Locate the engine assembly first–this is the core of your machine. Use the official breakdown schematic from Stihl’s service manual (part 5910-108-4500) to match each element. The air filter housing sits directly above the carburetor, secured by two Torx T25 screws. Replace this every 50 operational hours if working in dusty conditions.
The guide bar mounting system includes a sprocket nose, which requires inspection after each use. Measure wear with a depth gauge–exceeding 0.5mm means immediate replacement. Stihl’s original bars (model numbers starting with 3002) fit precisely; third-party options often lack hardened rails, reducing lifespan by 30%.
For the chain tensioning mechanism, note the spring-loaded adjuster behind the clutch cover. Turn clockwise until the chain rests snugly against the bar’s bottom, then extend it by one full rotation. Over-tightening strains the crankshaft bearings–monitor engine temperature after adjustment. Use only Stihl’s RS or PM3 chains; alternatives stretch 15% faster under load.
Fuel system components demand exact matches. The impulse line (black, 3mm diameter) connects the carburetor to the fuel pump–swap it annually. The fuel filter (part 0000-358-5120) traps debris; clogging reduces RPM by 400-600 under load. Store petrol with Stihl MotoMix to prevent varnish buildup; standard fuel leaves deposits in 40 hours.
The clutch assembly engages at 2,800 RPM. Worn shoes reduce cutting efficiency–listen for slipping under throttle. Replace the entire clutch (part 1121-640-2050) if grooves exceed 1.5mm. The flywheel’s cooling fins must remain unobstructed; clean them with compressed air every 10 hours to prevent overheating.
For electrical troubleshooting, test the ignition module (part 1119-400-1302) with a multimeter: resistance should read 5-7 kΩ at 20°C. Replace the spark plug (NGK BPMR7A) every 25 hours–fouling drops power by 20%. Always gap at 0.5mm; incorrect spacing causes misfires.
Understanding the Engine Assembly Breakdown for Stihl’s 251 Chainsaw Model
Begin by locating the crankcase cover–marked as component #1125 020 2000–secured with three Torx T27 bolts. Removal requires an impact driver due to factory-applied thread locker; without it, bolts may shear. Note the orientation of the gasket (part #1125 000 5200) during disassembly, as improper reinstallation risks oil leaks from the bar lubrication system.
Inspect the piston (#1125 030 0500) by rotating the crankshaft manually after removing the flywheel housing. Look for scoring or carbon buildup on the piston skirt–excessive wear (>0.1mm deviation from OEM specs) indicates urgent replacement. The cylinder (#1125 020 0100), held by four M6x35 bolts, should show crosshatch honing marks; polished areas suggest overheating and mandate a rebuild.
Critical Components Hidden Behind the Crankcase
The crankshaft (#1125 020 1000) features dual sealed bearings (#6204-2RS); verify play with a dial indicator–readings above 0.03mm necessitate bearing replacement. The connecting rod (#1125 020 1200) uses a floating wrist pin; lubricate it with Stihl’s HP Ultra oil during reassembly to prevent galling. Ignore generic alternatives–viscosity mismatch accelerates wear.
Examine the ignition module (#1130 400 1401) by checking resistance across the coil (8–12 kΩ) and spark plug terminal (0.8–1.2 kΩ). Deviations signal coil failure; substitute only with OEM components, as aftermarket units often lack the flyback diode, risking ECM damage. The flywheel (#1125 020 0800) must sit flush–misalignment disrupts ignition timing, causing hard starts.
Replace the air filter (#1111 120 0600) every 20 hours of operation in dusty conditions; a clogged filter richens the fuel mix, fouling the spark plug (#NGK BPMR7A) within 5 operating cycles. Use the OEM spark plug gauge (#5910 700 8700) to set the gap at 0.5mm–over-tightening (beyond 22 Nm) cracks the insulator.
Lubrication and External Systems Checklist
Flush the oil pump (#1125 640 0610) with kerosene if the chain oiler spurts inconsistently; residue buildup starves the bar groove, increasing friction. The bar adjuster screw (#1125 640 1300) should turn smoothly–apply anti-seize to the threads to prevent galling under high torque (18–22 Nm). For the fuel system, the carburetor (#1111 120 0200) requires calibration only after verifying the fuel filter (#1130 150 0900) is unobstructed; ethanol-blended fuels degrade the filter membrane in under 100 hours.
Finding Critical Elements in the Stihl Model 251 Exploded View
Begin by identifying the air filter assembly (item 12 on most schematics). It sits directly behind the carburetor intake and is secured by a single screw or quick-release latch. Cleaning or replacing this component every 25 operating hours prevents engine damage from debris.
The flywheel (typically item 35-40) is located on the engine’s left side beneath the starter housing. Verify its condition during maintenance–cracks or bent fins require immediate replacement to avoid ignition timing issues. Use a flywheel puller (spec #1110-890-7003) for safe removal.
Trace the fuel line from the tank (item 5) to the carburetor (item 6). Ensure the fuel filter (item 10) is seated properly; a loose fit allows air leaks, causing erratic idling. Replace the filter if fuel flow diminishes–symptoms include sputtering or failure to start.
Locate the cylinder (item 22) by following the piston’s path downward from the spark plug. The cylinder’s fins must be free of dirt; blocked airflow reduces cooling efficiency. Inspect for scoring–deep grooves indicate worn piston rings, necessitating a rebuild kit (spec #1121-400-3340).
The chain brake band (item 53) wraps around the clutch drum. Check for fraying or excessive wear, which reduces braking force. Adjust tension via the brake’s adjusting screw (item 54) if the chain engages sluggishly.
For gear-related repairs, focus on the drive sprocket (item 33). It connects to the clutch drum–removal requires a sprocket extractor (spec #5910-200-7000). Worn teeth lead to chain slippage; compare to a new sprocket (spec #1123-640-2000) for wear patterns.
Electrical Components
- Ignition module (item 9): Positioned beneath the flywheel, it’s critical for spark timing. Test resistance (8-12 kΩ typical) with a multimeter; irregular readings signal failure.
- Stop switch (item 7): Wiring harnesses often corrode at terminals. Apply dielectric grease during reassembly to prevent oxidation.
Tools for Efficient Disassembly
- Torx driver set (T25, T30)–essential for housing screws.
- Feeler gauges–adjust spark plug gap to 0.020 in (0.5 mm).
- Barbecue torch–heat stubborn crankshaft nuts (item 27) for removal.
Label each sub-assembly’s screws by size and location. M5 bolts secure the handlebar; M6 bolts fasten the cylinder. Mixing them risks thread stripping. Refer to the torque specs listed in the schematic–crankcase bolts require 18 ft-lbs, while clutch housing bolts need 12 ft-lbs.
Step-by-Step Guide to Locating Components in Stihl’s 251 Powerhead Assembly
Start by securing the unit on a stable workbench, removing the spark plug wire to prevent accidental ignition. Flip the housing sideways to expose the side cover–this often reveals screws or clips holding the outer casing in place. Use a T25 Torx driver for most fasteners; newer models may require a T30 for tighter tolerances. Mark each screw with tape or a magnetic tray to track their original positions, as mixing them up during reassembly can damage threads.
Cylinder and crankcase separation demands caution: tap gently along the seam with a rubber mallet to avoid cracking the aluminum. If sealed with gasket adhesive, apply a heat gun (120°F for 30 seconds) to soften the bond–never pry with metal tools. Inside, note the orientation of the piston rings (gaps at 120° intervals) and crankshaft seals (lip facing outward). Label wires from the ignition module and cooling fins to ensure correct reconnection; misrouting can cause overheating.
Inspect the flywheel magneto gap next–adjust to 0.2mm using a non-magnetic feeler gauge. The recoil starter assembly detaches via three screws; check the spiral spring for fraying before reusing. Replace the air filter cartridge every 20 operating hours if working in dusty conditions, as debris here accelerates wear on the carburetor diaphragms. Store removed components in order of disassembly to streamline reassembly.
Mastering the Chainsaw Fuel System Blueprint
Locate the reference grid printed along the edges of the schematic. Each component is tied to an alphanumeric label–e.g., “G-7” for the idle speed screw. Cross-reference these with the exploded view’s numerical list to pinpoint exact placement before disassembly. Ignoring this step risks swapping parts between similar models, compromising calibration.
Key Annotations Demystified
| Symbol | Meaning | Action |
|---|---|---|
| Dashed lines | Flexible hoses or linkages | Trace path before cutting to avoid kinks |
| Solid arrows | Fuel/airflow direction | Ensure orientation matches arrow when reinstalling |
| Number in circle | Torque specification (Nm) | Use a torque wrench for fasteners marked this way |
Remove the air filter cover first; the diagram’s layers follow a front-to-back sequence mirroring disassembly order. Components like the diaphragm (item 12) are often drawn with partial transparency–this indicates they sit behind visible parts. Store screws with their matching washers as shown; mixing them causes leaks. Note the choke plate’s serrated edge aligns only one way–reverse installation floods the engine.
Check for shadowed outlines around critical components like the main jet (item 23). These indicate press-fit or interference assembly; force required is 5-8 N·m–exceeding this cracks the housing. For gaskets (item 8), the diagram shows material thickness in millimeters–OEM replacements must match within ±0.1 mm to prevent vacuum leaks. If the schematic includes a magnified inset, it highlights stress points; clean these areas with 1200-grit paper, never wire brush.
Final Verification Before Reassembly
Compare the actual fuel bowl’s float height against the side-view cross-section. Deviations over 0.5 mm from the drawn position require adjustment–bend the tang using needle-nose pliers, never a screwdriver. Before sealing the carburetor, rotate the throttle shaft through its full range; binding at any point signals misaligned linkage or deformed gasket. Recheck all connections against the legend–omitting this step voids the manufacturer’s warranty.