Maharashtra HSC Scooter & Motorcycle Servicing (Bifocal) cheat sheet

Micrometer least count = screw pitch ÷ thimble divisions; a 0.5 mm pitch screw with a 50-division thimble reads to 0.01 mm.

Vernier caliper least count = 1 main scale division − 1 vernier scale division; a 50-division vernier spanning 49 mm on a 1 mm main scale gives 0.02 mm.

Torque T = F × L; 1 kgf·m = 9.807 N·m and 1 N·m = 0.738 lbf·ft. Always convert the manufacturer's unit before setting the wrench.

Bore gauge readings are taken at three heights (top, middle, bottom) and in two planes (parallel and perpendicular to the crank axis); taper = top − bottom, ovality = max − min.

Feeler gauge blades are marked in mm or inches and are used for valve clearance, spark plug gap, and piston ring side clearance; the blade must slide with light drag, never be forced.

A ring spanner or correct-size socket must be used on hex fasteners above about 10 mm; open-ended and adjustable wrenches round the flats and should be a last resort.

Fire classes: A solids, B flammable liquids, C gases, D metals. Petrol and electrical fires need CO₂ or dry powder; water spreads burning fuel and conducts electricity.

Special tools include the bearing puller, magneto/flywheel puller, clutch holder, valve spring compressor, ring compressor, and torque wrench. A hammer is never a substitute for a puller.

The four-stroke cycle (intake, compression, power, exhaust) takes 720° of crankshaft rotation; the two-stroke cycle completes the same gas exchange in 360°.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume; a 150 cc engine with 16.7 cc clearance volume gives CR ≈ 10:1.

Valve clearance is set on a cold engine; typical figures are 0.08–0.12 mm inlet and 0.10–0.15 mm exhaust. Too tight burns valves, too loose causes rattle and lost lift.

Piston ring end gap is measured with the ring squared in the bore using a piston crown as a pusher; too small a gap causes ring butting and cylinder scoring, too large causes blow-by.

Wet-sump lubrication path: strainer → trochoid pump → filter or centrifuge → main gallery → crankpin, camshaft and rocker gear → gravity drain back to the sump.

JASO MA/MA2 oils are specified for wet clutches; API 'Energy Conserving' or ILSAC GF-5 oils contain friction modifiers that make the clutch slip under load.

Air-cooled engines shed heat through cylinder and head fins. A 1 mm mud layer or a broken fin raises head temperature sharply and can cause piston seizure.

Oil level is checked with the vehicle upright and the dipstick resting on the threads, not screwed in, on most Indian two-wheelers; overfilling causes seal leakage and oil aeration.

Carburettor metering ranges: pilot (idle) circuit 0–1/4 throttle, jet needle and needle jet 1/4–3/4, main jet 3/4 to full throttle.

Stoichiometric air–fuel ratio for petrol is 14.7:1 by mass; below about 12:1 the mixture is rich, above about 16:1 it is lean.

On a fuel-mixture screw, turning in leans the idle circuit; on an air screw, turning in richens it. Identify the screw type before adjusting.

Float height is set by bending the float tang with the needle valve just seated; a high float level richens the mixture and causes flooding and fuel smell.

In EFI the ECU calculates injector pulse width from rpm, throttle position, MAP, intake air temperature and oxygen sensor feedback; closed-loop correction begins only after the sensor reaches roughly 300 °C.

Typical port fuel injection pressure on a small two-wheeler is 250–350 kPa; low pressure produces lean misfire, poor power and a lean fault code.

A three-way catalytic converter oxidises CO and HC and reduces NOx only while the mixture oscillates near 14.7:1; a rich or lean bias destroys conversion efficiency.

Bharat Stage VI norms apply to all new two-wheelers sold in India from 1 April 2020, and a valid PUC certificate is required for on-road use.

Ohm's law V = I × R and P = V × I; a 12 V 35 W headlamp draws about 2.9 A, so its fuse must be rated above that current.

A 12 V lead-acid battery has six cells of about 2.1 V each; specific gravity is 1.260–1.280 when fully charged and 1.120 or below when discharged.

Charge a lead-acid battery at roughly one-tenth of its Ah rating (0.5 A for a 5 Ah battery); gassing begins above about 14.4 V.

The regulator-rectifier converts alternator AC to DC and clamps the system at about 14.2–14.8 V; a reading above 15 V means it has failed and will boil the battery.

CDI ignition: the source coil charges a capacitor, the pickup coil triggers a thyristor, and the capacitor discharges into the ignition coil primary to produce the secondary spark.

Spark plug reach, thread and heat range must match the engine; a hot plug in a high-speed engine causes pre-ignition, a cold plug fouls at low speed.

Typical ignition coil resistance is 0.5–2 Ω on the primary and 5–15 kΩ on the secondary; an open secondary gives no spark.

Voltage-drop testing under load finds high-resistance earths; more than 0.5 V drop on the earth side of a 12 V circuit indicates a bad connection.

Clutch lever free play is typically 10–20 mm at the lever end; too little lets the release bearing ride and the clutch slips, too much causes drag and hard gear engagement.

Gear ratio = driven teeth ÷ driving teeth; overall ratio = primary ratio × gearbox ratio × final drive ratio.

Chain slack is normally 20–30 mm at the mid-point of the lower run with the vehicle on its wheels, not on the centre stand.

In chain size 428 the first digit gives pitch in eighths of an inch (4/8 in = 12.7 mm); the remaining digits define roller width, so replacement chain must match the number.

A worn chain shows tight and loose spots and hooks on the rear sprocket teeth; chain and sprockets should be replaced as a set to avoid rapid wear of the new chain.

Scooter CVT: variator rollers wear flat, raising rpm and reducing top speed; a belt narrower than the wear limit changes the effective ratio range.

Wet multiplate clutch slip is confirmed by holding the brake in a high gear and releasing the clutch; rising rpm without vehicle movement means slip.

Shaft final drive uses bevel or hypoid gears and requires the specified hypoid gear oil; ordinary engine oil causes whine and rapid wear.

Trail is the horizontal distance between the steering-axis ground intersection and the front tyre contact patch; more trail gives straight-line stability and heavier steering.

Tyre size 90/90-17 means 90 mm nominal section width, 90% aspect ratio (81 mm sidewall height) and a 17-inch rim diameter.

Tyre pressures are set cold; under-inflation increases sidewall flex, heat, rolling resistance and shoulder wear, and can cause a blowout.

Typical service limits for wheel runout are about 2 mm radial and 2 mm axial; beyond this the wheel must be trued or replaced.

Front fork oil level and viscosity set the damping; unequal oil levels in the two legs cause uneven damping and a pull to one side.

Static sag is set by spring preload; too much sag makes the machine sit low and bottom out, too little makes the ride harsh and the wheel skip on bumps.

Front and rear wheels must lie in the same plane; a bent fork tube, twisted triple clamp or skewed swingarm causes off-centre tyre wear and pulling.

Spoked wheels need even spoke tension, a correctly seated rim band and a tube free of pinches; a loose spoke lets the rim deflect and the tube fail.

Braking distance rises with the square of speed: doubling speed from 40 to 80 km/h roughly quadruples the stopping distance.

Weight transfer puts 70–80% of braking force on the front wheel; the front brake is the primary brake on a two-wheeler and the rear supplements it.

Drum brake adjustment: set lever or pedal free play to 10–20 mm and align the brake cam index mark with the hub mark; a worn lining needs replacement, not more adjustment.

DOT 3 and DOT 4 brake fluids are glycol based and hygroscopic; absorbed moisture lowers the boiling point and causes fade and a spongy lever.

Air is removed from a hydraulic disc circuit by bleeding at the caliper nipple with the reservoir kept topped up; expelled fluid is never reused.

Minimum disc pad friction material thickness is about 1 mm; a scored disc or glazed pads reduce friction and must be machined or replaced.

CBS applies part of the front braking force when the rear pedal is pressed; ABS prevents wheel lock by modulating pressure using wheel-speed sensor signals.

Roadworthiness inspection items include brake efficiency, tyre tread depth (minimum 1 mm for two-wheelers), headlamp aim, horn, mirrors, and a valid PUC certificate.

First service falls at 500–1,000 km (commonly 500–750 km); later services follow the manufacturer's table, typically every 3,000–6,000 km or 3–6 months.

Compression test: warm engine, fully open throttle, fully charged battery, crank until the gauge peaks; a healthy small two-wheeler reads roughly 9–12 bar and holds it.

A wet compression test (a few ml of oil in the bore) that raises pressure points to worn rings; no change points to valves or the head gasket.

Diagnosis follows symptom → system → component. Confirm the complaint with a road test before dismantling, and record speed, gear and engine temperature conditions.

Vacuum gauge: a steady low reading suggests an intake leak or late valve timing; a fluctuating needle suggests a sticking valve or weak valve spring.

A job card records customer complaint, odometer reading, diagnosis, work performed, parts and part numbers, labour, and the next service due; it is the record used for warranty claims.

Road test sequence: cold start and idle quality, low-speed clutch and gear engagement, acceleration through the gears, braking from about 40 km/h, and a slow figure-of-eight.

A PUC certificate is valid for one year on a new two-wheeler and is renewed every six months thereafter; the test measures CO and HC at idle.