Small Engine Code

Briggs & Stratton Intek 15.5 HP (Model 31R777) - Specs and Service Data

The later R-series tune of the 501 cc block - rated 15.5 HP in Briggs manual-derived service data.

Specs last verified Oct 28, 2026 against the sources shown.

501cc
Displacement (Briggs & Stratton)
OHV, single cylinder
15.5 HP (gross)
Power (Briggs & Stratton)
1 in
Shaft diameter (Briggs & Stratton)
3-5/32 in long
65.0 lb (29.5 kg)
Dry weight (Briggs & Stratton)
1.5 qt (48 oz) with filter
Oil capacity (Briggs & Stratton)
SAE 30 or 10W-30 (5W-30 below 40 F)

Full specifications

Specifications grouped by system.

01 Engine

  • Bore x Stroke90.49 x 77.77 mm (Briggs & Stratton)
  • ConfigurationOHV, single cylinder, vertical shaft, air-cooled (Briggs & Stratton)
  • Displacement501 cc (Briggs & Stratton)
  • Power15.5 HP (gross) (Briggs & Stratton)

02 Dimensions and Mounting

  • Dry weight65.0 lb (29.5 kg) (Briggs & Stratton)
  • MountingRider bolt pattern, 10 in. bolt circle class (Dealer engine listings)

03 Capacities and Service

  • Oil capacity1.5 qt (48 oz) with filter (Briggs & Stratton)

04 Shaft and Fitment

  • Keyway1/4 in keyway, 7/16-20 tapped end (Dealer engine listings)
  • Shaft diameter1 in (Briggs & Stratton)
  • Shaft length3-5/32 in (Briggs & Stratton)
  • RotationCounterclockwise (from PTO side) (Dealer engine listings)

Service data

Service specifications for rebuilds and tune-ups.

  • Valve clearance - intake0.003-0.005 in (Briggs & Stratton)
  • Valve clearance - exhaust0.005-0.007 in (Briggs & Stratton)
  • Cylinder head bolt torque220 in-lb (Briggs & Stratton)
  • Flywheel nut torque100 ft-lb (Briggs & Stratton)
  • Spark plug gap0.030 in (Briggs & Stratton)
  • Spark plugChampion RC12YC (Champion (via Briggs service data))
  • Air filter796031 / 698083 (Briggs & Stratton)
  • Oil typeSAE 30 or 10W-30 (5W-30 below 40 F) (Briggs & Stratton)

Service figures are for this model stamp as published - check your operator manual before a rebuild.

Find and decode your stamp

This engine's stamp reads: Model 31R777, Type 0042-B1 (documented, eReplacementParts model listing), the Code is stamped per engine and gives its build date.

On Intek single-cylinder OHV engines the Model-Type-Code is stamped into the metal valve (rocker) cover on most production, and on later engines it also appears on a printed label on the blower housing. Check the valve cover first; if the cover has been replaced, check the blower-housing label and the edge of the blower housing above the spark plug. (Per Briggs & Stratton engine-identification literature and dealer ID guides.). Bring the full stamp - model, type, and code - to the decoder.

Read the Intek (single-cylinder OHV) decode page or paste the whole stamp into the Briggs & Stratton decoder for the build date and what each part means.

Repower - what it replaces

Same rider footprint class as PowerBuilt singles; check shaft diameter, crankshaft length, bolt pattern, and charging output before ordering.

Same footprint class as:

Briggs & Stratton PowerBuilt single-cylinder classBriggs & Stratton Professional Series single class
Check 1 - Shaft diameterYour clutch, pulley, or pump must fit the shaft on the new engine. A different diameter stops the swap cold.
Check 2 - Crankshaft lengthToo short and the driven part cannot seat; too long and it fouls the cover or deck.
Check 3 - Base bolt patternBase holes must land on the equipment's mounting plate or deck. Slotted bases forgive a little; drilled plates do not.
Check 4 - Fuel tank clearanceCheck the new tank clears the hood, seat, and frame through the full steering and tilt range.
Check 5 - Charging outputMatch the alternator output to the equipment's lights and electric PTO clutch, or the battery will not keep up.
Check 6 - Muffler sideConfirm the exhaust exits on the same side, so it clears the frame and hood.

Common uses

lawn tractorsriding mowersgarden tractors

Quick answers

How much oil does the 31R777 take?

1.5 qt (48 oz) with an oil filter change; 42-44 oz without a filter change, per the Briggs & Stratton operator manual specifications table. Always finish by checking the dipstick on level ground.

What oil should I use in the 31R777?

SAE 30 or 10W-30 for normal mowing temperatures, and 5W-30 for cold-weather use, per Briggs & Stratton oil recommendations for this family.

What are the valve clearances on the 31R777?

Intake 0.003-0.005 in. and exhaust 0.005-0.007 in., set cold with the piston 1/4 in. past top dead center on the compression stroke (Briggs & Stratton checking specifications).

What spark plug and gap does the 31R777 use?

Gap is 0.030 in. See the service block on this page for the plug part number verified for this model.

What does the head bolt torque spec mean on the 31R777?

Cylinder head bolts torque to 220 in-lb in stages in a star pattern, per the Briggs & Stratton single-cylinder OHV repair manual torque table. Always fit a new head gasket.

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