1997-2001 Honda CR250 – Top End Service – Part 5: Piston Inspection

How To Inspect The Piston On Your 1997 – 2001 Honda CR250.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection

The Tools You Will Need Are:

  • 4mm Hex Wrench
  • Needle Nose Pliers
  • Snap Ring Pliers
  • Calipers
  • Telescoping Bore Gauges

If you don’t have a set of Telescoping bore Gauges, you can get them HERE.

Click Here To Open The Top End Specifications Table In A New Window

This piston has perfect carbon build up, meaning the pre-mix ratio is correct, the carburetor is tuned right, and the transfer ports are flowing freely.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Perfect Carbon PatternThis piston did make contact on the intake side of the cylinder, indicating the piston warmed up before the cylinder, and expanded too fast.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Cylinder ContactThis 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 in order to re-use it, so one thing wrong and it needs to be replaced.

You will however need new wrist pin clips regardless of your piston’s condition.

Piston Inspection

Expand the rings, and move them straight up off the piston. Do not try to “unwrap” the rings.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Remove RingsYou can use a ring spreader, but your fingernails will work just fine.

Label the top and bottom rings accordingly.  You will not re-use these rings, but you will need them later.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Label RingsClean the carbon from the crown of the piston with parts cleaner and a plastic scraper.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Clean CrownClean the pin bore and ring grooves with a soft brush and a pick.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Clean Ring Grooves 1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Check Locating PinsInspect the piston crown for damage. If you find any, replace the piston.

Inspect the ring grooves and locating pins, if they are damaged, or you have missing pins, 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?

Pin, Needle Bearing & Connecting Rod Inspection

Clean and dry the wrist pin and bearings, then check for excessive wear or damage.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Clean Wrist Pin and BearingsMeasure the pin at the three contact points, and the pin bore.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Measure Wrist PinThe pin should measure no LESS than 0.707 inches.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Measure Pin BoreAnd the pin bore should measure no GREATER than 0.709 inches.

The piston to pin clearance can be no GREATER than 0.0016 inches.

To calculate the piston to pin clearance, subtract the smallest wrist pin measurement from the largest piston bore measurement.

The largest bore measurement for this piston was 0.708 inches, and my smallest pin measurement was 0.707 inches, resulting in a clearance of 0.001 inches, which is within spec.

Inspect and measure the connecting rod bore.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Measure Connecting Rod BoreThe connecting rod bore can be no greater than 0.867 inches.

If the connecting rod is out of spec, you will need to replace the crank assembly.

Insert the needle bearing into rod bore, if the conrod bore is in spec and there is play, the needle bearing needs to be replaced.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Install Needle BearingsPiston Ring Inspection

Even though you will not be re-using your piston rings, we can use them to see what the engine has been up to.

Take the old pistons and roll them around their respective grooves and feel for any snags or binding.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Check Ring GroovesRe-Install the piston rings.

Lubricate the crown and grooves with assembly lube or 2-stroke oil.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Lubricate Crown And GroovesPosition the bottom ring with the IT mark facing UP. Spread and install the ring straight down over the piston. Make sure the ring is seated against the locating pin.

1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Install RingsRepeat with the top ring.

Press the rings into the groove, and measure around the piston with a feeler gauge.1997 - 2001 Honda CR250 - Top End Service - Part 5 - Piston Inspection - Measure Ring To PistonThe top ring should have a clearance no greater than 0.0037 inches.

And the bottom ring should have a clearance no greater than 0.026 inches.

Repeat this process with the new rings. If the clearance exceeds spec with new rings, the piston must be replaced.

Part 4: Exhaust Valve Removal
Part 5: Cylinder Head Inspection
1997 - 2001 Honda CR250 - Top End Service - Part 4 - Exhaust Valve Removal - Featured1997 - 2001 Honda CR250 - Top End Service - Part 6 - Cylinder Head Inspection

Keep Your ’99-’01 CR250 Running Right!

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