STIHL TS420 Repair Manual

The STIHL TS420 is a professional-grade cut-off saw designed for demanding construction, masonry, and rescue applications. This manual provides comprehensive repair, maintenance, and troubleshooting information for the TS420's engine, fuel system, cutting system, and safety features. Below are key sections covering safety, specifications, disassembly, component repair, assembly, and troubleshooting procedures.

Safety Precautions & Warnings

Before performing any repair, disconnect the spark plug boot to prevent accidental starting. Wear appropriate PPE (safety glasses, gloves). Work in a clean, well-ventilated area. Use only genuine STIHL replacement parts.

Hazard	Precaution
Fuel Fire/Explosion	Drain fuel tank before repair. Work away from ignition sources.
Hot Surfaces	Allow engine and muffler to cool completely before disassembly.
Sharp Edges	Handle cutting blade, chain, and metal parts with cut-resistant gloves.
Spring Tension	Use proper tools when handling recoil starter or clutch springs.
Chemical Exposure	Avoid skin contact with fuel, oil, and cleaning solvents.

Specifications & Technical Data

• Model: STIHL TS420 Cut-Off Saw
• Engine Type: 2-stroke, single cylinder, air-cooled
• Displacement: 64.1 cm³ (3.91 cu. in.)
• Power Output: 3.1 kW (4.2 HP)
• Fuel/Oil Mix: 50:1 (STIHL HP Ultra or equivalent)
• Idle Speed: 2,800 RPM
• Max. RPM (Cutting): 9,500 RPM
• Cutting Wheel Diameter: 300 mm (12 in.) max
• Weight (without blade): 9.8 kg (21.6 lbs)
• Ignition System: Electronic, CDI
• Carburetor: Zama C1Q-S267 or equivalent

Tools & Required Equipment

Standard mechanic's tool set, STIHL special tools (piston stop, flywheel puller, clutch tool), torque wrench, compression tester, spark tester, feeler gauges, cleaning brushes, parts cleaning solvent.

1. STIHL Special Tools: Flywheel puller (1110 890 4500), Piston stop (1108 890 4500), Clutch holding tool (0000 893 4303).
2. General Tools: Socket set (8mm-17mm), Torx bits (T20-T27), Screwdrivers, Needle-nose pliers.
3. Consumables: STIHL Ultra oil, High-temp grease, Gasket sealant, Clean rags.

WARNING! Using incorrect tools may damage components and void warranty.

Engine System Repair

Cylinder, piston, crankshaft, and crankcase service procedures.

1. Cylinder & Piston Removal: Remove muffler, intake manifold. Use piston stop to hold crankshaft; remove cylinder bolts; lift cylinder off carefully.
2. Piston Inspection: Check for scoring, ring groove wear. Measure piston skirt clearance (spec: 0.045-0.055 mm).
3. Crankshaft Seal Replacement: Remove clutch/flywheel; pry out old seals; press new seals flush with case.
4. Crankcase Disassembly: Split case after removing all fasteners; note bearing positions.
5. Compression Test: Should read 120-150 PSI (warm engine). Low compression indicates worn rings, cylinder, or leaking seals.

CAUTION! Label all parts during disassembly. Keep mating surfaces clean.

Fuel System Repair

Carburetor, fuel lines, tank, and filter service.

1. Carburetor Removal: Disconnect fuel line, throttle linkage. Remove mounting nuts.
2. Carburetor Rebuild: Disassemble; clean jets, diaphragms, gaskets in solvent. Replace diaphragms and gaskets with rebuild kit.
3. Fuel Line Replacement: Replace cracked or stiff lines. Ensure proper routing to avoid kinks.
4. Fuel Filter: Replace in-tank filter annually or if clogged.
5. Fuel Tank Vent: Clean vent hole in cap; replace if damaged.

Tip: Use compressed air to blow out passages. Never use wire to clean jets.

Cutting System & Drive Train

Blade spindle, clutch, gearbox, and guard maintenance.

Component	Service Procedure
Clutch	Remove with clutch tool; inspect shoes for wear; replace if worn or glazed.
Gearbox	Drain old grease; inspect gears for chipping; repack with STIHL gear grease.
Blade Spindle	Check for straightness and thread damage. Replace bent spindles.
Blade Guard	Inspect for cracks; ensure mounting points are secure.
Drive Shaft	Check for wear at coupling points; lubricate splines with grease.

WARNING! Always use a STIHL-approved cutting wheel. Never operate without guard.

Ignition & Electrical System

Spark plug, ignition coil, stop switch, and wiring harness.

1. Spark Plug: Gap 0.5 mm (0.020 in.). Replace if electrodes are worn or fouled.
2. Ignition Coil: Check air gap (0.2-0.4 mm). Test primary/secondary resistance with multimeter.
3. Stop Switch: Test for continuity when pressed. Replace if circuit remains open/closed.
4. Wiring Harness: Inspect for frayed wires, loose connections, especially near vibration points.
5. CDI Module: If no spark after coil/wire check, CDI may be faulty (requires replacement).

CAUTION! Do not crank engine with spark plug removed and boot disconnected—risk of CDI damage.

Cooling & Air Intake System

Air filter, cooling fins, and muffler maintenance for optimal performance and overheating prevention.

1. Air Filter: Clean with soapy water, dry thoroughly, re-oil with STIHL filter oil. Replace if torn.
2. Cooling Fins: Remove debris from cylinder head and flywheel fins with brush and air.
3. Muffler: Remove carbon deposits with wire brush. Check for cracks at mounting points.
4. Spark Arrestor: Clean screen with wire brush or replace if clogged (required for some worksites).
5. Intake Boot: Inspect for cracks that could cause air leaks and lean running.

Tip: Overheating is often caused by clogged cooling fins or lean fuel mixture.

Assembly & Torque Specifications

Critical fastener torque values for safe reassembly.

Fastener	Torque (Nm)	Torque (ft-lbs)
Cylinder Bolts	10-12	7.4-8.9
Clutch Nut	45-50	33.2-36.9
Flywheel Nut	40-45	29.5-33.2
Spark Plug	20-25	14.8-18.4
Blade Nut	45-55	33.2-40.6
Crankcase Bolts	8-10	5.9-7.4

Assembly Sequence: 1. Install crankshaft seals. 2. Join crankcase halves. 3. Install piston/cylinder. 4. Install clutch/flywheel. 5. Install carburetor/intake. 6. Install cooling shrouds.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, fuel issue, compression	Check spark, fuel flow, compression. Clean carburetor.
Engine starts then dies	Clogged fuel filter, tank vent, carb issue	Replace fuel filter, clean vent, rebuild carb.
Loss of power	Clogged air filter, exhaust, carb adjustment	Clean air filter, muffler. Adjust H/L carb screws.
Excessive vibration	Bent blade, unbalanced blade, loose mounts	Inspect/replace blade. Tighten all mounts.
Overheating	Lean mix, clogged cooling, bad fuel	Check for air leaks, clean fins, use proper fuel mix.
Clutch slipping	Worn shoes, greasy shoes, weak springs	Replace clutch assembly. Clean drum.

Reset: After repair, reset carburetor to factory settings: L screw 1 turn open, H screw 1 turn open, then fine-tune.

Parts Diagrams & Ordering

Use genuine STIHL parts for optimal performance and safety. Reference the following part groups:

• Group 1: Crankcase, Cylinder, Piston (Part #s vary by serial number)
• Group 2: Carburetor & Fuel System (Zama C1Q-S267 kit)
• Group 3: Ignition System (Coil: 4130 400 1301)
• Group 4: Clutch & Gearbox (Clutch assembly: 1141 160 2001)
• Group 5: Cutting Attachment (Spindle: 1141 160 2600)

Ordering: Contact an authorized STIHL dealer with your model and serial number for exact parts. Serial number located on crankcase.

Support: STIHL USA – 1-800-467-8445 | www.stihlusa.com