1994 – 2001 Yamaha YZ125 Piston – Inspection

How to Inspect your 1994 through 2001 Yamaha YZ125 Piston

The Tools You Will Need

• 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
• Socket Extension
• Feeler Gauge → https://amzn.to/4nrpkwx

The Parts You Will Need

• New Piston Kit → https://amzn.to/3KXHY1w (Check Your Size)
• Piston Ring → https://amzn.to/47cDyLx
• Wrist Pin Clips → 93450-16068-00
• Top End Gasket Set → https://amzn.to/4hlHG0D

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 Piston Inspection

• Before purchasing a new piston, ensure that yours needs to be replaced. And if you do need a new piston, follow the same inspection steps before installation. 
• You never know if there’s a flaw, and taking 10 minutes to measure beats the hell out of grenading your freshly rebuilt engine.
  • If you need a new piston, just get a stock replacement. With this small of an engine, you won’t see any advantages of an ultralight forged aluminum race piston from Wiesco, or a heavy cast Pro-X piston for trail riding. The only time an aftermarket piston is needed is when your engine has undergone significant modifications.
• This piston exhibits perfect carbon buildup, indicating that the pre-mix ratio is correct, the carburetor is tuned correctly, and the transfer ports are flowing freely. 
• This piston made contact on the intake side of the cylinder, indicating that the piston warmed up before the cylinder and expanded too quickly.
• This is what happens when you don’t let your engine warm up.
• I have to replace this piston, but if your piston still looks good, it may still be within spec, and if it is, all you need are new rings.
• Your piston must be in perfect condition to reuse it; if one thing is wrong, it needs to be replaced.
• You will, however, need new wrist pin clips regardless of your piston’s condition.

Piston Measurement

• Before you bother cleaning your piston, make sure it’s within spec.
• Measure your piston 10mm from the skirt and 90° to the wrist pin.
• This measurement should be between 53.957 and 53.972 mm.
• If your piston is out of spec, it will need to be replaced, but don’t throw it out just yet.

Piston Inspection

• Expand the piston ring and move it straight up off the piston.  
• Do not try to “unwrap” the ring.
• You can use a ring spreader, but your fingernails will work just fine.
• Clean the carbon from the crown of the piston with parts cleaner and a plastic scraper.
• Clean the pin bore and ring grooves with a soft brush and a pick.
• Inspect the piston crown for damage. If you find any, replace the piston.
• Inspect the ring grooves and locating pin; if the pin is damaged or missing, replace the piston.
• Inspect the piston skirt; if you find any abrasions or cracking, replace the piston.
• Inspect the interior of the piston and pin bore for wear or cracks. Guess what you need to do if you find any?
• Measure the piston diameter perpendicular to the wrist pin bore 10mm from the bottom of the piston skirt, and record that number for your clearance calculations.

Piston Ring Inspection

• Even though you will not be reusing your piston ring, we can use it to see what the engine has been up to.
• Take the old ring and roll it around the piston in its groove, and feel for any snags or binding.
• Reinstall the piston ring.
• Press the ring into the groove, and then measure the ring-to-groove clearance with a feeler gauge.
• The ring should have a clearance no greater than .004 inches.
• If the ring-to-piston clearance is out of spec, the piston is most likely worn, not the rings.
• Repeat this process with the new ring. 
• If the clearance exceeds spec with the new ring, the piston must be replaced.
• This type of wear is caused by ring deflection, which occurs due to improper lubrication from a poor air-to-fuel ratio, incorrect premix ratio, engine braking, or a combination of all three.

Pin, Needle Bearing & Connecting Rod Inspection

• Clean and dry the wrist pin and bearings, and check for excessive wear or damage.
• Measure the pin at the three contact points and the pin bore.
• There is no manufacturer service limit for the pin, piston bore, connecting rod bore, or clearance limits. If it’s tight, use it; if it’s not tight, don’t use it.
• Insert the needle bearing into the rod bore. If there is play, the needle bearing needs to be replaced.
• Inspect the crankshaft side clearance by inserting a feeler gauge between the crankshaft and connecting rod. 
• My crankshaft has been removed for the rebuild, but it can be easily accessed inside the engine.
• The connecting rod to crankshaft clearance should be no more than 0.026 inches.

Piston Assembly

Gap New Rings

• Before installing the piston ring, we need to ensure that the installed end gap is within spec.
• Insert the piston ring in the bottom of the cylinder and push it down with the piston so it inserts evenly.
• Measure the ring gap with a feeler gauge.
• The end gap should be no greater than 
  • 0.031 inches on 1994 – 1995 model years
  • 0.047 on 1996 – 2001 model years

• If the rings are too tight, you can file the ends with a fine metal file or sandpaper until they are within spec.

Piston Pin Clip

• Begin by installing your safety glasses.
• Install the first piston pin clip into the pin bore groove. 
• Your thumbs and a small pick should work just fine, but be careful not to gouge the cylinder with the end of the clip.
• Rotate the clip so the opening faces up.

Piston Ring Installation

• Double-check that the ring is clean and dry.
• Lubricate the piston edge and ring grooves with 2-stroke oil.
• If your ring has any markings, install it with the markings facing up.
• Spread the ring just enough to clear the piston, and install straight down; do not try to wrap the ring into the groove.
• Align the ring with the alignment pin.
• Check that the ring is free in the groove.
• Place your piston, pin, clip, and bearings in a ziplock to keep them clean and organized.

If you have any questions, please let me in the comments, or send me a DM!

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