1994 – 2001 Yamaha YZ125 Yamaha YZ125 Cylinder Head – Inspection

Nov 11, 2025

How to Inspect your 1994 through 2001 Yamaha YZ125 Cylinder Head.

I recommend opening the Service & Torque Specs in a new tab for easy reference.
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Pictures with captions are below after step.
If you need any help don’t hesitate to ask!

Clean the entire head with parts cleaner and a soft brush.
Clean the cylinder head mounting surface with a plastic scraper.
If you have carbon buildup in the combustion chamber, soak it in parts cleaner, then gently scrub it off with a scouring pad.
You can use a wire brush on a drill, but be very careful not to apply too much pressure.
With the surface cleaned, inspect for any signs of obvious wear or damage.
Check that the water jackets are free of obstruction, and the mating surface is smooth.
You can smooth out small burs and pits with 600-grit emery paper, but be very careful, 1000th of an inch could take your head out of spec.
I like to buff my combustion chamber and mating surface to a shine, but I’m extra.
Gently run your spark plug thread follower all the way through the spark plug hole to correct any thread damage.
If your threads are stripped out and your follower cannot correct them, you will need to install a helicoil.
- A helicoil is a screw thread insert. It’s basically an extra set of threads made out of steel that can replace damaged threads in soft materials, like aluminum

Lay your straight edge across two adjacent mounting holes, and try to fit your feeler gauge underneath.
If you find any gaps, double-check the head for smoothness and try again.
If the gap still exists, record the measurement.
Repeat this measurement around the head.
The warp limit for your cylinder head is 0.0012 inches.
You can bring your head into spec with a sheet of 600-grit emery paper and a piece of glass.
Tape the emery paper to a sheet of glass, and run your cylinder head in a figure eight motion against the paper, using light pressure.
Stop and measure your clearance every three passes.
If you cannot bring your head back to spec, you will need to take it to a machine shop.
Clean your cylinder head with HOT soapy water, and rinse thoroughly.
Dry with compressed air and double-check the mating surface for smoothness.
If everything looks good, apply a light coat of WD-40 to avoid oxidation, and wrap your cylinder head in plastic wrap to avoid contaminants until you are ready for assembly.
Finally, check the hanger plates for damage.
If the plates are bent or the holes are elongated, they need to be replaced.
These have some dents on the top, but they are still straight with round holes.