Briggs & Stratton Professional Series 21.0 HP (Model 33M777) - Specs and Service Data
The top-rated Professional Series single - 21.0 gross HP on the 540 cc block for large tractors and zero-turns.
Variant: 540 cc / 21.0 HP gross
Specs last verified Oct 28, 2026 against the sources shown.
Dimensions
Our own illustrative drawing, from the published overall dimensions - not to scale.
Width (front to back in this view): 16.18 in (Briggs & Stratton). Leave clearance on the recoil side for the pull start.
Diagrams on this site are our own illustrative drawings, drawn from published dimensions (Briggs & Stratton) - not official or technical drawings. Verify dimensions before fitment.
Full specifications
Specifications grouped by system.
01 Engine
- Bore x Stroke94.0 x 77.8 mm (Briggs & Stratton)
- ConfigurationSingle cylinder, 4-stroke, air-cooled, OHV, vertical shaft; Dura-Bore cast iron sleeve; full-pressure lubrication (plain bearing, pressure lubrication per model decode); float carburetor; electric starter with alternator (Briggs & Stratton)
- Displacement540 cc (Briggs & Stratton)
- Power21.0 HP (gross, SAE J1940) (Briggs & Stratton)
02 Dimensions and Mounting
- Dry weight65 lb (29.5 kg) (Briggs & Stratton)
- Mounting4-bolt rider base pattern (0.350 in mounting holes, Briggs service-engine diagram) (Briggs & Stratton)
- Height12.87 in (Briggs & Stratton)
- Length18.86 in (Briggs & Stratton)
- Width16.18 in (Briggs & Stratton)
03 Capacities and Service
- Fuel capacityNone - no fuel tank fitted (remote equipment tank) (Briggs & Stratton)
- Oil capacity48 oz (1.4 L) with filter; 42-44 oz without filter (Briggs & Stratton)
04 Shaft and Fitment
- Keyway1/4 in keyway, 7/16-20 drilled and tapped end (Briggs & Stratton)
- 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 (Briggs resistor plug 491055) (Briggs & Stratton)
- Air filter793569 cartridge (793685 pre-cleaner) (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 33M777, Type 0111-B1, the Code is stamped per engine and gives its build date.
On Professional Series single-cylinder OHV engines the Model-Type-Code is stamped into the metal on the left side of the overhead valve (OHV) cover - Briggs & Stratton's own identification guide lists the valve cover first for OHV engines. Some builds also carry the numbers on the blower housing or a metal tag above the starter, and later engines add a printed label; Briggs lists five possible locations across its engines, so if the valve cover has been swapped, check the blower housing and the area above the spark plug next. (Per Briggs & Stratton 'How To Locate Your Engine Model Number' identification literature, corroborated by dealer ID guides.). Bring the full stamp - model, type, and code - to the decoder.
Read the Professional Series (single-cylinder) 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
Professional Series singles are the upgrade repower path for Intek and PowerBuilt riders on the same 310000/330000 footprint: same base pattern and 1 in x 3-5/32 in shaft on verified types. Confirm the shaft, charging output, and muffler side on the exact Type before ordering.
Same footprint class as:
Common uses
Quick answers
How much oil does the 33M777 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 33M777?
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 33M777?
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 operator manual specifications).
What spark plug and gap does the 33M777 use?
Champion RC12YC (Briggs resistor plug 491055), gapped to 0.030 in. Air filter is the 793569 cartridge with a 793685 pre-cleaner.
What size is the 33M777 crankshaft?
1 in diameter x 3-5/32 in long, with a 1/4 in keyway and a 7/16-20 drilled and tapped end (Briggs & Stratton service-engine catalog, crankshaft table for this model).
What equipment did the 33M777 come on?
Large garden tractors and zero-turns that needed the strongest single-cylinder tune - documented fits include the Snapper SC 2450 (type 0111-B1) and Snapper ZT 2142 zero-turn (type 0126-G5), plus Craftsman ZTS 7500 tractors.