Craftsman Motorcycle Vehicles Maintenance key terms

A diagnostic procedure that cranks the engine with the throttle open and measures peak cylinder pressure; comparing dry readings with readings taken after adding oil reveals whether leakage comes from rings or from valves and head gasket.

A test that pressurizes a cylinder at top dead center with both valves closed and measures the percentage of air escaping; the location of escaping air (crankcase breather, intake, exhaust, radiator) pinpoints the faulty sealing component.

The specified gap between the camshaft lobe or rocker arm and the valve stem tip, measured with a feeler gauge when the engine is cold or hot as the manufacturer requires; incorrect clearance causes noise, hard starting, or burned valves.

Index marks on the camshaft sprocket, cylinder head, and crankshaft that must align during assembly so valve events occur at the correct piston position; misalignment can cause piston-to-valve contact and severe engine damage.

The exact piston position at the highest point of its stroke, used as the reference for ignition timing, cam timing, and valve adjustment; it is identified by rotor marks or a timing plug on the crankcase.

A sensor and lamp or gauge circuit that alerts the rider when lubricating oil pressure falls below a safe threshold; diagnosis involves checking the sensor, wiring continuity, and actual pressure with a mechanical gauge.

The engine-driven pump, usually a trochoid or gear type, that circulates oil from the sump through the filter and galleries to bearings and the cylinder head; low output causes overheating and bearing failure.

The complete path oil follows from the sump through the pickup, pump, filter, main galleries, crankshaft, transmission shafts, and cylinder head before draining back; restrictions or leaks anywhere reduce protection for downstream components.

Selecting pilot, needle, and main jet sizes to deliver the correct air-fuel ratio across throttle openings; changes are verified by reading spark plug color and exhaust gas analysis after test runs.

The fuel level setting inside a carburetor float bowl, measured with the float tang just touching the needle valve; an incorrect height makes the mixture rich or lean at all throttle positions.

The mass ratio of air to fuel entering the engine, with about 14.7:1 being chemically correct for gasoline; ratios richer or leaner than optimum affect power, emissions, and combustion temperature.

The time in milliseconds an injector stays open per combustion cycle, calculated by the engine control unit from airflow, rpm, and sensor feedback; longer pulses add fuel and shorter pulses lean the mixture.

An oxygen sensor in the exhaust that generates a voltage indicating whether the mixture is richer or leaner than stoichiometric; the engine control unit uses this signal to trim fuel delivery in closed-loop operation.

The crankshaft angle, measured in degrees before top dead center, at which the spark plug fires; too advanced causes detonation and too retarded causes overheating and power loss.

A number indicating how quickly a plug sheds heat from its insulator tip; a plug too hot for the engine can pre-ignite, while one too cold fouls under light load.

A transformer that steps battery voltage up to the thousands of volts needed to jump the spark plug gap; failures show as weak or absent spark, especially when hot or damp.

An exhaust component coated with precious metals that converts hydrocarbons, carbon monoxide, and nitrogen oxides into less harmful gases; it requires correct mixture control and must not be contaminated by lead or excess oil.

A sealed system of tank venting, a charcoal canister, and purge valves that captures fuel vapor instead of releasing it; leaks or a saturated canister cause fuel smell and failed emission tests.

A system that routes blow-by gases from the crankcase back into the intake to be burned, reducing hydrocarbon emissions and preventing pressure from forcing oil past seals.

A potentiometer or contactless sensor reporting throttle plate angle to the engine control unit; its signal influences fuel pulse width and ignition advance, so a dead spot causes hesitation or surging.

A magnetic or hall-effect sensor that reads teeth or a reluctor on the crankshaft to tell the engine control unit engine speed and position, which it needs to schedule spark and injection.

A stack of friction and steel plates running in engine or transmission oil, engaged by spring pressure and released by the clutch actuator; measured stack height and plate flatness determine whether it slips or drags.

The small amount of lever travel before the clutch begins to disengage, specified by the manufacturer; too little causes slipping and too much causes incomplete release and difficult neutral selection.

The outer hub that carries the clutch plates and is driven by the primary gear; its tangs wear into notches that prevent plates from sliding freely, causing grabby engagement and noise.

The gear or chain and sprocket set that transfers crankshaft torque to the transmission input shaft; ratio and backlash affect acceleration and noise, and wear shows as metal debris in the oil.

The relationship between front and rear sprocket tooth counts, or the bevel gear ratio on shaft-drive machines, that converts engine speed into road speed and multiplies torque at the wheel.

The measured vertical movement at the middle of the lower chain run, adjusted by moving the rear axle; incorrect slack overloads countershaft bearings or lets the chain jump the sprocket.

The visible shape of sprocket teeth, which hook or sharpen as they wear; worn teeth accelerate chain stretch and should be replaced together with the chain to avoid rapid repeat failure.

The tendency of glycol-based brake fluid to absorb moisture from the air, which lowers its boiling point and promotes corrosion; this is why manufacturers specify periodic fluid replacement regardless of mileage.

The temperature at which brake fluid vaporizes in the caliper, creating compressible gas and a spongy or failed lever; wet fluid has a much lower boiling point than fresh fluid.

Loss of braking force caused by heat, either from pad friction material outgassing or from fluid vaporization; the lever may feel normal while stopping power drops.

The procedure of forcing fresh fluid through the hydraulic system to expel air bubbles, usually with a bleed nipple and hose at the caliper; air remaining in the circuit makes the lever feel spongy.

A sensor at each wheel that reports rotational speed to the antilock brake module; a damaged sensor or dirty tone ring disables ABS and may trigger a warning lamp.

The difference between fully extended and rider-loaded suspension length, used to set spring preload so the bike sits at the intended ride height and keeps adequate travel in both directions.

The hydraulic control of how quickly a spring extends (rebound) and compresses (compression); adjusters change oil flow through shims or needles to tune stability and comfort.

The air gap measured from the top of the fork tube to the oil surface with the fork fully compressed and springs removed; it controls the air spring effect and must match the service specification.

The adjustment of tapered or ball bearings in the frame head tube so the front end turns freely without play; too tight causes weave and notchy steering, too loose causes clunking under braking.

The lateral and radial deviation of a spinning wheel measured with a dial gauge; excessive runout indicates a bent rim, worn bearings, or poor spoke tension and causes vibration and uneven tire wear.

The tightness of each spoke nipple, which must be even around the wheel to keep the rim true and share load; loose spokes allow the rim to crack or distort under impact.

A measurement of voltage and current at the battery with the engine running, checking that the stator and regulator-rectifier deliver the specified charge; low output leaves the battery flat after short rides.

The alternator winding that produces alternating current and the solid-state unit that rectifies it to direct current and limits voltage to about 14 volts; failures cause overcharging or no charging.

A measurement of current flowing from the battery with the ignition off, used to find accessories or shorts draining it overnight; normal draw is usually a few milliamps.

A check that applies a heavy current load while monitoring voltage to confirm the battery holds charge under cranking demand; a battery that drops below specification needs replacement.

A two-wire serial network that lets the engine control unit, instrument cluster, and ABS module exchange data; a shorted or open bus disables multiple systems at once.

A stored alphanumeric identifier set by a control module when a monitored circuit or value falls outside limits; it guides testing but does not by itself identify the failed part.

A measurement of frame alignment using datum points, a straightedge, or a dedicated fixture after a crash; bent frames cause persistent handling problems that cannot be corrected by suspension adjustment.

The manufacturer's tightening value and order for fasteners, especially cylinder heads and axle nuts; following both prevents distortion, loosening, and stripped threads.

An anaerobic adhesive applied to fastener threads to resist vibration loosening; strength grades differ, and the wrong grade can make future disassembly difficult.

Numbers or radial marks on a bolt head showing its strength class; replacing a high-grade fastener with a lower grade risks failure in critical joints such as brake calipers and suspension.

A four-digit stamp on the sidewall giving the week and year of manufacture; tires age even when unused, and old rubber should be replaced regardless of tread depth.

Raised bars inside the tread grooves that become level with the surface at the legal minimum depth; reaching them means the tire must be replaced for safety in wet conditions.

Inflation pressure measured before the tire has warmed from riding; it is the value manufacturers specify, since hot pressures read higher and should not be used for adjustment.

The mileage or time at which valve clearances must be checked and adjusted; skipping it leads to hard starting, rough idle, and burned valves on shim-under-bucket designs.

A short systematic check of tires, controls, lights, fluid levels, chain or belt tension, and leaks before every ride; it catches faults that would otherwise become failures at speed.