Short, direct instructions from a tech’s viewpoint.

Important first note: Most MF255/265/270/275/290 tractors were built with diesel engines and use glow plugs, not spark plugs. If you have a gasoline/petrol variant (less common) use the “spark plug” procedure below. For diesel tractors follow the “glow plug” procedure.

Tools & supplies
- Metric socket set, extensions, universal joint (3/8" drive works best)
- Spark plug socket(s): 5/8" (16 mm) or 13/16" (21 mm) for petrol plugs; deep, thin-walled glow-plug socket for glow plugs (use correct size)
- Ratchet and breaker bar
- Torque wrench (0–100 Nm scale) — essential to avoid over/under-torque
- Spark-plug gap gauge (feeler gauge) — petrol only
- Small wire brush/soft-bristle brush and compressed air or shop vacuum
- Penetrating oil (e.g., PB Blaster) for seized plugs
- Dielectric grease (petrol wire boots) and light anti-seize (use sparingly; check plug manufacturer guidance)
- Multimeter (for testing glow plugs)
- Replacement spark plugs or glow plugs (correct part numbers for your engine)
- Shop rags, gloves, safety glasses
- Battery terminal wrench

Safety precautions
- Park on level ground, set parking brake, block wheels.
- Engine must be cold (cooling head/plug area) to avoid thread damage and burns.
- Remove ignition key and disconnect negative battery terminal before starting work (prevents shorts and accidental cranking).
- Work in well-ventilated area if you'll run the engine after service.
- Wear gloves and eye protection. Do not apply excessive force that could break a plug in the head.

Procedure — Spark plugs (petrol engines)
1. Prepare: disconnect battery negative. Remove any obstructing intake or air cleaner parts to access the plug wells.
2. Clean: blow compressed air or brush around each plug well to remove dirt/debris. Do not let debris fall into cylinder.
3. Remove lead/boot: firmly grasp the boot (not the wire) and twist/pull to remove. If stuck, use a boot puller or apply penetrating oil to the boot seam, then pull.
4. Loosen: fit the correct spark-plug socket with extension and ratchet. Break plug loose turning counterclockwise. Use steady controlled force — do not use cheater bars on ratchet.
5. Extract: remove plug using the socket with rubber insert or magnet to hold it. Inspect electrode and porcelain for signs (oil, carbon, overheating).
6. Set gap: check the replacement plug gap with a feeler gauge and adjust to the engine spec (if you don’t have the factory spec, typical small petrol engines use 0.025–0.035" / 0.6–0.9 mm — verify). Never file center electrode aggressively.
7. Install new plug: apply a very light smear of anti-seize on threads only if recommended by plug manufacturer (note: many modern plugs are fine without anti-seize). Hand-thread the plug several turns to avoid cross-threading.
8. Torque: tighten with a torque wrench to the manufacturer spec. Typical spark-plug torque is in the 20–30 Nm (15–22 lb-ft) range; verify if possible. If you don’t have a torque wrench, tighten snugly by hand then 1/8–1/4 turn with ratchet — less reliable.
9. Reattach boot: apply a small dab of dielectric grease inside the boot, push on until it clicks/seats. Ensure correct firing order if multiple leads/wires.
10. Reconnect battery negative, start engine and check operation.

Common petrol pitfalls
- Pulling on the wire instead of the boot tears the lead.
- Cross-threading plugs — always hand-start threads.
- Over-torquing or using excessive anti-seize leads to plug or head damage.
- Not cleaning the well first — debris can fall into the cylinder.
- Wrong plug heat range or gap causes poor running.

Procedure — Glow plugs (diesel engines, most MF2xx tractors)
1. Prepare: engine cold. Disconnect negative battery terminal and remove any covers/air intake parts obstructing access to the head.
2. Locate glow plugs: typically one per cylinder in the cylinder head. You’ll see a small plug with electrical connector.
3. Clean: blow compressed air and brush around each glow-plug area. Remove dirt to prevent contamination when plug is removed.
4. Disconnect electrical connector: carefully release the clip/connector. Do not yank wiring.
5. Test (optional but recommended): use a multimeter to measure resistance across the glow plug. Typical good resistance values are low (roughly 0.5–2 Ω) — consult specs. You can bench-test by applying 12 V briefly to check heating, but do so safely.
6. Remove glow plug: use the correct deep glow-plug socket and extension. Break it loose with a ratchet, remove straight out. If it’s seized, apply penetrating oil, let soak; you can gently heat the area with a heat gun (not open flame) to help free it. Move slowly to avoid snapping the plug.
7. Inspect threads and seats for damage or carbon buildup; clean gently.
8. Install new glow plug: hand-thread only initially. Lightweight anti-seize is optional — check manufacturers; don’t overdo it in aluminum heads. Hand-thread then torque to spec. Glow-plug torque is lower/sensitive — typical values ~10–25 Nm (7–18 lb-ft) depending on plug type — use manual if available.
9. Reconnect electrical connector. If the harness has corrosion, clean terminals.
10. Reconnect battery negative, start tractor. Diesel glow plugs don’t produce a spark — confirm quick starting and no fault codes; test glow plug relay if there are starting issues.

Common glow-plug pitfalls
- Breaking a glow plug off in the head (very difficult to remove). Take care, use penetrating oil and patience.
- Over-torquing fragile glow plugs or using the wrong socket that crushes the hex — use proper thin/well-fitting socket.
- Not testing plugs before replacement — sometimes only one is bad, and harness/relay may be at fault.
- Using excessive anti-seize in aluminum heads, which can alter torque and sealing behavior.
- Reconnecting wrong or corroded connectors causing intermittent operation.

Replacement parts required
- Correct spark plugs or glow plugs for your engine (use OEM or quality aftermarket; match thread size, reach, heat range and connector type).
- Optional: new plug wires/boots (petrol), dielectric grease, anti-seize, replacement electrical connectors/harness if corroded.

How tools are used (quick notes)
- Spark-plug socket: rubber insert holds the plug so it doesn’t drop into the head on removal. Deep socket required for recessed plugs and most glow plugs.
- Torque wrench: set to correct value and tighten slowly to that exact torque to avoid stripping head threads or snapping plugs.
- Multimeter: resistance check across glow-plug terminals; very low resistance indicates heater intact. Compare across cylinders.
- Feeler gauge: measure gap between center and ground electrode for petrol plugs. Turn the ground electrode slightly if adjustment is necessary (do not damage insulator).
- Penetrating oil + heat: used on seized glow plugs only; allow soak time and don’t force.

Final notes (practical tech tips)
- If a glow plug snaps, stop and plan for extraction—don’t over-torque hoping it will come out. A broken plug often requires evasive repair (left in place may allow compression leak).
- Replace all glow plugs as a set if one fails and engine has high hours — uneven heating causes rough running.
- Keep track of order and orientation of wires/boots. Mark leads if necessary.
- Keep service manual or OEM torque specs handy; these tractors have variations so exact torque and plug types can vary by engine model.

Done.
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Massey Ferguson 255 PTO engage issue solved PTO would grind when attempting to engage on the engine side, easy fix this time.

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