Briggs and Stratton 31Q777

Briggs and Stratton 31Q777The Briggs and Stratton 31Q777 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 31Q777 engine a total of 501 cc of displacement. This engine produced 18.6 PS (13.7 KW; 18.5 HP) at 3,300 rpm of horsepower.

The B&S 31Q777 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 31Q777
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 3,300 rpm
Max. torque
Cooling system Forced air
Air cleaner Dual element
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)
ATTENTION! Dear visitors, this is an informational database of technical specifications, we are not a parts supplier or a marketplace.

We try to use official documentation and verify the published data, but errors may occur. In addition, specifications of machinery and equipment may vary depending on markets and modifications. We do not recommend the use of this data for spare parts or repairs. Use official service manuals or parts catalogs for your product.

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