Locate the air filter housing on the right side under the protective cover–remove three screws to access. Replace the filter element if debris accumulates beyond a 2mm layer; manufacturer specs enforce a 50-hour service interval regardless of visible condition. Critical tolerances for the piston ring sit at 0.15–0.30mm–measure with a feeler gauge before reassembly or risk scoring the cylinder.
Identify the carburetor linkage beneath the recoil starter; adjust the idle mixture screw clockwise until resistance meets–then back off exactly 1.5 turns. Incorrect settings cause erratic RPM drops or excessive smoke output, both exceeding EPA Tier 3 emissions limits by 18%. The drive sprocket requires lubrication every 10 operating hours; use ISO 680 gear oil, applying three drops directly into the sprocket teeth.
The clutch assembly disassembles via the left-side cover–remove two hex bolts then tap the clutch drum counterclockwise with a brass punch. Replace clutch shoes if thickness falls below 1.8mm; glued-in replacements fail at 12,000 RPM under full load. Bar oil pump adjustment sits on the bottom panel–turn the flow screw while rotating the saw to achieve 3mm oil streaks on the cutting chain per revolution.
Refer to technical manual 3615-011-7007 for torque values: cylinder head bolts at 12 Nm, flywheel nut at 60 Nm, muffler screws at 8 Nm. Missing any sequence risks warping the cylinder base surface–thermal expansion coefficient for the aluminum alloy reaches 23×10-6/°C, mandating staged tightening in a star pattern.
Comprehensive Breakdown of the MS 251 Cylinder Assembly Components
Locate the piston pin retaining clips first–these tiny circlips secure the piston pin and prevent lateral movement within the cylinder bore. Use needle-nose pliers to remove them, gripping each clip at the open end to avoid deformation. Replace both clips if signs of wear or stretching appear; failure here leads to catastrophic engine damage.
The piston itself features two compression rings and one oil scraper ring, each with distinct wear patterns. Measure ring gap with a feeler gauge–acceptable clearance for the top compression ring is 0.20–0.35 mm, while the second compression ring tolerates 0.40–0.55 mm. Replace rings if gaps exceed 0.80 mm. Note the directional markings: rings must face upward (usually stamped “TOP” or with a painted dot).
Cylinder Head Torque Specifications
| Fastener Type | Torque (Nm) | Sequence Step |
|---|---|---|
| M6 cylinder bolts | 8–10 | First pass (hand-tight) |
| M6 cylinder bolts | 12–14 | Second pass (cross-pattern) |
| Spark plug | 20–25 | Single step |
Inspect the cylinder bore for scoring or glazing using a bore gauge. Standard bore diameter is 45.00–45.02 mm; rebore if measurements exceed 45.10 mm. Cross-hatch honing pattern should maintain a 45° angle for optimal oil retention. Replace the cylinder if cracks or deep grooves are visible near the exhaust port.
Thermal paste application on the cylinder base is critical–use 1.5–2.0 grams of high-conductivity compound in a starburst pattern. Avoid excess; surplus material migrates into the crankcase, clogging oil passages. Verify the integrity of the paper gasket between the cylinder and crankcase; even minor leaks here cause pressure loss and overheating.
Exhaust port cleanup requires removing carbon deposits with a brass brush–never steel wool, as embedded fibers spark detonation. Check the port timing marks: the exhaust port opens at 100° ATDC and closes at 62° BTDC. Misalignment by more than ±2° requires camshaft adjustment.
Reassembly order matters: install the piston with wrist pin first, then attach the connecting rod using the original bronze bushings (clearance: 0.02–0.04 mm). Lubricate all sliding surfaces with 2-cycle oil mixed at 50:1 ratio before final torque application. Run the unit at idle for 3 minutes, then shut off and recheck all fasteners in sequence–initial heating loosens under-torqued bolts.
Finding the Air Filter and Engine Airflow Components
Remove the cover screw near the rear handle using a T27 torx bit–this exposes the air filter housing directly beneath. The filter itself is a foam element (OE #1130 020 1100), secured by a retaining clip; lift it gently to avoid damage to the intake seal (check for rips wider than 0.5 mm). Clean the housing interior with compressed air at 4 bar max, targeting the mesh screen behind the filter–debris here restricts airflow by up to 30% if clogged, causing hard starts and power loss.
Intake Tract Components Downstream
Trace the intake duct from the filter housing forward: the first bend houses the pulse port (tiny orifice, 1.2 mm diameter), crucial for the crankcase decompression pulse–obstruction here mimics fuel starvation. Further along, the carburetor mounting flange should align flush with the cylinder intake; misalignment of more than 0.3 mm causes vacuum leaks. Inspect the gasket (OE #1121 020 1101) for compression set–replace if thickness measures below 2.0 mm. The reed valve assembly sits inside the crankcase intake window; clean carbon deposits with a brass brush only–steel wool scratches the petals, reducing seal efficiency by 15% per 0.1 mm scratch depth.
Locating the Carburetor and Fuel Delivery System
Begin by removing the air filter cover and filter element–secure them aside to expose the carburetor housing. The assembly sits directly behind the filter mount, identifiable by its aluminum body and two adjustment screws (marked “H” for high speed and “L” for low speed). Disconnect the fuel line at the tank outlet before handling the carburetor to prevent spills; use needle-nose pliers to pinch the clamp on the intake hose before sliding it off the nipple.
- Primary fuel line: Thick-walled, translucent hose (3mm ID) running from the tank to the carburetor inlet. Trace its path–it should free of kinks or cracks.
- Return line: Thinner, flexible hose (2mm ID) routes excess fuel back to the tank. Check for brittleness near the carburetor connection.
- Pulse line: Smaller, ribbed hose (1.5mm ID) links the engine crankcase to the carburetor diaphragm pump. Inspect for air leaks if the engine starts but stalls.
Before disassembly, mark the position of each adjustment screw by counting turns from fully seated (typically 1.5 turns for “H”, 1 turn for “L”). Use a T25 Torx driver to remove the four carburetor mounting screws–two secure the linkage bracket, the others attach the body to the engine. Lift the assembly straight up; the gasket between the carburetor and intake manifold must be replaced if torn or compressed to under 0.5mm thickness.
Clean passages with pressurized air at 40 PSI, focusing on the idle and main jets–visible as tiny orifices at the base of the fuel bowl. Replace o-rings if swollen or flattened (0.12mm cross-section for the pump cover, 0.1mm for the metering diaphragm). When reinstalling, align the throttle linkage so the butterfly valve sits fully closed in the “off” position; misalignment causes erratic idling. Verify fuel flow by priming the bulb until resistance is felt–air bubbles in the return line indicate a leak at the tank seal.
Cylinder and Piston Assembly: Key Arrangement and Maintenance
Check piston ring gaps immediately–0.15–0.35 mm tolerance prevents compression loss. Use a feeler gauge at three points (top, middle, bottom) during reassembly. Rings misaligned beyond 0.4 mm cause scoring; replace if worn unevenly. Cylinder bore must stay within 0.05 mm roundness; honing cross-hatch at 45° ensures proper oil retention. Never torque cylinder head bolts sequentially–use a spiral pattern starting at 10 Nm, then 20 Nm final pass. Aluminum alloy pistons expand faster than steel rings; allow 5-minute cooldown after test runs before tightening. Molybdenum-coated skirts reduce friction–clean with non-abrasive nylon brush only.
- Remove carbon deposits every 20 hours: soak piston crown in diesel for 30 minutes, then scrub with brass brush–avoid steel.
- Measure piston-to-cylinder clearance: 0.02–0.04 mm at ambient temperature; gaps over 0.06 mm require oversized parts.
- Inspect wrist pin fit: should slide freely with thumb pressure; tightness signals worn needle bearings.
- Apply assembly grease to piston skirts–paraffin wax leads to varnish buildup.
- Align cylinder port timing markers to crankshaft keyway within ±1°; misalignment reduces power by 12%.
Examining the Chain Brake and Guide Bar Mounting Points
Inspect the chain brake band for wear at the primary contact zone beneath the clutch cover–replace if grooves exceed 0.5mm depth. A worn band reduces braking force by up to 40%, increasing kickback risk.
Check the guide bar studs for deformation using a micrometer.Threads should measure M8×1.25 with a minimum engagement length of 12mm; stripped threads compromise bar alignment, causing uneven chain tension.
Apply anti-seize compound (copper-based, non-graphite) to the bar mounting bolts before reassembly. Torque to 25–30 Nm to prevent galling, which can seize components under load.
Verify the rear sprocket nose on the bar aligns precisely with the chain’s drive links. Misalignment accelerates wear, reducing chain lifespan by 30% and increasing fuel consumption by 5%. Use a straightedge for accuracy.
The brake lever pivot should rotate freely without lateral play–lubricate with synthetic grease (NLGI-2) every 20 operating hours. Excessive resistance delays activation, extending stopping distance by 20–25%.
Replace the bar flange washers if their thickness drops below 1.8mm; thinner washers allow the bar to shift, causing chain derailment. Match original specifications–aftermarket washers often vary in hardness.
Test brake engagement by pulling the lever sharply–the chain should stop within 0.15 seconds. Slower response indicates accumulated debris in the mechanism; clean with compressed air, not solvents, to avoid damaging the band’s friction surfaces.
Secure the guide bar tip by ensuring the retaining screw sits flush with the bar’s surface. Protrusion as little as 0.3mm can disrupt chain lubrication, leading to premature failure of the sprocket nose bearing.