Working With a Craftsman 18 42cc Chainsaw — Parts, Diagrams, and What Actually Goes Wrong
Most people who own one of these chainsaws end up needing a parts diagram at some point. The Craftsman 18 inch 42cc model was sold under a lot of different house brands over the years — Sears, Craftsman, sometimes no brand at all. That alone makes finding the right diagram frustrating. The engine is a generic Chinese 2-stroke design that's been copied and rebranded for two decades. Understanding how the parts diagram fits into real maintenance rather than treating it like a reference document is what separates people who keep their saw running from people who buy another cheap saw every two years. The parts diagram for this saw breaks down into several subsystems. The engine section covers the cylinder, piston, crankshaft, connecting rod, and the carburetor. The ignition system includes the flywheel, spark plug, and the magneto coil. The fuel system has the tank, primer bulb, fuel lines, and the carburetor itself. The bar and chain assembly is separate from the engine but shown on most diagrams. Then there are the housing pieces — the starter assembly, the air filter cover, the muffler, and the chain brake mechanism. Each diagram labels parts with numbers that correspond to a parts list. That list tells you the part name and usually a manufacturer part number. Getting those numbers right is the entire point of using a diagram in the first place. Here's the practical reality that most diagrams don't tell you. The numbering system is not standardized across manufacturers. A part labeled "7" on one diagram might be the carburetor on one diagram and the air filter on another. The numbers are arbitrary within each individual diagram's key. Always cross-reference the number with the corresponding label in the key before ordering anything. I learned this the hard way after ordering a carburetor based on the number alone and receiving an intake manifold gasket instead. It took me about an hour to figure out the mismatch, and another three days waiting for the correct part to arrive.
Let me walk through how I actually use a parts diagram when something breaks. Take the carburetor. On this saw, the carb is almost always a diaphragm-type carburetor, typically a Zama or a generic copy. The diagram will show the main body, the fuel inlet, the throttle linkage, and the diaphragms. When I'm troubleshooting a carb issue, I don't just look at the diagram — I photograph the disassembled carb while it's still apart. That way if I forget which orientation a gasket goes in, I have a reference. The fuel pump diaphragm on these carbs is tiny. It's about the size of a dime. Dropping one into the grass during a repair is not uncommon. I keep a small magnetic tray beside me now. It saves about twenty minutes of searching through grass that I'd otherwise spend. The choke and throttle linkage on this saw is another area where diagrams are useful but incomplete. The diagram shows the lever, the spring, and the adjustment screw. What it doesn't show is how the throttle cable tension affects the idle speed screw. If your saw dies at idle but runs fine under load, the problem is almost never the carburetor itself. It's the idle mixture or the cable tension. I adjust the idle speed screw a quarter turn at a time and test between adjustments. Each full rotation changes the idle noticeably on these engines. The mixture screws are softer than they look. Stripping one takes about three seconds of overtightening. I recommend using a properly fitted screwdriver and not an impact driver. I've replaced two mixture screws this way over the years, and each one runs about eight dollars shipped. The spark arrestor screen on the muffler is another component that shows up on diagrams but gets ignored until it becomes a problem. This is a fine mesh screen inside the muffler that prevents sparks from escaping. Over time it gets clogged with carbon buildup. A clogged spark arrestor reduces power and makes the saw run rich because exhaust flow is restricted. Cleaning it takes about five minutes with a wire brush or a flame. Removing the muffler takes roughly ten minutes if you know which bolts hold it on. The diagram helps here because the bolt sizes vary — usually a 10mm and an 8mm. Using the wrong socket will round off the bolt head. I learned that one during a job where I only had a cheap socket set available. Rounding off a muffler bolt is one of those moments that makes you want to throw the saw into a tree.
When it comes to actually finding a reliable parts diagram for your specific saw, the model number matters more than anything else. Look on the top of the engine housing for a silver or black sticker. The Craftsman model number is usually something like CM118 or similar. There will also be a manufacturer model number from the actual maker — often a number starting with 216 or 358. Search for that manufacturer number along with "parts diagram." You'll find PDFs on sites like Partstown, SearsPartsDirect, and various chainsaw-specific parts dealers. Some diagrams are only available through authorized Craftsman service centers, and those usually require you to provide proof of ownership. A receipt or the serial number on the engine housing will do. One thing worth noting about these diagrams is that they don't always reflect every variation. Craftsman sourced these saws from multiple manufacturers over the production run. An earlier model might have a different carburetor than a later one even though the external appearance is nearly identical. If you're ordering parts based on a diagram, compare the diagram's part numbers against your actual saw's components. A carburetor diagram showing a single jet setup won't match a saw that was later upgraded to a two-jet carb. This happens more often than you'd think. The saw ran fine for years and then you needed a rebuild carb, and the diagram you found online was for an earlier revision. The chain tension adjustment mechanism is another area where the diagram is helpful but limited. The diagram shows the tensioning screw, the guide bar nut, and the lock mechanism. What you need to know in practice is that the chain should sit snugly against the bar but still pull freely by hand. Over-tightening is the most common mistake, and it kills bearings faster than anything else. A properly tensioned chain on this saw will sag about a quarter inch below the bar midpoint when you pull it down. If you're cutting wood and the chain jumps off the bar frequently, check the tension first, then the bar groove, then the drive sprocket wear. Three separate issues that all produce the same symptom.
Get the Full Details
I should mention the limits of what a parts diagram can do for you. It can't tell you which parts are genuine versus aftermarket copies. A carburetor kit labeled as OEM might contain diaphragms that fail within a season. The diagram won't warn you about that. Similarly, gasket sets sold as complete rebuild kits sometimes skip the crankcase seals. Those seals are critical. A missing crank seal means the engine won't hold compression. Check what's actually included in any kit you order before you start disassembling the saw. I once received a rebuild kit that was missing the main gasket and both crank seals. I had to order them separately and wait another week. The diagram I was working from didn't list individual seal part numbers, so I spent an afternoon tracing through multiple supplier pages to find the right ones. If you're looking to download a diagram, search for the manufacturer model number followed by "parts manual PDF." Several reputable sources host these files for free. The Sears catalog archives also still have many of the original diagrams available. Don't rely on random YouTube videos that claim to have the parts list — those are often outdated or from different revisions of the same saw. A PDF diagram from an official or semi-official source will have the accurate numbering system for your specific model year. The bottom line is that a parts diagram is a tool, not a solution. It tells you what parts exist and how they relate to each other. It doesn't diagnose problems or guarantee that replacement parts will work. Use it alongside actual mechanical knowledge, and keep a small notebook or phone photo log of each repair. That habit alone will save you more time than any diagram ever will.
