1994 – 2001 Yamaha YZ125 Cylinder – Installation

Fix Your Dirt Bike

Nov 12, 2025

How to Install your 1994 through 2001 Yamaha YZ125 Cylinder

The Tools You Will Need

The Tools You Will Need To Service Your YZ125 Top End:
Foot-Pound Torque Wrench → https://amzn.to/3FVEdr3
Safety Glasses → https://bulltru.com
Dremel with Sanding Disc → https://amzn.to/407t6l3
2-Stroke Oil → https://amzn.to/3WnyXSl
Assembly Lube → https://amzn.to/4ar8mty
Calipers → https://amzn.to/3BRoCXX
Brake Cleaner → https://amzn.to/4jv9v7a
Telescoping Bore Gauges → https://amzn.to/4g5reiE
Feeler Gauge → https://amzn.to/4hd7qen
Cylinder Brush → https://amzn.to/3W7Trhv
8mm Socket
10mm Socket
¼” Socket Wrench
⅜” Socket Wrench

The Specialty Tools You Will Need Are:

Motion Pro Torque Wrench Adapter → https://amzn.to/3IUQnSg
Machinists Straight Edge → https://amzn.to/4g4fSM1

Pre-Service Notes

I recommend opening the Service & Torque Specs in a new tab for easy reference.
If you need parts and tools, make sure to buy them here! → https://amzn.to/4h4lDKq
Pictures with captions are below after step.
If you need any help don’t hesitate to ask!

Yamaha YZ125 Cylinder Installation

Double-check the cylinder bore and mating surfaces for cleanliness.

Apply lithium grease to the exhaust valve pinion joint.

Install your new gasket.

Lubricate the piston with 2-stroke oil, and rotate the crankshaft so the piston is at top dead center.

Double-check that the piston ring is seated against the locating pins, and position your cylinder.

Hold the cylinder as level as you can with the piston, and compress the ring against the alignment pin.

The cylinder should slide right over the ring.

It is extremely important that you do NOT rotate the cylinder!

As you can see, you might need to fight the piston and ring to get it into the bottom of the cylinder, but whatever you need to do, make sure the cylinder goes in straight.

With your cylinder on the gasket, rotate the crankshaft several times to feel for any binding or dragging.

If your cylinder travel is anything but smooth, remove the cylinder and recheck everything.

Thread the cylinder mounting nuts finger tight.

I highly recommend using ARP Ultra Torque to get the most accurate torque specification.

Begin torquing the nuts in the same cross pattern.

Turn the nuts a quarter turn at a time, taking as many passes as needed to reach torque.

Torque the cylinder mounting nuts to 21 ft-lbs.

Remember, when using the torque adapter, you will NOT need to compensate for length if the adapter remains at a right angle to your torque wrench.
1 – Install your new gasket
2 – Hold the cylinder as level as you can with the piston, and compress the ring against the alignment pin
3 – I highly recommend using ARP Ultra Torque to get the most accurate torque specification
4 – Begin torquing the nuts in the same cross pattern
5 – Torque the cylinder mounting nuts to 21 ft-lbs