Yamaha MZ125

Yamaha MZ125The Yamaha MZ125 is a 123 cc (7.5 cu-in) single-cylinder air-cooled 4-stroke internal combustion general-purpose gasoline engine with horizontal shaft manufactured by Yamaha Motor Co., Ltd.

The Yamaha MZ125 engine has an OHV (overhead valve) design; float type carburetor; centrifugal flyweight governor; mechanical splashing lubrication system; Transistor Controlled Ignition (T.C.I.) and recoil or electrical (option) starter. The Yamaha MZ125 used aluminum cylinder block with the spiny cast-iron sleeve. For exhaust valve used high heat resistant SUH3 steel.

The cylinder bore is 56.0 mm (2.20 in) and the piston stroke is 50.0 mm (1.97 in), compression ratio rating is 8.3:1. The engine produced 3.4 PS (2.5 kW) at 3,600 rpm of maximum horsepower and 2.9 PS (2.1 kW) at 3,600 rpm of continuous rated power. Maximum torque is 7.3 N·m (0.7 kg·m, 5.4 ft·lb) at 2,400 rpm.

General information

Engine Specifications
Type4-stroke, OHV, single cylinder
Displacement123 cm 3 (7.5 cu-in)
Max. horsepower3.4 PS (2.5 kW) at 3,600 rpm
Max. torque (crank PTO)7.3 N·m (0.7 kg·m, 5.4 ft·lb) at 2,400 rpm
CarburetorFloat type
Cooling systemForced-air
Ignition systemTransistor Controlled Ignition (T.C.I.)
Lubricating systemMechanical splashing
Starting systemRecoil or Electrical (option)
Fuel usedUnleaded gasoline (octane number 86 or higher)
Fuel tank capacity4.5 L (1.18 US Gal)
PTO shaft rotationCounterclockwise (from PTO shaft side)

Dimensions and Weight

Yamaha MZ125
Length, mm (in)325 (12.8)
Width, mm (in)352 (13.8)
Height, mm (in)370 (14.6)
Dry weight, kg (lb)15.5 (34.2)

Cylinder Block and Head

Cylinder block
Compression ratio:8.3:1
Bore x Stroke56 X 50 mm (2.20 x 1.97 in)
Sleeve internal diameter56.005-56.015 mm (2.0249-2.2053 in)
Piston skirt outer diameter56.000 mm (2.205 in)
Number of piston rings (compression / oil):2 / 1
Ring side clearanceTop0.040-0.080 (0.0016-0.0030 in)
Second0.020-0.060 (0.0008-0.0024 in)
Ring end gapTop0.20-0.40 mm (0.008-0.016 in)
Second0.20-0.40 mm (0.008-0.016 in)
Oil0.20-0.40 mm (0.008-0.016 in)
Piston pin outer diameter15.995-16.000 mm (0.6297-0.6299 in)
Connecting rod small end inner diameter16.006-16.020 mm (0.6301-0.6307 in)
Connecting rod big end inner diameter28.000-28.015 mm (1.1023-1.1029 in)
Crankpin outer diameter28.00 mm (1.10 in)
Cylinder Head
Warpage (limit)0.10 mm (0.004 in)
Valve Arrangement:OHV
Valve head diameterINTAKE21.0 mm (0.83 in)
EXHAUST19.0 mm (0.75 in)
Stem outer diameter (standard):INTAKE5.5 mm (0.22 in)
EXHAUST5.5 mm (0.22 in)
Cam height:INTAKE26.9 mm (1.06 in)
EXHAUST26.68 mm (1.05 in)
Camshaft journal diameter:14.193-14.989 mm (0.5895-0.5901 in)

Tightening torque specs

Tightening torque specs
Cylinder head20 Nm; 2.0 kg·m; 14 ft·lb
Cylinder head cover10 Nm; 1.0 kg·m; 7 ft·lb
Crankcase cover22 Nm; 2.2 kg·m; 16 ft·lb
Connecting rod12 Nm; 1.2 kg·m; 9 ft·lb
Flywheel magneto65 Nm; 6.5 kg·m; 47 ft·lb
Governor arm8 Nm; 0.8 kg·m; 6 ft·lb
TCI unit10 Nm; 1.0 kg·m; 7 ft·lb
Oil drain plug17 Nm; 1.7 kg·m; 12 ft·lb

Service Information

Valve clearance
Intake valve clearance0.10 mm (0.004 in)
Exhaust valve clearance0.10 mm (0.004 in)
Oil system
Oil typeYamalube 4-stroke or an equivalent (SE or SF)
Recommended oilSAE 10W-30, 10W-40
Oil capacity0.6 L (0.63 US. qt, 0.53 Imp. qt.)
ModelMikuni BV-18-11
Bore size18/11
Main jet#78.8
Pilot jet#78.8
Float height16.0 mm (0.63 in)
Pilot screw opening2 turns out
Ignition system
Spark plugNGK: BPR4ES
Spark plug tightening torque18 Nm; 1.8 kg·m; 13 ft·lb
Spark plug gap0.7-0.8 mm (0.028-0.031 in)
Ignition coil air gap0.5 mm (0.02 in)
Author: Sergei Wers
About: Co-founder and CEO of a multi-brand auto repair shop (engine repair, chassis repair, body repair and painting) for over 15 years. I have been developing internet sites with technical specifications for more than seven years.

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