The Briggs and Stratton 31P777 is a 501 cc, (30.6 cu·in) natural aspirated single-cylinder four-stroke air-cooled internal combustion small gasoline engine with a vertical shaft, manufactured by Briggs & Stratton for general-purpose applications.
A 90.6 mm (3.57 in) cylinder bore and 77.8 mm (3.06 in) piston stroke give the B&S 31P777 engine a total of 501 cc of displacement. This engine produced 18.6 PS (13.7 KW; 18.5 HP) at 3,600 rpm of horsepower.
The B&S 31P777 features:
- Cast iron sleeve
- OHV (overhead) design
- Plain bearing
- Electronic ignition system
- Float type carburetor
- Mechanical governor
- Full pressure lubrication system
- Forced air cooling system
- Electric starter
General information
| Engine Specifications | |
| Engine model | 31P777 |
| Engine type | 4-stroke, OHV, single-cylinder |
| Displacement | 501 cm3, (30.6 cu·in) |
| Max. horsepower | 18.6 PS (13.7 KW; 18.5 HP) at 3,600 rpm |
| Max. torque | – |
| Cooling system | Forced air |
| Air cleaner | Dual element (paper and foam) |
| Lubricating system | Full pressure lubrication system |
| Ignition system | Electronic |
| Fuel system | Float type carburetor |
| Fuel used | Gasoline |
| Starting system | Electric starter |
| Governor | Mechanical governor |
| Dimensions (L x W X H) | – |
| Dry weight | – |
Cylinder Block and Head
| Cylinder block | |
| Cylinder bore: | 90.6 mm (3.57 in) |
| Piston stroke: | 77.7 mm (3.06 in) |
| Cylinder internal diameter (standard): | 90.500-90.600 mm (3.563-3.5669 in) |
| Number of piston rings (compression / oil): | 2/1 |
| Connecting rod small end diameter (standard): | 20.330-20.370 mm (0.8004-0.802 in) |
| Piston pin diameter: | 20.310-20.290 mm (0.7996-0.7988 in) |
| Side clearance (Top): | 0.05-0.12 mm (0.002-0.0047 in) |
| Side clearance (Second): | 0.05-0.12 mm (0.002-0.0047 in) |
| Side clearance (Oil): | 0.05-0.12 mm (0.002-0.0047 in) |
| Ring end gap (Top): | 0.150-0.230 mm (0.0059-0.0091 in) |
| Ring end gap (Second): | 0.350-0.450 mm (0.0138-0.0177 in) |
| Ring end gap (Oil): | 0.150-0.250 mm (0.0059-0.0098 in) |
| Crankshaft main journal diameter: | 35.000 mm (1.378 in) |
| Crankpin journal diameter: | 38.050 mm (1.498 in) |
| Crankshaft center distance: | 38.9 mm (1.53 in) |
| Cylinder head | |
| Valve arrangement: | OHV |
| Valves: | 2 |
| Valve steam diameter (INTAKE): | 5.920-5.970 mm (0.2331-0.235 in) |
| Valve steam diameter (EXHAUST): | 5.920-5.970 mm (0.2331-0.235 in) |
| Camshaft lobe height (INTAKE): | 31.490 mm (1.2398 in) |
| Camshaft lobe height (EXHAUST): | 31.490 mm (1.2398 in) |
| Camshaft journal diameter: | 12.670 mm (0.4988 in) |
Tightening torque
| Tightening torque specs | |
| Air cleaner base | 6.0 Nm; 4.4 ft·lb |
| Alternator | 2.0 Nm; 1.5 ft·lb |
| Armature | 3.0 Nm; 2.2 ft·lb |
| Blower housings | 10.0 Nm; 7.4 ft·lb |
| Blower housing brackets | 10.0 Nm; 7.4 ft·lb |
| Breather | 3.0 Nm; 2.2 ft·lb |
| Carburetor to intake manifold | 8.0 Nm; 5.9 ft·lb |
| Carburetor solenoid | 5.0 Nm; 3.7 ft·lb |
| Connecting rod | 17.0 Nm; 12.6 ft·lb |
| Control bracket | 5.0 Nm; 3.7 ft·lb |
| Crankcase sump | 25.0 Nm; 18.5 ft·lb |
| Cylinder head | 25.0 Nm; 18.5 ft·lb |
| Cylinder shield | 5.0 Nm; 3.7 ft·lb |
| Exhaust manifold / adapter | 19.0 Nm; 14.0 ft·lb |
| Flywheel nut | 135.0 Nm; 100.0 ft·lb |
| Flywheel fan | 16.0 Nm; 11.9 ft·lb |
| Fuel pump | 10.0 Nm; 7.4 ft·lb |
| Governor lever nut | 5.0 Nm; 3.7 ft·lb |
| Intake manifold to cylinder head | 10.0 Nm; 7.4 ft·lb |
| Muffler | 10.0 Nm; 7.4 ft·lb |
| Oil drain plug | 11.0 Nm; 8.1 ft·lb |
| Oil fill tube | 10.0 Nm; 7.4 ft·lb |
| Oil pump cover | 10.0 Nm; 7.4 ft·lb |
| Regulator / Rectifier | 5.0 Nm; 3.7 ft·lb |
| Rewind starter | 3.0 Nm; 2.2 ft·lb |
| Rocker ball set screw or nut | 5.0 Nm; 3.7 ft·lb |
| Rocker cover | 7.0 Nm; 5.2 ft·lb |
| Starter motor | 16.0 Nm; 11.9 ft·lb |
Service Data
| Valve clearance | |
| Intake valve clearance: | 0.08-0.13 mm (0.003-0.005 in) |
| Exhaust valve clearance: | 0.13-0.17 mm (0.005-0.007 in) |
| Oil type: | SF, SG, SH, SJ or higher |
| Recommended oil: |
Summer: SAE 30, 10W-30 Winter: SAE 5W-30 |
| Oil capacity: | 1.42 L (1.5 US. qt, 1.25 Imp. qt.) |
| Fuel system | |
| Type: | Float type carburetor |
| Fuel pump | Pulse type |
| Ignition system | |
| Type: | Electronic |
| Spark plug: | Champion RC12YC |
| Spark plug gap: | 0.7-0.8 mm (0.0276-0.0315 in) |
| Spark plug torque: | 20 N·m (2 kg·m, 14.7 ft·lb) |
| Armature air gap | 0.25-0.36 mm (0.0098-0.0142 in) |
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