Fixing a stalling or unresponsive saw starts with knowing your hardware. The primary keyword target is the Craftsman 18 42cc chainsaw parts diagram, a map used by DIY mechanics to locate components like the carburetor assembly, clutch drum, ignition coil, and oil pump gear. When your saw loses compression, floods on startup, or fails to oil the bar, tracking down the exact part number prevents costly diagnostic mistakes at a small engine repair shop.
This guide breaks down every core sub-assembly of the Craftsman 42cc 18-inch gas chainsaw, details common failure points for each zone, and provides step-by-step diagnostic paths to get your equipment running reliably.
Engine Cylinder and Piston Assembly
The top-end assembly forms the core of your two-stroke gas chainsaw. It houses the combustion chamber where fuel ignition drives the piston downward.
Looking at the cylinder and piston schematic, you'll spot several critical components:
- Cylinder head and fins: Designed to dissipate extreme combustion heat. Caked-on sawdust and pitch reduce cooling efficiency, leading to overheating and scoring.
- Piston and piston rings: The piston moves vertically within the cylinder bore, sealed by one or two metal piston rings that maintain compression.
- Wrist pin and needle bearing: Connects the piston to the upper end of the connecting rod.
When examining parts diagrams for a drop in compression, check the cylinder walls through the exhaust port. Vertical scratches indicate that foreign debris entered the combustion chamber or that straight gas was run through the engine instead of a proper 40:1 or 50:1 two-stroke mix. Replacing a scored piston requires pulling the shroud, detaching the muffler, and unbolting the four cylinder base screws.
Carburetor and Fuel Delivery System
Fuel delivery failures account for the vast majority of chainsaw starting issues. The Craftsman 42cc fuel system routes gasoline from the tank through a weighted fuel filter and flexible fuel line directly into the carburetor inlet.
Inside the carburetor, a system of internal diaphragms, tiny metering jets, and check valves balances fuel pressure and air intake.
- Primer bulb and fuel lines: Over time, ethanol-blended gasoline degrades rubber fuel lines, causing them to crack, harden, or leak air. A spongy or cracked fuel line prevents the carburetor from drawing fuel.
- Carburetor diaphragms: Stiff or warped internal gaskets prevent proper fuel metering, causing the saw to bog down when you squeeze the throttle.
- High and low speed adjustment screws (H and L): These needle valves regulate the fuel-to-air ratio at idle and wide-open throttle.
If your saw starts with starting fluid but dies immediately, trace the fuel line on your diagram from the tank to the carburetor inlet. A clogged inline fuel filter or a varnished carburetor throat requires a complete teardown and cleaning with carburetor cleaner and compressed air.
Ignition and Starter Assembly
When you pull the starter cord, you're spinning a flywheel past a stationary ignition module to generate electrical current for the spark plug.
The ignition subsystem relies on three primary parts:
- Recoil starter assembly: Features a starter rope, pulley, and rewind spring housed in a plastic cover. If the rope fails to retract, the internal rewind spring has slipped or snapped.
- Flywheel and ignition coil (module): As the flywheel spins, its internal magnets pass the ignition coil, inducing high voltage. The air gap between the flywheel and coil must be precisely set—typically around 0.010 to 0.012 inches—to produce a strong spark.
- Spark plug: Fires the compressed fuel mixture. A fouled, carbon-crusted, or improperly gapped spark plug kills combustion entirely.
If you test for spark and get nothing, disconnect the stop switch wire temporarily to rule out a shorted kill switch before ordering a replacement ignition module. For official safety specs and maintenance guidelines, consult the Craftsman Official Website.
Clutch, Chain Brake, and Drive Sprocket
Located beneath the clutch cover on the side of the saw, the drive system transfers engine rotation to the cutting chain while protecting the operator from kickback.
The assembly features distinct mechanical zones:
- Centrifugal clutch: Consists of clutch shoes held together by a garter spring. As engine RPM rises, centrifugal force throws the shoes outward against the clutch drum, driving the sprocket and chain. At idle, the shoes retract so the chain stops moving.
- Chain brake band: A steel strap that wraps tightly around the clutch drum. When you push the front hand guard forward, the linkage snaps the brake band tight, stopping the chain in milliseconds.
- Rim or spur sprocket: The toothed gear welded to or floating behind the clutch drum that engages the drive links of the saw chain.
A common mechanical failure occurs when the clutch spring snaps, causing the chain to spin constantly at idle. Replacing the clutch requires locking the piston with a piston stop tool and unthreading the clutch nut clockwise (note that many two-stroke clutches use left-hand threads).
Guide Bar, Chain, and Oiling System
The cutting attachment relies on continuous lubrication to prevent destructive friction between the steel guide bar and the fast-moving chain links.
- Automatic oil pump: Driven by a worm gear attached to the crankshaft, the pump draws bar and chain oil from the onboard reservoir and forces it through a discharge port onto the guide bar pad.
- Guide bar groove: The channel running around the perimeter of the bar. Wood dust and resin pack into this groove, blocking oil flow. Clean out this channel regularly using a thin metal scraper.
- Saw chain drive links and cutters: Each cutting tooth requires proper sharpening angles (usually 25 to 30 degrees) maintained with a round file and depth gauge tool.
If your guide bar is smoking or the chain runs dry, inspect the oil pickup port behind the clutch cover. Clogged oil holes or a sheared oil pump drive gear will starve the bar of lubrication within minutes. You can also review general small engine safety and maintenance protocols via the Occupational Safety and Health Administration.
Common Maintenance Mistakes to Avoid
DIY chainsaw repairs often fail due to simple procedural errors during reassembly. Avoid these frequent missteps:
- Ignoring fuel line degradation: Replacing a carburetor without checking old, brittle fuel lines leaves air leaks intact, preventing the saw from holding a proper tune.
- Over-tightening bar nuts: Excessive torque strips the mounting studs out of the plastic or magnesium crankcase. Tighten them firmly, but respect torque limits.
- Neglecting the spark arrestor screen: A carbon-clogged spark arrestor screen inside the muffler chokes exhaust flow, causing the engine to lose power at high RPMs. Clean the screen with a wire brush every 25 hours of operation.
- Mixing incorrect fuel ratios: Running standard gas without two-stroke oil scores the cylinder walls permanently within minutes of operation. Always mix fresh fuel in a certified container using a verified 2-stroke synthetic oil blend.
Frequently Asked Questions
How do I find the exact part number for my Craftsman 42cc chainsaw?
Locate the model number tag stamped into the plastic housing or crankcase (usually starting with 358.XXXXXX). Enter this exact number into an authorized parts lookup database or consult the manufacturer's illustrated parts list to match component IDs with the correct engineering numbers.
Why does my Craftsman chainsaw bog down when I apply full throttle?
This usually points to a lean fuel mixture caused by a clogged carburetor jet, a restricted fuel filter, or air leaking past a torn intake boot. Clean the carburetor jets and inspect all rubber fuel pathways for splits or looseness.
How often should I clean the spark arrestor and air filter?
Inspect and clean the foam air filter and metal spark arrestor screen every 10 to 15 hours of heavy use. Wash foam filters in warm soapy water, let them dry completely, and apply a few drops of clean filter oil before reinstalling.
What causes the saw chain to keep spinning when the engine is idling?
A spinning chain at idle indicates that the idle speed screw (T-screw) is set too high, or that the centrifugal clutch springs have stretched out and lost tension, failing to pull the clutch shoes away from the drum at low RPMs.
