Replace worn components precisely by referencing a detailed schematic of your professional-grade chainsaw model. The AV system–particularly older but reliable variants–requires exact part identification for optimal performance. Obtain the official exploded view from an authorized dealer or technical manual; third-party diagrams often omit critical tolerances, leading to misalignment.
Step 1: Locate the crankcase assembly in the diagram–typically Section 4 in manufacturer schematics. Verify each gasket thickness matches OEM specifications (0.2mm for base models, 0.3mm for reinforced versions). Even minor deviations disrupt compression ratios, increasing fuel consumption by up to 18%.
Step 2: Cross-reference piston rings (Part Group 6.1) against the cylinder bore measurement. Use a micrometer to confirm wear limits: 0.15mm maximum for standard conditions, 0.1mm for high-altitude variants. The diagram’s numeric labels correspond to torque values–ignore these at your peril. For instance, the flywheel nut (Item 12) demands 45 Nm; over-tightening shears the keyway.
Electrical components–spark plug (7.3) and ignition module (8)–rarely appear in aftermarket schematics. The official diagram includes proprietary resistor values essential for cold-start reliability. Replace the spark plug every 50 hours of operation, regardless of visual condition. The wiring harness (Section 9.7) uses color-coded leads not reflected in generic PDFs: blue/red stripe denotes the kill switch circuit, while solid green grounds the condenser.
Procure a physical copy of the schematic if possible. Digital versions often compress layered assemblies into single views, obscuring small but vital parts like the carburetor diaphragm (3.4a) or the needle valve spring (3.4c). The diaphragm, though seemingly insignificant, regulates fuel flow within ±0.02mm; errant replacements cause erratic idle or engine flooding.
Visual Breakdown of the MS 660 Chainsaw Components
Obtain the official exploded-view schematics directly from the manufacturer’s repair manual–version 15 or later–for precise part numbers. Ignore third-party reproductions; they often mislabel gaskets and retaining rings, leading to incorrect reassembly.
Focus on sheet 3 of the manual first. It details the crankcase assembly, including the piston, cylinder, and flywheel. Note the orientation marks on the piston pin clips–improper seating will score the cylinder bore within ten operation hours.
Locate the carburetor diagram on sheet 7. The MS 660 uses a Walbro WT-690 variant; the throttle shaft bushing differs from earlier WT models. Replace both the diaphragm and gasket simultaneously–failure to do so risks lean mixture calibration errors.
When servicing the clutch, refer to sheet 5. The drive sprocket bearing (part 1136-120-4003) requires precise torque: 45 Nm. Over-tightening strips the crankshaft threads; under-tightening causes play, accelerating sprocket wear.
Check the ignition module alignment on sheet 9. The air gap between the flywheel magnets and ignition coil must measure 0.3mm. Use a non-magnetic feeler gauge–standard steel gauges distort readings.
Replace the starter rope only with OEM-spec braided nylon. Aftermarket ropes fray at the pulley contact point, causing uneven retraction. Lubricate the recoil spring with molybdenum disulfide grease during reassembly to prevent corrosion.
Study sheet 12 for the fuel tank assembly. The pickup filter screen (part 0000-417-6200) clogs rapidly in dusty conditions–clean it every 20 operating hours with compressed air, not brushes.
For final driveshaft servicing, consult sheet 4. The needle bearing (1137-010-2000) requires a dedicated extraction tool–hammering damages the housing. Pack bearings with high-temperature grease before installation; regular lubricants degrade above 150°C.
Locating Critical Powerhead Elements in the Saw’s Schematic
Begin with the cylinder assembly marked near the top of the exploded view–item number typically falls between 12 and 18 depending on the revision. Identify the piston first: it should show two compression rings and a single oil-scraper ring. Verify ring gaps align with 0.2–0.4 mm specifications; anything outside this range requires immediate replacement to prevent pressure loss.
Trace fuel system components next. The carburetor sits adjacent to the intake manifold, often depicted as a silver housing with three adjustment screws. The central screw controls idle speed–initial setting: 1 full turn from lightly seated. Left and right screws govern low- and high-end fuel mix; base setting: 1.25 turns open. Locate the impulse diaphragm beneath the mounting plate–cracks mandate replacement.
- Air filter housing connects via rubber grommet–inspect for hardening.
- Fuel pump diaphragm sits beneath the carb plate–test by blowing into impulse port.
- High-pressure fuel line threads into the metering chamber; torque 3 Nm.
Examine ignition group elements placed under the flywheel cover. Flywheel magnets must sit flush against the stator plate–measure gap with 0.3 mm feeler gauge. Ignition coil mounts above the crankshaft seal; resistance across terminals should read 3–5 kΩ. Spark plug connects via boot; torque 20 Nm and gap 0.5 mm.
Inspect crankshaft seals marked on the rear side of the block. Output side seal dimensions: 18×30×7 mm–replace alongside main bearings if play exceeds 0.05 mm. Input side seal mirrors dimensions but features a sealing lip facing outward. Retainers must sit flush against crankcase halves–uneven seating causes oil bypass.
Diagnosing Common Wear Points
Check connecting rod small-end needle bearing for brinelling–rotate piston manually to detect roughness. Rod cap torque: 8 Nm in cross pattern; reuse bolts only if thread stretch stays below 4%. Big-end bearing clearance must not exceed 0.03 mm–use plastigage to confirm.
- Wrist pin diameter: 10 mm–measure with micrometer at three points.
- Cylinder bore wear limit: 48.2 mm–honing marks should retain cross-hatch.
- Exhaust port flange: 8 screws–torque 6 Nm in spiral sequence.
Identify cooling shroud assembly mounted over the cylinder. Impeller blades must sit centered–misalignment reduces air velocity by 15%. Recoil starter engages via pawl spring–test spring tension by rotating starter 30°; smooth return indicates proper tension.
How to Find and Swap the Fuel Mixing Unit with the Chainsaw’s Schematic
Begin by identifying the air filter housing–locate it on the right side of the powerhead, secured with two screws. Remove these screws with a T27 torx driver and set the housing aside. The carburetor sits directly beneath, connected by two fuel lines (marked “L” for low-speed and “H” for high-speed) and a throttle linkage. Use needle-nose pliers to gently pinch and slide off the fuel line clamps, then pull the lines free. Disconnect the linkage by rotating it counterclockwise until it detaches from the control arm.
- Label each component as you remove it–masking tape and a marker work best.
- Photograph the orientation of the fuel lines and linkage before disassembly to avoid reassembly errors.
- Check the gasket between the carburetor and engine block; replace if cracked or flattened.
Install the replacement unit by aligning it with the mounting studs, then hand-tighten the nuts to avoid cross-threading. Reattach the fuel lines, ensuring the “L” and “H” ports match their original positions–swap them, and the saw will run lean or flood. Secure the lines with new clamps to prevent vacuum leaks. Reconnect the throttle linkage, rotating it clockwise until it snaps into place. Reinstall the air filter housing and tighten the screws to 8 Nm torque. Prime the carburetor with 5-6 bulb presses, then start the saw and adjust the idle screw until the chain stops spinning.
How to Locate Engine Core Components in Schematic Drawings
Begin by identifying the exploded view section labeled “Power Unit” or equivalent–manufacturers often group major internal assemblies under this heading. In most manuals, the piston assembly appears near the top of this segment, directly below the crankcase but above the connecting rod.
Scan for numeric identifiers adjacent to the illustrations. Piston rings are typically marked with sequential numbers (e.g., 4-6), while the piston itself carries a single primary designation. Cylinder liners usually follow immediately after, distinguished by their cylindrical shape with visible port windows.
| Component | Typical Identifier Range | Visual Clues |
|---|---|---|
| Piston | 1200-1250 | Grooved crown, wrist pin bore |
| Cylinder | 1251-1300 | Reed valves, exhaust ports |
| Rings | 1301-1320 | Split gaps, varying thicknesses |
Check for exploded callouts where individual elements are pulled away from their assembled positions. The piston should appear separated from the cylinder sleeve, revealing the full extent of its skirt and compression ring grooves. Look for dashed lines indicating removal direction–these help distinguish front/back orientation.
Cross-reference any text annotations. Manufacturers often include material specifications or tolerances next to components; cylinders may be labeled “Nikasil” or “chrome-plated,” while pistons might specify anodized surfaces for heat resistance. These details narrow mistaken substitutions.
If dimensions aren’t printed, measure the physical reference print using calipers against the schematic. Piston diameters usually scale 1:1 in mm on A3-sized manuals. Cylinders often include bore measurements in side-profile views where port heights and widths are visible.
Compare adjacent diagrams if available–some manuals split assemblies into sub-views for clarity. The crankshaft end might appear in one illustration while the piston top receives its own detailed section showing ring arrangements and wrist pin offset.
Request specific microfiche frames if working from archival scans. Cylinder ports and piston crown details blur in low-resolution copies; frame numbers like “14-A-7” direct archivists to high-definition close-ups of each component’s machined surfaces.