⚠️ Safety Warning: Valve clearance must only be checked and adjusted when the engine is completely cold (room temperature). Checking clearances on a warm engine will result in incorrect measurements, potentially leading to burnt valves or poor performance. Always rotate the engine by hand using the crankshaft bolt; never use the starter motor to align timing marks.
Yamaha YZF-R125 Valve Clearance Adjustment
The 125cc Minarelli engine used in the YZF-R125 and WR125 uses a screw-and-locknut adjuster system. This makes maintenance easier as you do not need to replace shims to change the clearance.
Specifications (Cold Engine)
Intake Valves: 0.10 mm – 0.14 mm
Exhaust Valves: 0.20 mm – 0.24 mm
Step-by-Step Summary
Access: Remove the seat, fuel tank, and upper fairings to access the cylinder head. Remove the spark plug to make rotating the engine easier.
Top Dead Center (TDC): Remove the timing marks access caps on the left-side engine cover. Rotate the crankshaft counter-clockwise until the "T" mark aligns with the pointer. Ensure the rocker arms have slight play (this confirms you are on the compression stroke).
Measurement: Slide a feeler gauge between the valve stem and the rocker arm adjuster screw.
Correct fit: The gauge should slide with a slight "drag" (like pulling a piece of paper from under a fridge magnet).
Adjustment: If the gap is outside the specs:
Loosen the locknut.
Turn the adjuster screw until the feeler gauge has the correct drag.
Hold the screw in place and tighten the locknut.
Re-check the clearance after tightening the nut, as the gap often closes slightly during tightening.
Reassembly: Inspect the valve cover gasket; replace it if it is flattened or cracked to prevent oil leaks.
Parts & Tools
Feeler Gauges:
Precision Metric Feeler Gauges on eBay UK Replacement Gaskets:
YZF-R125 Valve Cover Gaskets on eBay UK
To begin with set the timing marks and tcd and get the valve closed and rockers on the rock, see this post on here to learn how to do this then test the valve clearances with your feeler gauge its 0.1mm for Inlet side and 0.2mm for exhust side valves.
See video of this whole process below
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