The 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
Model
MZ125
Type
4-stroke, OHV, single cylinder
Displacement
123 cm 3 (7.5 cu-in)
Max. horsepower
3.4 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
Carburetor
Float type
Cooling system
Forced-air
Ignition system
Transistor Controlled Ignition (T.C.I.)
Lubricating system
Mechanical splashing
Starting system
Recoil or Electrical (option)
Fuel used
Unleaded gasoline (octane number 86 or higher)
Fuel tank capacity
4.5 L (1.18 US Gal)
PTO shaft rotation
Counterclockwise (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 Stroke
56 X 50 mm (2.20 x 1.97 in)
Sleeve internal diameter
56.005-56.015 mm (2.0249-2.2053 in)
Piston skirt outer diameter
56.000 mm (2.205 in)
Number of piston rings (compression / oil):
2 / 1
Ring side clearance
Top
0.040-0.080 (0.0016-0.0030 in)
Second
0.020-0.060 (0.0008-0.0024 in)
Ring end gap
Top
0.20-0.40 mm (0.008-0.016 in)
Second
0.20-0.40 mm (0.008-0.016 in)
Oil
0.20-0.40 mm (0.008-0.016 in)
Piston pin outer diameter
15.995-16.000 mm (0.6297-0.6299 in)
Connecting rod small end inner diameter
16.006-16.020 mm (0.6301-0.6307 in)
Connecting rod big end inner diameter
28.000-28.015 mm (1.1023-1.1029 in)
Crankpin outer diameter
28.00 mm (1.10 in)
Cylinder Head
Warpage (limit)
0.10 mm (0.004 in)
Valve Arrangement:
OHV
Valves:
2
Valve head diameter
INTAKE
21.0 mm (0.83 in)
EXHAUST
19.0 mm (0.75 in)
Stem outer diameter (standard):
INTAKE
5.5 mm (0.22 in)
EXHAUST
5.5 mm (0.22 in)
Cam height:
INTAKE
26.9 mm (1.06 in)
EXHAUST
26.68 mm (1.05 in)
Camshaft journal diameter:
14.193-14.989 mm (0.5895-0.5901 in)
Tightening torque specs
Tightening torque specs
Cylinder head
20 Nm; 2.0 kg·m; 14 ft·lb
Cylinder head cover
10 Nm; 1.0 kg·m; 7 ft·lb
Crankcase cover
22 Nm; 2.2 kg·m; 16 ft·lb
Connecting rod
12 Nm; 1.2 kg·m; 9 ft·lb
Flywheel magneto
65 Nm; 6.5 kg·m; 47 ft·lb
Governor arm
8 Nm; 0.8 kg·m; 6 ft·lb
TCI unit
10 Nm; 1.0 kg·m; 7 ft·lb
Oil drain plug
17 Nm; 1.7 kg·m; 12 ft·lb
Service Information
Valve clearance
Intake valve clearance
0.10 mm (0.004 in)
Exhaust valve clearance
0.10 mm (0.004 in)
Oil system
Oil type
Yamalube 4-stroke or an equivalent (SE or SF)
Recommended oil
SAE 10W-30, 10W-40
Oil capacity
0.6 L (0.63 US. qt, 0.53 Imp. qt.)
Carburetor
Model
Mikuni BV-18-11
Bore size
18/11
Main jet
#78.8
Pilot jet
#78.8
Float height
16.0 mm (0.63 in)
Pilot screw opening
2 turns out
Ignition system
Spark plug
NGK: BPR4ES
Spark plug tightening torque
18 Nm; 1.8 kg·m; 13 ft·lb
Spark plug gap
0.7-0.8 mm (0.028-0.031 in)
Ignition coil air gap
0.5 mm (0.02 in)
Author:Sergei WersAbout: 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.
The 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. 
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