Disassembly/Inspection and Service Scuffi ng and scoring of pistons and cylinder walls occurs when internal engine temperatures approach welding point of piston. Temperatures high enough to do this are created by friction, which is usually attributed to improper lubrication and/or overheating of engine. Normally, very little wear takes place in piston boss- piston pin area. If original piston and connecting rod can be reused after new rings are installed, original pin can also be reused but new piston pin retainers are required. Piston pin is included as part of piston assembly – if pin boss in piston or pin are worn or damaged, a new piston assembly is required.
Ring failure is usually indicated by excessive oil consumption and blue exhaust smoke. When rings fail, oil is allowed to enter combustion chamber where it is burned along with fuel. High oil consumption can also occur when piston ring end gap is incorrect because ring cannot properly conform to cylinder wall under this condition. Oil control is also lost when ring gaps are not staggered during installation. When cylinder temperatures get too high, lacquer and varnish collect on pistons causing rings to stick, which results in rapid wear. A worn ring usually takes on a shiny or bright appearance.
Scratches on rings and pistons are caused by abrasive material such as carbon, dirt, or pieces of hard metal. Detonation damage occurs when a portion of fuel charge ignites spontaneously from heat and pressure shortly after ignition. This creates two fl ame fronts which meet and explode to create extreme hammering pressures on a specifi c area of piston. Detonation generally occurs from using low octane fuels. Preignition or ignition of fuel charge before timed spark can cause damage similar to detonation. Preignition damage is often more severe than detonation damage.
Preignition is caused by a hot spot in combustion chamber from sources such as glowing carbon deposits, blocked cooling fi ns, an improperly seated valve, or wrong spark plug(s). Replacement pistons are available in STD bore size, and in 0.25 mm (0.010 in.), and 0.50 mm (0.020 in.) oversize. Replacement pistons include new piston ring sets and new piston pins. Replacement ring sets are also available separately for STD, 0.25 mm (0.010 in.), and 0.50 mm (0.020 in.) oversize pistons. Always use new piston rings when installing pistons. Never use old rings. Some important points to remember when servicing piston rings: Piston Style A 1.
Cylinder bore must be de-glazed before service ring sets are used. 2. If cylinder bore does not need re-boring and if old piston is within wear limits and free of score or scuff marks, old piston may be reused. 3. Remove old rings and clean up grooves. Never reuse old rings. 4. Before installing new rings on piston, place top 2 rings, each in turn, in its running area in cylinder bore and check end gap. Top and middle compression ring end gap clearance is 0.25/0.56 mm (0.0100/0.0224 in.) for a new bore, or 0.94 mm (0.037 in.) for a used bore. 5.
After installing new compression (top and middle) rings on piston, make sure top compression ring-to- groove side clearance is 0.025/0.048 mm (0.0010/0.0019 in.) and middle compression ring-to- groove side clearance 0.015/0.037 mm (0.0006/0.0015 in.). If side clearance is greater than specifi ed, a new piston must be used. Piston Style B 1. Cylinder bore must be de-glazed before service ring sets are used. 2. If cylinder bore does not need re-boring and if old piston is within wear limits and free of score or scuff marks, old piston may be reused. 3. Remove old rings and clean up grooves. Never reuse old rings. 4.
Before installing new rings on piston, place top 2 rings, each in turn, in its running area in cylinder bore and check end gap. 80 mm bore engines: Top compression ring end gap clearance is 0.100/0.279 mm (0.0039/0.0110 in.) for a new bore, or 0.490 mm (0.0192 in.) for a used bore. Middle compression ring end cap is 1.400/1.679 mm (0.0551/0.0661 in.) for a new bore, or 1.941 mm (0.0764 in.) for a used bore. 83 mm bore engines: Top compression ring end gap clearance is 0.189/0.277 mm (0.0074/0.0109 in.) for a new bore, or 0.531 mm (0.0209 in.) for a used bore. Middle compression ring end cap is 1.519/1.797 mm (0.0598/0.0708 in.) for a new bore, or 2.051 mm (0.0808 in.) for a used bore. 5.
After installing new compression (top and middle) rings on piston, make sure top compression ring-to- groove side clearance and is 0.030/0.070 mm (0.0010/0.0026 in.) and middle compression ring-to- groove side clearance 0.030/0.070 mm (0.0010/0.0026 in.). If side clearance is greater than specifi ed, a new piston must be used. Install New Piston Rings NOTE: Rings must be installed correctly. Ring installation instructions are usually included with new ring sets. Follow instructions carefully. Use a piston ring expander to install rings. Install bottom (oil control) ring fi rst and top compression ring last. To install new piston rings, proceed as follows: 1. Oil control ring (bottom groove):
Install expander and then rails. Make sure ends of expander are not overlapped. 2. Middle compression ring (center groove): Install center ring using a piston ring installation tool. Make sure identifi cation mark is up or dye colored stripe (if contained) is to left of end gap. 3. Top compression ring (top groove): Install top ring using a piston ring expender. Make sure identifi cation mark is up or dye colored stripe (if contained), left of end gap. 66 690 01 Rev. C KohlerEngines.com 55