STIHL HT 75 Service Manual
The STIHL HT 75 is a professional-grade pole pruner designed for demanding arborist and landscaping work. This manual provides comprehensive service, maintenance, and repair information for the HT 75 engine and cutting system, including safety procedures, technical specifications, troubleshooting, and detailed disassembly/assembly instructions. Below are key sections for safe operation, maintenance schedules, engine service, cutting attachment service, and parts identification.
Safety Information & Warnings
Before performing any service, read and understand all safety instructions. The HT 75 is a powerful tool that can cause serious injury or death if improperly handled.
| Hazard | Precaution |
|---|
| Fuel Fire/Explosion | Service in well-ventilated area away from sparks or flames. Use only approved fuel containers. |
| Hot Surfaces | Allow engine to cool completely before servicing. The muffler and cylinder can cause severe burns. |
| Sharp Cutters | Always wear cut-resistant gloves. Ensure cutting attachment is stopped and disconnected before handling. |
| Spring Force | The starter recoil spring is under tension. Use proper tools and procedures for disassembly. |
| Carbon Monoxide | Never run engine indoors or in enclosed spaces. Exhaust gases are poisonous. |
Specifications & Identification
Key identifiers and technical data for the STIHL HT 75.
- Engine Type: STIHL single-cylinder, 2-stroke, air-cooled.
- Displacement: 31.8 cc (1.94 cu. in.).
- Power Output: 1.4 kW (1.9 hp).
- Fuel/Oil Mix: 50:1 (STIHL HP Ultra or equivalent ISO-L-EGD certified oil).
- Idle Speed: 2,800 RPM (factory set).
- Maximum Power Speed: 9,500 RPM.
- Cutting Attachment: STIHL Polycut™ head or circular saw blade.
- Drive System: Flexible drive shaft with right-angle gearhead.
- Weight (Powerhead): 5.7 kg (12.6 lbs).
- Serial Number Location: Stamped on crankcase near the muffler.
Maintenance Schedule
Regular maintenance is critical for performance, longevity, and safety.
- Before Each Use: Check for loose fasteners, fuel leaks, and cutter sharpness/condition.
- Daily (or every 5 hours): Clean air filter, check spark plug, inspect cutting attachment for damage.
- Weekly (or every 25 hours): Clean spark arrestor screen (muffler), check fuel filter, inspect drive shaft housing.
- Monthly (or every 100 hours): Clean carburetor (external), check all fuel lines, inspect gearhead grease.
- Annually (or every 300 hours): Replace fuel filter, inspect piston/cylinder through exhaust port, check clutch springs.
WARNING! Use only genuine STIHL replacement parts. Non-approved parts may void warranty and cause unsafe operation.
Fuel System Service
Components: Fuel tank, filter, lines, carburetor (Zama C1Q-S series).
- Fuel Line Replacement: Drain tank. Remove tank cover. Pull old lines from tank grommet and carburetor. Install new lines, ensuring they reach the bottom of the tank.
- Carburetor Adjustment (H/L Screws): Initial settings: Turn H and L screws gently clockwise to stop, then counterclockwise 1 turn each. Fine-tune at operating temperature.
- Fuel Filter: Replace by pulling from fuel line inside tank. Soak new filter in fuel before installation.
- Diaphragm Service: Remove carburetor cover. Check pump and metering diaphragms for cracks or stiffness. Replace as a set.
CAUTION! Gasoline is extremely flammable. No smoking during service.
Ignition System Service
Components: Flywheel, ignition module (coil), spark plug (Bosch WSR6F or equivalent), stop switch.
- Spark Plug: Gap: 0.5 mm (0.020 in). Replace if electrodes are worn or ceramic is cracked.
- Ignition Module Air Gap: Set with feeler gauge (0.3 mm / 0.012 in) between coil legs and flywheel magnets. Loosen mounting screws to adjust.
- Stop Switch Test: Disconnect wire from module. Check continuity to ground; switch should show open circuit when "ON" and short to ground when "OFF".
- Flywheel Removal: Use STIHL flywheel holder (1129 893 4500) and puller. Torque on reinstallation: 40 Nm (30 ft-lb).
Engine & Cylinder Service
Procedures for top-end service.
- Cylinder Removal: Remove muffler, carburetor, and intake manifold. Unbolt cylinder base (4x 6mm bolts). Lift cylinder off carefully.
- Piston Inspection: Check for scoring, ring groove wear, and pin bearing play. Replace if deep vertical scratches are present.
- Cylinder Inspection: Look for scoring, plating flaking (Nikaseal), or excessive wear. Measure bore diameter.
- Piston Ring Installation: Ring end gap should be 0.2-0.4 mm. Ensure ring locating pins are in place in ring grooves.
- Crankcase Seals: Check for leaks. Replace by prying out old seals and pressing new ones flush.
Tip: Lubricate piston and rings with 2-stroke oil during assembly.
Cutting Attachment Service
Service for Polycut™ cutting head and drive components.
- Line Head Disassembly: Remove cover screw. Replace worn cutting line or entire spool assembly. Ensure line is wound in direction of arrow.
- Drive Coupling: Inspect for worn or cracked dogs. Replace if engagement is sloppy.
- Clutch Inspection: Remove clutch drum. Check clutch shoes for wear and springs for fatigue or stretching.
- Shaft Bearing: At gearhead end, check for smooth rotation. Replace if gritty or loose.
WARNING! Always disconnect spark plug wire before servicing cutting attachment.
Drive Shaft & Gearhead Service
The flexible drive shaft transmits power from the engine to the cutting head.
- Shaft Removal: Disconnect from engine and gearhead. Inspect for kinks or broken wires in the core.
- Gearhead Disassembly: Remove retaining clips and housing. Inspect bevel gears for chipped teeth. Repack with STIHL grease (Part # 0781 400 0805).
- Seal Replacement: Replace shaft seals at both ends if grease is leaking or contamination is present.
- Bushing Wear: Check bushings in the shaft tube. Excessive play can cause vibration and premature failure.
Troubleshooting Guide
| Symptom | Possible Cause | Corrective Action |
|---|
| Engine will not start | No spark, flooded, fuel issue | Check spark, dry plug, verify fuel flow and mix. |
| Engine starts then dies | Clogged fuel filter, carburetor adjustment, air leak | Replace filter, adjust L screw, pressure test crankcase. |
| Loss of power | Clogged air filter, exhaust screen, poor compression | Clean/replace filter, clean spark arrestor, check compression (≥ 120 psi). |
| Excessive vibration | Bent drive shaft, unbalanced cutter, loose engine mounts | Inspect shaft, check cutter, tighten all fasteners. |
| Cutting head does not spin | Sheared drive pin, clutch failure, shaft disconnect | Replace shear pin, inspect clutch, ensure shaft is engaged. |
Torque Specifications
Critical fasteners and their torque values. Use a calibrated torque wrench.
- Cylinder Base Nuts (6mm): 10 Nm (88 in-lb)
- Muffler Bolts: 8 Nm (71 in-lb)
- Spark Plug: 20 Nm (15 ft-lb)
- Clutch Nut: 40 Nm (30 ft-lb) - Left-hand thread
- Flywheel Nut: 40 Nm (30 ft-lb)
- Gearhead Housing Screws: 6 Nm (53 in-lb)
Parts Diagrams & Ordering
Refer to the official STIHL HT 75 Parts Catalog (Document # 991 500 0650) for exploded views and part numbers. Always provide the serial number when ordering parts.
Common Replacement Parts:
- Air Filter: 0000 120 1620
- Spark Plug: 0000 400 1305 (Bosch WSR6F)
- Fuel Filter: 0000 350 3501
- Carburetor Repair Kit: 0000 350 0700
- Piston Kit: 0000 030 2015
- Drive Shaft: 4130 713 2600
Service Support: Contact an authorized STIHL servicing dealer or visit www.stihlusa.com for technical bulletins and dealer locator.
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