Briggs and Stratton 31P777

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)