massive 18mm spark plugs. These engines were originally engineered to burn low-grade 'tractor fuel' or kerosene distillate, which required hot-running plugs to burn off heavy petroleum deposits.
Late-model styled John Deere A tractors (serial numbers 584000 and above, built from late 1947 to 1952) introduced redesigned high-compression 'cyclone' cylinder heads that transitioned to standard 14mm spark plug threads. Installing an 18mm plug into a late head is physically impossible, and using thread reducer bushings on an early head alters the spark gap depth within the combustion chamber, leading to sluggish flame front propagation.
Compare spark plug thread sizes, original heat ranges, and modern cross-references for John Deere Model A tractors:
| Tractor Era / Serial Number | Spark Plug Thread Size | Original Vintage Part Spec | Modern Champion Equivalent | Modern NGK Equivalent |
|---|---|---|---|---|
| Early Model A (Unstyled & Early Styled) | 18mm x 1.5mm Thread (1/2 in reach) | Champion W14 / Commercial 7 | Champion W14 (Stock) / W18 (Hotter) | NGK A-6 (Standard 18mm) |
| Late Model A (Styled Serial #584000+) | 14mm x 1.25mm Thread (3/8 in reach) | Champion D16 / D21 | Champion D16 (Stock) / D21 (Hotter) | NGK AB-6 / B-4L |
| All-Fuel / Kerosene Distillate Heads | 18mm Thread (Deep combustion well) | Champion W20 (Ultra-Hot) | Champion W18 (Prevents carbon fouling) | NGK A-6 Hot Range |
| High-Altitude / Gasoline Converted | 14mm Thread (Cyclone cylinder head) | Champion D14 / D16 | Champion D16 / RD16 | NGK B-6L |
Heat Ranges and Fuel Types: Distillate Versus Modern Gasoline
Understanding spark plug heat ranges is vital for vintage John Deere two-cylinder tractors. Today, virtually all preserved John Deere A tractors burn modern 87-octane unleaded pump gasoline rather than the heavy, low-octane kerosene distillate used in the 1930s. Modern gasoline burns significantly cooler than heavy distillate in low-compression (4.5:1 to 5.5:1) engines, especially when tractors are paraded at low idle rather than worked hard under a continuous multi-bottom plow load.
If you run a colder factory-original plug (like a Champion W14) on a parade tractor, the plug tip temperature will never reach the 800°F self-cleaning threshold. Carbon and unburnt fuel rapidly coat the porcelain insulator, shorting out spark current and causing the tractor to miss on one cylinder ('one-lunging'). Experienced two-cylinder collectors step up one or two heat ranges hotter—switching to a Champion W18 or D21—which runs hot enough to burn off carbon deposits at low parade speeds.
Review troubleshooting spark plug reading conditions and corrective actions on a John Deere Model A:
| Spark Plug Tip Condition | Combustion Diagnostic Cause | Engine Operational Symptom | Corrective Action Required |
|---|---|---|---|
| Dry Fluffy Black Carbon Coating | Plug heat range too cold; rich carburetor | Misfires; runs on one cylinder at idle | Switch to hotter heat range (W18/D21); lean fuel needle |
| Wet Black Oily Deposits | Worn piston rings or intake valve guides | Blue smoke from vertical exhaust stack | Clean plug; evaluate cylinder bore hone and rings |
| Tan or Light Gray Porcelain | Perfect stoichiometric air-fuel combustion | Smooth distinctive 2-cylinder 'pop-pop' idle | Optimal condition; maintain gap at 0.030 in |
| Blistered White Porcelain / Erosion | Plug heat range too hot; severe lean mix | Engine pinging; overheating head | Switch to colder plug; richen load carburetor needle |
| Bridged Gap with Carbon Flake | Carbon chunk dislodged from piston dome | Sudden dead cylinder under load | Clear gap; scrape heavy combustion chamber deposits |
Spark Plug Gap and Magneto Ignition Tuning
The spark plug gap specification for a John Deere Model A is universally set at 0.030 inches (0.76 mm). Setting the gap accurately using a wire feeler gauge is critical for vintage ignition systems. Unlike modern automotive electronic ignitions that produce 40,000 to 60,000 volts, vintage Wico (Wico C or Wico X) and Fairbanks-Morse impulse-coupling magnetos generate between 12,000 and 18,000 volts at cranking speeds.
If the spark gap is widened beyond 0.030 inches (e.g. 0.035 or 0.040 in), the weak magneto voltage will fail to jump the gap under cylinder compression pressure, resulting in extended, exhausting hand-flywheel cranking. Always use solid copper-core spark plug wires rather than modern automotive carbon-suppression wires, as carbon wires create excessive electrical resistance that suffocates vintage magneto secondary coils.
How to Service Spark Plugs on a John Deere Model A in 5 Steps
Follow these five mechanical steps to remove, inspect, gap, and install spark plugs on a John Deere A tractor.
Blow Out Debris from Cylinder Head Pockets
Use compressed air to blow accumulated dirt, straw, and grease out of the deep spark plug wells before removal.
Unscrew Spark Plugs Using Deep Socket
Use a 7/8-inch deep socket for 18mm plugs or a 13/16-inch deep socket for 14mm plugs, turning counterclockwise.
Inspect Tip Color and Measure Gap
Examine the porcelain color and use a round wire feeler gauge to measure and set the electrode gap to exactly 0.030 inches.
Install New Solid Copper Crush Gasket
Always fit a fresh solid copper crush washer onto the plug base to ensure an airtight seal against the cast iron cylinder head.
Thread by Hand and Torque Snugly
Hand-thread plugs to avoid cross-threading in cast iron, then torque snugly (approx. 25-30 lb-ft for 18mm, 20-25 lb-ft for 14mm).
Frequently Asked Questions (8 Questions Answered)
Q1: What spark plug does a John Deere Model A take?
Early Model A tractors use 18mm plugs (Champion W14 or hotter W18); late styled models (serial #584000+) use 14mm plugs (Champion D16 or hotter D21).
Q2: What is the spark plug gap on a John Deere A?
The factory spark plug gap for a John Deere Model A is exactly 0.030 inches (0.76 mm) for both magneto and battery distributor ignitions.
Q3: Why does my John Deere A foul spark plugs quickly?
Plugs foul when running too cold during low-idle parade use or from a rich carburetor mixture; switching to a hotter plug like a Champion W18 solves this.
Q4: Can you use automotive resistor spark plugs on a John Deere A?
Avoid modern resistor plugs (marked with 'R'); vintage magnetos require non-resistor plugs and solid copper-core plug wires for maximum spark voltage.
Q5: What socket size removes John Deere 18mm spark plugs?
An 18mm vintage spark plug requires a large 7/8-inch deep-well spark plug socket, while 14mm plugs take a standard 13/16-inch socket.
Q6: How can I tell if my John Deere A has 18mm or 14mm plugs?
Check the tractor serial number: serials below 584000 use large 18mm plugs, while serials 584000 and above use 14mm plugs.
Q7: What is the NGK equivalent for a Champion W14 spark plug?
The direct 18mm NGK equivalent for the Champion W14/W18 series is the NGK A-6 spark plug.
Q8: Why does my John Deere A only run on one cylinder?
Running on one cylinder ('one-lunging') is almost always caused by a fouled spark plug, a cracked distributor cap, or an unseated copper head gasket.
Final Thoughts & Key Takeaways
In conclusion, understanding john deere a spark plug provides essential clarity, practical strategies, and actionable advice. By incorporating these foundational insights, adhering to verified safety guidelines, and following structured best practices, you ensure reliable, long-term outcomes while preventing common mistakes. Stay informed, consult certified professionals when needed, and maintain consistent quality care.