1984-2007 Honda CR80R/CR85R/CR125R/CR250R Cylinder/Piston Overhaul

CR80/CR85/CR125/CR250-R Cylinder & Piston

Always refer to a service manual before attempting to do any work on your motorcycle, important data including step-by-step procedures, diagrams, pictures, illistrations, specifications cleaning & maintenance information will help you care for and lower the chances of premature parts failurer. Below is a great reference source available for instant download straight to your computers in just seconds.
All manuals are simple to use pdf downloads and are delivered straight to your computer in just seconds. Not sure what a pdf is? Here is a sample of what a  Service Manual looks like (sample is restricted to one page only).
Service Manuals


Honda CR125R, CR125-R, CR-125-R, CR125 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 & 2007 models.

Honda CR250R, CR250, CR-250R, CR250-R 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 00, 01, 02, 03, 04, 05, 06 & 07 year CR250R models.

Honda  CR80R, CR85R, CR80, CR85, CR-80-R, CR-85-R 1984, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 00, 01, 02, 03, 04, 05, 06 & 07 year CR-80/85-R models.

This cylinder-piston guide describes the service procedures as general procedures and what is involved in the replacement, service and overhaul of top end piston & cylinder on Honda CR80R / CR85R / CR125R & CR250R motorcycles. It is only recommended to follow the cylinder & piston guide in the reference guide links listed above as they contain the same information mechanics and technicians use to service your bike.

Important information contained in this page like special notes, tips, proper procedures, complete specifications, torque specs, additional torque specs and recommendations must be followed in the reference links to ensure that the task is performed in the proper flow and to manufacture spec's recommendations.

The piston & cylinder can be serviced with the engine installed in the frame. Take care not to damage the cylinder wall and piston. Be careful not to damage the mating surfaces when removing the cylinder. Do not strike the cylinder too hard during removal.


When might it be time to service and overhaul the piston and cylinder? The following is a list of troubleshooting areas that should be performed to dianosis the causes.

  • Compression too low, hard starting or poor performance at low speed
    • Compression too high, overheating or knocking
    • Excessive smoke
    • Abnormal noise

Troubleshooting and possible causes:

  • Leaking cylinder head gasket
  • Worn, stuck or broken piston ring
  • Worn or damaged cylinder and piston
  • Bent connecting rod
  • Excessive carbon built-up on piston head or combustion chamber
  • Worn cylinder, piston or piston rings
  • Improper installation of piston rings
  • Scored or scratched piston or cylinder wall
  • Worn piston pin or piston pin hole
  • Worn connecting rod small end
  • Worn cylinder, piston or piston rings



Remove the cylinder head (see diagrams in reference download) and remove the following:

Dowel pins

Do not strike the cylinder too hard and do not damage the mating surface with a screwdriver.


Place a clean shop towel over the crankcase to prevent the clip from falling into the crankcase then remove the piston pin clips with pliers. Push the piston pin out of the piston and connecting rod, and remove the piston. Spread each piston ring and remove it by lifting up at a point opposite the gap. Clean carbon deposits from the ring grooves with a ring that will be discarded. Never use a wire brush; it will scratch the groove.



Inspect the cylinder bore for scratch(s) or wear. Measure the cylinder I.D. at three levels in an X and Y axis. Take the maximum reading to determine the cylinder wear (see reference download for service limits for exact model and year).

SERVICE LIMIT: Always refer to the reference links above for service wear limits.

Calculate the cylinder-to-piston clearance. Refer to your manual download for measurement of the piston O. Calculate the cylinder taper and out-of-round at three levels in an X and Y axis. Take the maximum reading to determine the taper and out-of-round. The cylinder must be rebored and an oversize piston fitted if the service limits are exceeded. The four oversize pistons are available from 0.25 mm piston to 1.0 mm piston in intervals of 0.25 mm (0.010 in).

The cylinder must be rebored so that the clearance for an oversize piston is (see reference links)  mm (see reference links). Check the top of the cylinder for warpage with a straight edge and feeler gauge across the studs and bolt holes.


Inspect the piston rings for movement by rotating the rings. The rings should be able to move in their grooves without catching. Push the ring until the outer surface of the piston ring is nearly flush with the piston and measure the ring-to-ring groove clearance. Insert each piston ring into the bottom of the cylinder squarely using the piston. Measure the ring end gap. Measure the piston pin O.D. 90° to the piston pin hole and at point 15 mm (0.6 in) from bottom of the piston skirt. Compare this measurement against the maximum cylinder I.D. measurement and calculate the piston-to- cylinder clearance.

Measure piston pin hole and take the maximum reading to determine the I.D. Measure the piston pin O.D. at three points and calculate the piston-to-piston pin clearance. Measure the connecting rod small end I.D. Calculate the connecting rod-to-piston pin clearance.


Remove the stud bolts from the cylinder and install new stud bolts in their proper positions and tighten them. After installation, measure the stud height from the cylinder surface.



Carefully install the piston rings into the piston ring grooves with the markings facing up.

NOTE: Do not confuse the top and second rings. To install the oil ring, install the spacer first, then install the side rails. Stagger the piston ring end gaps 120° degrees apart from each other. Stagger the side rail end gaps as shown. Be careful not to damage the piston and rings.



Place a clean shop towel over the crankcase to prevent the clip from falling into the crankcase. Apply molybdenum oil solution to the piston pin outer surface. Apply engine oil to the piston pin hole and connecting rod inner surface. Install the piston with the ‘‘IN’’ mark toward the intake side and insert the piston pin through the piston and connecting rod. Install new piston pin clips into the grooves in the piston pin hole. Make sure that the piston pin clips are seated securely. Do not align the piston pin clip end gap with the piston cutout.


Clean the gasket surfaces of the cylinder and crankcase thoroughly, being careful not to damage it, and being careful not to allow gasket material into the crankcase. Blow through the oil passage (stud bolt hole) in the cylinder with compressed air. Apply liquid sealant to the crankcase mating areas. Install the dowel pins and a new gasket and apply engine oil to the cylinder wall, piston and piston ring outer surfaces. Install the cylinder over the piston while compressing the piston rings with your fingers. Make sure that the cylinder touches the crankcase evenly. Install the cylinder head (see diagrams in reference download).

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ring lands. Always check piston ring installed gap

before rings are installed on piston. See Page 3.36.

If the piston has been in service clean any

accumulated carbon from the ring grooves and oil

control ring holes.

1. Place the oil control ring expander in oil ring

groove with the end gap facing forward. The

expander has no up or down marking and can be

installed either way. The ends should butt

squarely together and must not overlap.

2. Install the oil ring top rail with the end gap at least


3. Install the bottom rail with the gap at least 30

from the end of the expander on the side opposite

the top rail gap.

Ring Profile


“R1” Mark up

“R” Mark up


4. Install the second ring with the “R”mark facing up.

Position the end gap toward the rear (intake) side

of the piston.

5. Install the top ring (chrome faced) with the “R1”

mark facing up and the end gap facing forward

(toward the exhaust).

6. Check to make sure the rings rotate freely in the

groove when compressed.

Apply clean engine oil to all ring surfaces and from the end of the expander.


1. Clean the gasket surfaces on the cylinder and

crankcase. Remove all traces of old gasket


2. Make sure the cylinder mounting bolt holes are

clean and free of debris.

3. Install a new circlip on one side of the piston with

the end gap facing

up or down.


removal process. Do not re-use old circlips. Do not

compress the new clip more than necessary upon

installation to prevent loss of radial tension. Severe

engine damage may result if circlips are re-used or

deformed during installation.

Circlips become deformed during the

4. Apply clean engine oil to the piston rings, ring

lands, piston pin bore, piston pin, and piston skirt.

Lubricate the connecting rod (both ends) and

crankshaft main bearing area.

5. I

the pin casting notch facing the rear of engine

(starter side). The piston pin should be a

push fit in the piston.

nstall the piston on the connecting rod with

6. Install the other circlip with the gap facing up or

down. (See Caution withStep 3 above). Push the

piston pin in both directions tomake sure the clips

are properly seated in the groove.

7. Place the dowel pins in the crankcase and install a

new cylinder base gasket.

8. Lubricate the piston and rings with assembly lube

and install a sleeve--type ring compressor on the

piston assembly. Verify that the ring gaps are 120

degrees apart from each other before installation.


1. Position the Piston Support Block (

(A) beneath the piston skirt to support the piston

during cylinder installation.


2. Apply clean engine oil liberally to the cylinder bore

and tapered area of the sleeve. Install the

cylinder with a slight rocking motion until the rings

are captive in the sleeve.

3. Remove the ring compressor and support block.

4. Push the cylinder downward until fully seated on

the base gasket.

PN 2870390)


rotating the engine to avoid damage to the chain, drive

sprocket teeth, or tensioner blade.

5. Install the two 6 mm bolts, but do not tighten.

If cam chain is installed, hold it up while



Clean the gasket surfaces on the cylinder head and

cylinder. Remove all traces of old gasket material.

1. Install the cam chain tensioner guide. Be sure

bottom end of guide is located properly in


2. Install the two dowel pins and a new cylinder head


3. Place the cylinder head on the cylinder. Apply a

film of engine oil to the cylinder head bolt threads

and washers, and hand tighten the bolts.