Quick overview
- The spark plug creates the spark that ignites the fuel/air in each cylinder. Replacing worn or fouled plugs restores reliable starting, smooth running, power and fuel economy.
- I’ll cover the parts involved, how the ignition system works, why plugs fail, a step‑by‑step replacement procedure (beginner friendly), checks and tests, common mistakes and how to avoid them.

What the main components are (detailed)
- Spark plug: metal threaded shell, insulator (porcelain), center electrode, ground electrode. Threads screw into the cylinder head; the gap between electrodes is where the spark jumps.
- Spark plug boot/terminal: rubber or silicone cap that fits over the plug top and connects the high‑tension lead to the plug terminal. Often contains a conductive insert.
- High‑tension lead (HT lead/plug wire): insulated wire that carries high voltage from the distributor/coil pack to the plug. Inside is a conductor and sometimes a carbon core or spiral.
- Distributor (if fitted on TE‑20): rotor and cap distribute high voltage from the ignition coil to each plug wire in turn. The rotor spins inside the cap; contacts transfer the high voltage to the appropriate cap terminal.
- Ignition coil: step‑up transformer that takes battery/point voltage and produces high voltage for sparks.
- Ignition points/condensor or electronic module: timing control; opens/closes to create the coil primary collapse that produces the secondary high voltage.
- Cylinder head: houses the spark plug threads, valves and combustion chamber.
- Tools/components for the job:
- Spark plug socket (deep, rubber insert) of correct size (commonly 5/8" or 13/16", or 14 mm depending on plug)
- Ratchet and appropriate extension
- Torque wrench (important to avoid over/under‑tightening)
- Feeler gauge or spark plug gap tool
- Compressed air or small brush and rag (to clean around plug hole)
- Dielectric grease (for boot) — very light, for boot only, not threads
- Anti‑seize (optional; see note below)
- Penetrating oil (for stuck plugs)
- Spark plug tester or multimeter (optional for testing coil/wires)
- Shop manual or OEM spec sheet (to confirm exact plug type, gap and torque)

Basic ignition theory (in simple terms and analogies)
- The ignition coil is like a small power station that turns low voltage into very high voltage.
- The distributor is like an air traffic controller that sends each spark to the correct cylinder at exactly the right time.
- The spark plug is the final delivery: it sits in the combustion chamber and makes a tiny, controlled lightning bolt between its center and ground electrode. That lightning ignites the compressed fuel/air mixture.
- Timing matters: the spark must occur when the piston and valves are positioned to get the best burn. If the plug is weak or the gap wrong, the spark will be weak or absent, causing misfires, poor starting and power loss.
- Analogy: think of each cylinder as a small camp stove. The spark plug is the flint/striker. If the flint is worn, the stove won’t light reliably. If the striker is too far from the flint (gap too wide), the spark can’t jump. If the striker is fouled with grease or oil, it won’t spark.

Why spark plugs need replacing and common failure modes
- Normal wear: electrode material erodes with use; gap widens and spark energy decreases.
- Fouling: carbon (black dry) from a rich mixture, fuel additives, or poor combustion; oil (wet black) from worn rings/valve guides; or coolant (white deposits) from head gasket leaks.
- Overheating: white, blistered insulator or melted electrodes from too‑hot a plug or lean/over‑advanced ignition.
- Mechanical damage: cracked porcelain or bent electrodes, often caused by dropping a plug or hitting it with a tool or cross‑threading.
- Wrong heat range: a plug that stays too hot or too cold will foul or melt.
- Electrical problems: high resistance in wires or distributor cap causing weak spark, so replacing plugs alone won’t fix an ignition coil problem.

Symptoms indicating plugs need attention
- Hard starting or long cranking
- Rough idle, misfiring, or cylinder tapping
- Loss of power or poor throttle response
- Increased fuel consumption
- Visible fouling or electrode wear on removed plugs

Before you start — safety and prep
- Work with the engine cold to avoid burns and to minimize warping or thread damage.
- Disconnect the battery negative terminal if you’ll be working near electrical components, or at least ensure ignition is off and key removed.
- Clean around each spark plug base with compressed air or a brush; you don’t want dirt falling into the cylinder when the plug is removed.
- Do the job one plug at a time to avoid mixing up leads if you don’t mark them.

Step‑by‑step replacement procedure (beginner friendly)
1. Identify correct plug and specs
- Find the shop manual or the correct spark plug part number for the TE‑20, and note the correct gap and torque. If you don’t have that manual, many TE‑20s use M14 thread plugs; recommended gaps for older tractors are usually about 0.025–0.035 in (0.6–0.9 mm). Typical torque ranges: 20–35 ft‑lb (27–48 N·m) depending on thread size — confirm in manual.

2. Label and remove HT leads one at a time
- Remove one plug wire at a time by pulling the boot, not the wire. Twist the boot slightly to break the seal and pull straight off. If it’s stubborn, use a boot puller or gently pry at the boot lip. Do not yank the wire itself.

3. Clean around the plug
- Blow compressed air or brush debris away from the plug well. Preventing dirt in the cylinder is critical.

4. Remove the plug
- Use the correct deep spark plug socket and an extension. Turn counterclockwise. If a plug won’t budge, apply penetrating oil and let it soak; work gently to avoid breaking the porcelain. If the porcelain cracks, leave the thread in place and extract with proper tools or seek help.

5. Inspect the removed plug
- Note color and deposits: light tan/gray = healthy; black dry = rich/fouled; wet black = oil; white/ash = oil additives or coolant; glazed/burned = overheating. Record which cylinder’s plug shows which symptom.

6. Check and set the gap on the new plug
- Use a feeler gauge to set the electrode gap to the specified value (typically 0.025–0.035 in for many older tractors). Bend the ground electrode gently if needed; don’t hit the porcelain.

7. Prepare the plug threads
- If threads are dirty, clean them with a wire brush. On cast‑iron heads, anti‑seize is optional; many modern plugs come plated so anti‑seize isn’t necessary. If you apply a tiny smear of anti‑seize, reduce torque by ~20% because it increases ease of tightening. Don’t put anti‑seize on the electrode or porcelain.

8. Install the new plug
- Thread by hand to avoid cross‑threading. Once finger‑tight, use the socket and ratchet to snug. Finish to the correct torque with a torque wrench. If you don’t have a torque wrench, tighten snugly but cautiously — over‑tightening can strip the head threads or break the plug.

9. Apply dielectric grease to boot
- Put a very light dab of dielectric grease inside the boot to help prevent moisture and make removal easier later. Do not coat the plug terminal excessively.

10. Reattach the HT lead
- Push the boot fully onto the plug until it seats. Replace the next plug and repeat the process for all cylinders, one at a time.

11. Final checks and start
- Reconnect battery if disconnected. Start the engine and listen for smooth idle. If misfires occur, check that each lead is pushed on fully and that you didn’t swap leads. If misfires persist, verify coil, distributor cap/rotor and points/condensor.

Testing and what to check after replacement
- Visual: all boots seated and clean.
- Functional: smooth idle, easier starting, improved throttle response.
- If still misfiring: check plug wire continuity (multimeter), distributor cap and rotor for cracks or corrosion, coil secondary resistance per manual, and ignition timing/points gap if applicable.

Common mistakes and how to avoid them
- Cross‑threading the plug: always start by hand and don’t force it.
- Over‑tightening: use a torque wrench or be conservative. Stripped head threads are expensive to repair.
- Applying anti‑seize indiscriminately: it changes torque feel — use only if head material or old threads justify it, and use sparingly.
- Pulling on wires: always pull the boot to avoid damaging the wire.
- Not cleaning around the hole: debris falling into the cylinder can cause severe damage when the engine starts.
- Mixing up plug wires: replace one at a time or label wires to preserve firing order.

Troubleshooting plugs you removed
- Wet oily plug: suggests oil entering combustion (piston rings or valve guide seals) or over‑priming at start.
- Dry black carbon plug: running rich (float level, carburetor adjustment, choked air intake) or weak spark.
- White or blistered plug: overheating or incorrect plug heat range.
- Rounded electrodes and large gap: normal wear, replace plug and re‑check timing/ignition performance.

Maintenance interval guidance
- For a working TE‑20, check plugs every season or every 100–200 hours depending on use. Replace as needed if worn or fouled. If you run on poor fuel or oil blows into cylinders, inspect more often.

Final practical tips (no nonsense)
- One cylinder at a time. Clean well. Hand‑start threads. Correct gap and torque. Use dielectric grease only on boots. Record what each removed plug looked like — it’s a diagnostic snapshot of that cylinder.
- Keep the distributor cap and rotor, and plug wires, in good condition; new plugs won’t fix bad wires or cap cracks.

If you want, you can print this and have it beside you when working. Follow the torque/gap/specs in the TE‑20 shop manual for your exact engine variant for final numbers.
rteeqp73

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