________________________________________________________________________________

Wisconsin S12D Engine

The Wisconsin S12D is a heavy-duty, single-cylinder, four-stroke cycle, spark-ignition internal combustion engine engineered by the Wisconsin Motor Manufacturing Company. Designed as a high-RPM, lightweight cast-iron powerplant to compete in the commercial garden tractor and industrial equipment markets, this engine utilizes a horizontal-shaft layout. It features an integrated, automatic mechanical compression release mechanism built directly into the camshaft assembly. This feature reduces compression during cranking to permit fast, effortless manual or electric starting while preventing engine kick-back.

The engine employs an L-head (side-valve) design where both the intake and exhaust valves are mounted vertically within the cylinder block assembly. Valve Components: The exhaust valve configuration includes a premium valve seat insert paired with a mechanical valve rotator to ensure uniform heat distribution and minimize seat burning. Cylinder Bore Diameter: 3.50 inches (88.9 mm). Piston Stroke Length: 3.00 inches (76.2 mm). Piston Design: Standard factory pistons are cam-ground aluminum alloy, with service oversizes available at 0.010 inches and 0.030 inches for cylinder reboring procedures.

Fuel System: The fuel system consists of a gravity-fed or fuel-pump-assisted induction setup using a single-barrel carburetor. Depending on the specific engine production serial number, original units are equipped with Zenith side-draft or top-draft carburetors (such as the L86C, L86D, L95, L95A series, or the later LTA100 conversion assembly). Incoming air is filtered through a heavy-duty dry element or oil-bath air cleaner assembly. Fuel-air mixture regulation and engine speed stability are controlled by an adjustable, flyweight-actuated mechanical governor linked directly to the carburetor throttle shaft.

Lubrication is managed via a reliable splash-type system requiring no pressurized oil pump. A specialized oil dipper is rigidly attached to the cap of the connecting rod journal. As the crankshaft rotates, this dipper strikes the oil reservoir in the lower oil pan/crankcase, creating a continuous oil mist and spray that coats the cylinder walls, wrist pin, camshaft lobes, and main bearings. The engine uses a forced-air cooling system. A combination fan-flywheel, featuring cast-in cooling vanes, draws ambient air inward. This air stream is enclosed and pressurized by a heavy-gauge sheet metal flywheel shroud. Built-in air ducts and cylinder baffle plates divide and direct the high-velocity air flow uniformly over the cooling fins of the cylinder block and cylinder head to maintain optimal operating temperatures.

General Technical Data

Engine Specifications

Engine Model ............... Wisconsin S12D
Engine Type ............... 4-cycle single-cylinder heavy-duty air-cooled horizontal-shaft gasoline
Number of Cylinders ............... 1
Cylinder Arrangement ............... Single vertical cylinder
Engine Bore, in (mm) ............... 3.50 (88.9)
Engine Stroke, in (mm) ............... 3.00 (76.2)
Displacement, cu.in (cc) ............... 28.86 (473.0)
Rated Engine Power, hp (kW) ............... 12.5 (9.3) at 3600 rpm
Rated Engine Speed, rpm ............... 1600-3600
Peak Torque ............... 22.0 lb-ft (29.8 Nm) at 2400 rpm
Idle Speed ............... 1000-1200 rpm
Operating Speed ............... 2400-3600 rpm
Compression Ratio ............... 6.5:1
Compression Release Type ............... Automatic mechanical cam actuation device
Flywheel Type ............... Cast iron with integrated air blower vanes
Crankshaft Type ............... Forged steel counterbalanced design
Main Bearings Type ............... Adjustable tapered roller cone and cup configuration
Camshaft Type ............... Cast alloy iron with ground lobes and governor gear
Muffler Type ............... Low restriction exhaust silencer shell
Crankcase Vent Type ............... Reed valve element breather assembly
Cylinder Block Type ............... Cast iron heavy-duty integrated barrel
Cylinder Head Type ............... Aluminum alloy or cast iron removable flathead
Valve Location ............... Side valve L-head setup
Lubrication System ............... Splash lubrication with mechanical oil dipper
Oil Pump Type ............... Not Applicable (Splash mechanism via connecting rod dipper)
Oil Filter Type ............... Bypass type optional screen (No spin-on filter cartridge)
Cooling System ............... Forced air-cooled via flywheel blower shroud
Air Cleaner Type ............... Polyurethane foam element or dry element type
Ignition System ............... Flywheel magneto ignition with mechanical breaker points
Starting System ............... Electric Starter motor or rope start options
Electrical System ............... 12V flywheel alternator charging network
Alternator Rating ............... 10 A or 25 A

Fuel System

Carburetor Model ............... Zenith 1408 or Walbro 106 horizontal draft
Carburetor Type ............... Float feed adjustable needle single venturi design
Fuel Feed Method ............... Gravity feed or mechanical diaphragm pump optional configuration
Fuel Tank Capacity, gal (L) ............... 1.50 (5.68) standard setup up to 2.25 (8.52) optional
Fuel Strainer Type ............... Sediment bowl with fine mesh screen element setup
Governor Type ............... Centrifugal mechanical flyweight type gear-driven
Fuel Grade Requirement ............... Regular unleaded or low lead gasoline engine fuel
Minimum Fuel Octane Rating ............... 90 RON premium or midgrade standard grade
Choke Actuation Type ............... Manual cable linkage control device
Main Jet Configuration ............... Adjustable needle valve variable orifice assembly
Idle Jet Configuration ............... Fixed circuit with adjustable air bypass needle screw
Float Setting Height, in (mm) ............... 1.23 (31.2)
Throttle Valve Type ............... Butterfly disk plate connected to mechanical governor
Fuel Line Connection Diameter, in ............... 0.25
Maximum Inlet Pressure ............... 2.50 psi (17.2 kPa)
Fuel Filter Screen Mesh ............... 100 mesh brass wire screen filtering element

Dimensions and Weight

Overall Length, in (mm) ............... 15.7 (400)
Overall Width, in (mm) ............... 18.3 (465)
Overall Height, in (mm) ............... 19.5 (495)
Dry Weight, lbs (kg) ............... 115.0 (52.2)

Maintenance Data

Service Information

Compression Pressure Test, psi (kPa) ............... 110-120 (758.4-827.4)
Maximum Ambient Operating Temperature ............... 120 F (48.9 C)
Spark Plug Gap, in (mm) ............... 0.030 (0.762)
Breaker Point Gap, in (mm) ............... 0.020 (0.508)
Ignition Timing Setting Angle ............... 23 deg BTDC
Intake Valve Clearance, in (mm) ............... 0.007 (0.178)
Exhaust Valve Clearance, in (mm) ............... 0.016 (0.406)

Engine Oil

Engine Oil Type .................. SAE 30, SAE 20W, or SAE 10W
API Service Classifications ......................... MS, SD, SE, or SF
Crankcase Oil Capacity, qt (L) ............... 1.25 (1.18)
Reduction Gearbox Oil Capacity, pt (L) ............... 0.50 (0.24)

Service Intervals

Change engine oil ............... Every 50 hours
Inspect air cleaner ............... Every 8 hours
Service air cleaner ............... Every 50 hours
Clean fuel strainer ............... Every 100 hours
Valve clearance adjustment ............... Every 250 hours
Fuel system inspection ............... Every 100 hours
Clean spark plug ............... Every 100 hours
Replace spark plug ............... Every 300 hours
Clean cooling fin ............... Every 50 hours
Change reduction gear oil ............... Every 500 hours

Service Specifications

Cylinder Bore Inside Diameter, in (mm) ............... 3.5000-3.5010 (88.900-88.925)
Cylinder Bore Maximum Wear, in (mm) ............... 3.5050 (89.027)
Cylinder Bore Maximum Out of Round, in (mm) ............... 0.0020 (0.051)
Cylinder Bore Maximum Taper, in (mm) ............... 0.0025 (0.064)
Crankshaft Main Journal Diameter, in (mm) ............... 1.3750-1.3755 (34.925-34.938)
Crankshaft Crankpin Journal Diameter, in (mm) ............... 1.6250-1.6255 (41.275-41.288)
Crankshaft End Play, in (mm) ............... 0.0020-0.0060 (0.051-0.152)
Connecting Rod Big End Clearance, in (mm) ............... 0.0010-0.0025 (0.025-0.064)
Connecting Rod Side Clearance, in (mm) ............... 0.0050-0.0100 (0.127-0.254)
Piston-to-Cylinder Bore Clearance, in (mm) ............... 0.0035-0.0045 (0.089-0.114)
Piston Pin Diameter, in (mm) ............... 0.8750-0.8753 (22.225-22.233)
Piston Pin-to-Connecting Rod Bushing Clearance, in (mm) ............... 0.0003-0.0008 (0.008-0.020)
Piston Pin-to-Piston Pin Bore Clearance, in (mm) ............... 0.0001-0.0004 (0.003-0.010)
Top Ring Side Clearance, in (mm) ............... 0.0020-0.0035 (0.051-0.089)
Second Ring Side Clearance, in (mm) ............... 0.0015-0.0030 (0.038-0.076)
Oil Ring Side Clearance, in (mm) ............... 0.0015-0.0030 (0.038-0.076)
Top Ring End Gap, in (mm) ............... 0.0100-0.0200 (0.254-0.508)
Second Ring End Gap, in (mm) ............... 0.0100-0.0200 (0.254-0.508)
Oil Ring End Gap, in (mm) ............... 0.0100-0.0200 (0.254-0.508)
Intake Valve Stem Diameter, in (mm) ............... 0.3110-0.3120 (7.899-7.925)
Exhaust Valve Stem Diameter, in (mm) ............... 0.3095-0.3105 (7.861-7.887)
Intake Valve Stem-to-Guide Clearance, in (mm) ............... 0.0015-0.0030 (0.038-0.076)
Exhaust Valve Stem-to-Guide Clearance, in (mm) ............... 0.0030-0.0050 (0.076-0.127)
Valve Face Angle ............... 45 degrees
Valve Seat Angle ............... 45 degrees
Valve Seat Width, in (mm) ............... 0.0470-0.0620 (1.194-1.575)
Valve Spring Free Length, in (mm) ............... 1.7500 (44.450)
Camshaft Journal Diameter (Front), in (mm) ............... 1.1250-1.1255 (28.575-28.588)
Camshaft Journal Diameter (Rear), in (mm) ............... 0.8750-0.8755 (22.225-22.238)
Camshaft End Play, in (mm) ............... 0.0030-0.0070 (0.076-0.178)
Tappet Journal Diameter, in (mm) ............... 0.5000-0.5005 (12.700-12.713)
Tappet-to-Crankcase Bore Clearance, in (mm) ............... 0.0005-0.0020 (0.013-0.051)

Tightening Torques

Cylinder Head Nut Fasteners, lb-ft (Nm) ............... 22 to 24 (29.8 to 32.5)
Connecting Rod Bolt Fasteners, lb-ft (Nm) ............... 18 to 20 (24.4 to 27.1)
Crankcase Rear Main Cover Bolts, lb-ft (Nm) ............... 10 to 12 (13.6 to 16.3)
Flywheel Locknut Fastener, lb-ft (Nm) ............... 70 to 75 (94.9 to 101.7)
Spark Plug Component Fastener, lb-ft (Nm) ............... 24 to 28 (32.5 to 37.9)
Governor Lever Clamp Bolt Fastener, lb-in (Nm) ............... 60 to 70 (6.8 to 7.9)
Carburetor Mounting Nut Fasteners, lb-ft (Nm) ............... 8 to 10 (10.8 to 13.6)
Air Cleaner Support Bracket Bolts, lb-ft (Nm) ............... 6 to 8 (8.1 to 10.8)
Fuel Tank Bracket Attachment Bolts, lb-ft (Nm) ............... 12 to 14 (16.3 to 19.0)
Flywheel Shroud Sheet Metal Screws, lb-in (Nm) ............... 45 to 55 (5.1 to 6.2)
Breaker Point Mounting Assembly Screws, lb-in (Nm) ............... 15 to 20 (1.7 to 2.3)
Breaker Box Cover Clamp Screw, lb-in (Nm) ............... 10 to 15 (1.1 to 1.7)
Crankcase Oil Base Mounting Bolts, lb-ft (Nm) ............... 24 to 28 (32.5 to 37.9)
Oil Drain Plug Internal Pipe Threads, lb-ft (Nm) ............... 15 to 18 (20.3 to 24.4)
Starter Motor Frame Long Thru Bolts, lb-in (Nm) ............... 40 to 50 (4.5 to 5.6)
Starter Motor External Mounting Bolts, lb-ft (Nm) ............... 18 to 22 (24.4 to 29.8)
Alternator Stator Mounting Screws, lb-in (Nm) ............... 30 to 35 (3.4 to 4.0)
Manifold Exhaust Pipe Locknut Fastener, lb-ft (Nm) ............... 14 to 16 (19.0 to 21.7)
Valve Cover Breather Retaining Screw, lb-in (Nm) ............... 25 to 30 (2.8 to 3.4)
Camshaft Thrust Plate Retaining Screws, lb-in (Nm) ............... 65 to 75 (7.3 to 8.5)
Governor Flyweight Gear Nut Fastener, lb-ft (Nm) ............... 8 to 10 (10.8 to 13.6)
Blower Shroud Screen Retaining Screws, lb-in (Nm) ............... 35 to 40 (4.0 to 4.5)
Regulator Rectifier Mounting Screws, lb-in (Nm) ............... 20 to 25 (2.3 to 2.8)
Ignition Coil Bracket Support Bolts, lb-ft (Nm) ............... 6 to 8 (8.1 to 10.8)
Fuel Diaphragm Pump Flange Nut Screws, lb-in (Nm) ............... 45 to 55 (5.1 to 6.2)
Air Intake Horn Adapter Mounting Screws, lb-in (Nm) ............... 35 to 45 (4.0 to 5.1)
Choke Cable Clip Retaining Fastener, lb-in (Nm) ............... 15 to 20 (1.7 to 2.3)
Throttle Wire Connector Screw Terminal, lb-in (Nm) ............... 10 to 12 (1.1 to 1.4)
Oil Dipper Attachment Fastener Screw, lb-in (Nm) ............... 45 to 55 (5.1 to 6.2)
Crankcase Alignment Dowel Thread Pins, lb-ft (Nm) ............... 12 to 15 (16.3 to 20.3)
Auxiliary Power Takeoff Keyway Set Screws, lb-in (Nm) ............... 50 to 60 (5.6 to 6.8)
Idler Gear Spindle Shaft Locking Anchor, lb-ft (Nm) ............... 18 to 22 (24.4 to 29.8)
Timing Gear Cover Inspection Hole Plug, lb-ft (Nm) ............... 8 to 10 (10.8 to 13.6)
Speed Control Lever Friction Disk Nut, lb-in (Nm) ............... 25 to 30 (2.8 to 3.4)
Compression Release Mechanism Spring Anchor, lb-in (Nm) ............... 15 to 18 (1.7 to 2.0)
Deflector Shroud Cylinder Head Sheet Metal, lb-in (Nm) ............... 35 to 45 (4.0 to 5.1)
Flywheel Starter Ring Gear Retaining Screws, lb-ft (Nm) ............... 12 to 14 (16.3 to 19.0)
Ground Wire Terminal Chassis Hex Nut, lb-in (Nm) ............... 20 to 25 (2.3 to 2.8)
Crankcase Base Breather Inspection Port Plug, lb-ft (Nm) ............... 10 to 12 (13.6 to 16.3)
Main Bearing Cup Housing Support Cap Screws, lb-ft (Nm) ............... 20 to 24 (27.1 to 32.5)

________________________________________________________________________________________

• Wisconsin Engines Specs
  
• Wisconsin S7D Specs
  
• Wisconsin S8D Specs
  
• Wisconsin S10D Specs
  
• Wisconsin S14D Specs
  

________________________________________________________________________________

________________________________________________________________________________________

_______________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________

________________________________________________________________________________________