Note the IN mark which faces the inlet side of the cylinder

A Prise out the circlip ...

B ... then push out the pin and remove the piston

2«40 Engine, clutch and transmission

in boiling water then wring it out and wrap it around thepiston - this will expand the alloy piston sufficiently to release its grip on thepin. If the piston pin is particularly stubborn, extract it using a drawbolt tool, but be careful to protect thepiston's working surfaces.

Inspection

5Before the inspection process can be carried out, the pistons must be cleaned and the old piston rings removed.

6 Using your thumbs or a piston ring removal and installation tool, carefully remove the rings from the pistons (see illustration).Do not nick or gouge the pistons in the process. Note which way up each ring fits and in which groove. The top and second compression rings have different profiles and they must be installed in their original positions if being re­used. The upper surface of each ring is usually marked at one end. This information will also be required if you are fitting new rings (see illustration).

7Scrape 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

B Piston ring reference (arrowed)

is soft and will be eroded away by the wire brush.

8 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.

9 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.

10 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.

11 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.

Date: 2016-01-14; view: 525