Note: The pistons can be removed with the engine in the frame.
Removal
1. Remove the cylinder barrel(s) (see Section 15). Stuff clean rag into the crankcase aperture to support the connecting rod and to prevent anything falling in, such as the circlip.
2. Before removing the piston from the connecting rod. use a sharp scriber or felt marker pen to write the cylinder identity on the crown of each piston (or on the inside of the skirt if the piston is dirty and going to be cleaned). Each piston crown should already be marked IN on the intake side of the cylinder, though the mark is likely to be invisible until the piston is cleaned (see illustration).
16.2. Note the "IN" mark on the piston which faces the intake side
3. Carefully prise out the circlip on one side of the piston using needle-nose pliers or a small flat-bladed screwdriver inserted into the notch (see illustration). Push the piston pin out from the other side to free the piston from the connecting rod (see illustrations). Remove the other circlip and discard them as new ones must be used. When the piston has been removed, install its pin back into its barrel so that related parts do not get mixed up.
16.3a. Prise the piston pin circlip out from one side of the piston
16.3b. Push the piston pin out from the other side ...
16.3c ...then withdraw it and remove the piston
If a piston pin is a tight fit in the piston bosses, soak a rag in boiling water then wring it out and wrap it around the piston - this will expand the alloy piston sufficiently to release its grip on the pin. If the piston pin is particularly stubborn, extract it using a drawbolt tool, but be careful to protect the piston's working surfaces.
Inspection
4. Using your thumbs or a piston ring removal and installation tool, carefully remove the rings from the pistons (see illustrations 17.11, 10, and 8c, b and a). Do not nick or gouge the pistons in the process. Carefully note which way up each ring fits and in which groove as they must be installed in their original positions if being re-used. The upper surface of the top ring is marked with the letter R or N at one end, and the second (middle) ring is either marked RN or, or is unmarked (see illustration). The top and middle rings can also be identified by their different profiles (see illustration 17.12).
16.4. Note the marking on the top surface of the ring
5. Scrape all traces of carbon from the tops of the pistons. A hand-held wire brush or a piece of fine emery cloth can be used once most of the deposits have been scraped away. Do not. under any circumstances, use a wire brush mounted in a drill motor to remove deposits from the pistons; the piston material is soft and will be eroded away by the wire brush.
6. Use a piston ring groove cleaning tool to remove any carbon deposits from the ring grooves. If a tool is not available, a piece broken off an old ring will do the job. Be very careful to remove only the carbon deposits. Do not remove any metal and do not nick or gouge the sides of the ring grooves.
7. Once the deposits have been removed, clean the pistons with solvent and dry them thoroughly. If the identification previously marked on the piston is cleaned off, be sure to re-mark it with the correct identity. Make sure the oil return holes below the oil ring groove are clear.
8. Carefully inspect each piston for cracks around the skirt, at the pin bosses and at the ring lands. Normal piston wear appears as even, vertical wear on the thrust surfaces of the piston and slight looseness of the top ring in its groove. If the skirt is scored or scuffed, the engine may have been suffering from overheating and/or abnormal combustion, which caused excessively high operating temperatures. The oil pump should be checked thoroughly. Also check that the circlip grooves are not damaged.
9. A hole In the piston crown, an extreme to be sure, is an indication that abnormal combustion (pre-ignition) was occurring. Burned areas at the edge of the piston crown are usually evidence of spark knock (detonation). If any of the above problems exist, the causes must be corrected or the damage will occur again.
10. Measure the piston ring-to-groove clearance by laying each piston ring in its groove and slipping a feeler gauge in beside it (see illustration). Make sure you have the correct ring for the groove (see Step 5). Check the clearance at three or four locations around the groove. If the clearance is greater than specified, renew both the piston and rings as a set. If new rings are being used, measure the clearance using the new rings. If the clearance is greater than that specified, the piston is worn and must be replaced with a new one.
16.10. Measure the piston ring-to-groove clearance with a feeler gauge
11. Check the piston-to-bore clearance by measuring the bore (see Section 15) and the piston diameter. Make sure each piston Is matched to its correct cylinder. Measure the piston 10.0 mm (XL600V and XRV750 models) or 12 mm (XL650V models) up from the bottom of the skirt and at 90° to the piston pin axis (see illustration). Subtract the piston diameter from the bore diameter to obtain the clearance. If it is greater than the specified figure, and if not already done, check the cylinder for wear (see Section 15). If the cylinder is good but the piston Is worn, replace it with a new one. If the cylinder is worn it can be rebored, and then oversize pistons and rings, available from Honda, can be fitted.
16.11. Measure the piston diameter with a micrometer
12. Apply clean engine oil to the piston pin, Insert it into the piston and check for any freeplay between the two (see illustration). Measure the pin external diameter and the pin bore in the piston (see illustrations). Calculate the difference to obtain the piston pin-to-piston pin bore clearance. Compare the result to the specifications at the beginning of the Chapter. If the clearance is greater than specified, replace the components that are worn beyond their specified limits. If not already done (see Section 28), repeat the measurements between the pin and the connecting rod small-end (see illustration 28.6).
16.12a. Slip the pin into the piston and check for freeplay between them
16.12b. Measure the external diameter of the pin...
16.12c ...and the internal diameter of the bore in the piston...
13. Prise the oil jets out of the crankcase using a small screwdriver, taking great care not to damage them or drop them into the crankcase - make sure your rag covers the hole completely (see illustration). Remove tho O-rings and discard them (see illustration). Clean the jets with solvent and blow them through with compressed air if available. Fit new O-rings and press the jets back into the crankcase (see illustration).
16.13a. Prise out the oil jet
16.13b. Fit a new O-ring...
16.13c ...and press the jet into place
Installation
14. Inspect and install the piston rings (see Section 17).
15. Lubricate the piston pin, the piston pin bore and the connecting rod small-end bore with molybdenum disulphide oil (a 50/50 mixture of molybdenum disulphide grease and clean engine oil).
16. When installing the pistons onto the connecting rods, make sure the IN mark on the piston crown faces the intake side of the cylinder (see illustration 16.2).
17. Install a new circlip in one side of the piston (do not re-use old circlips). Line up the piston on its correct connecting rod (see illustration 16.3c). and insert the piston pin from the other side (see illustration). Secure the pin with the other new circlip (see illustration). When installing the circlips, compress them only just enough to fit them in the piston, and make sure they are properly seated in their grooves with the open end away from the removal notch.
16.17a. Slide the pin through the piston and connecting rod ...
16.17b ...and secure it with the circlip, locating the open end away from the notch in the piston
18. Install the cylinder barrel(s) (see Section 15).