F510 & F525 Cylinder Block – ANALYZE PISTON RING WEAR
Rings of the wrong size or rings having improper end gap will not conform to the shape of the cylinder. This results in high oil consumption and excessive blow-by.
Ring end gaps should be staggered on the piston during installation. End gaps in alignment can also cause oil consumption and blow-by.
Light scuffing or scoring (A) of both rings and piston occurs when unusually high friction and combustion temperatures approach the melting point of the piston material.
When this condition exists, it is due to one or more of the following probable causes:
• Dirty cooling shroud and cylinder head.
• Lack of cylinder lubrication.
• Improper combustion.
• Wrong bearing or piston clearance.
• Too much oil in crankcase causing fluid friction.
The engine operating at abnormally high temperatures may cause varnish, lacquer or carbon deposits (A) to form in the piston grooves making the rings stick. When this happens, excessive oil consumption and blow-by will occur.
Engine overheating and ring sticking is usually caused by one or more of the following:
• Overloading engine.
• Incorrect ignition timing.
• Lean fuel mixture.
• Dirty cooling fins.
• Incorrect oil.
• Low oil supply.
• Stale fuel.
Vertical scratches (A) across the piston rings are due to an abrasive in the engine. Abrasives may be airborne, may have been left in the engine during overhaul or may be loose lead and carbon deposits.
When this condition exists, check for one or more of the following:
• Damaged, collapsed or improperly installed air filter.
• Loose connection or damaged gasket between air cleaner and carburetor.
• Air leak around carburetor-to-cylinder block gasket.
• Air leakage around throttle shaft.
• Failure to properly clean cylinder bore after reconditioning engine.
Abrasive particles in engine oil cause scratches on side rails (A) of oil control ring. Inner spacer (B) wear or distortion may case:
• High oil consumption.
• Increased deposits in combustion chamber.
• Sticking compression rings.
Increased oil consumption may be caused by:
• Worn side rails with low tension.
• Worn or distorted inner spacer.
Detonation, is abnormal combustion causing excessive temperature and pressure in the combustion chamber.
Commonly called carbon knock, spark knock or timing knock, detonation occurs as the compressed fuel-air mixture ignites spontaneously to interrupt the normal ignition.
The following is a list of possible causes for detonation:
• Lean fuel mixture.
• Low octane fuel.
• Advanced ignition timing.
• Engine lugging.
• Build-up of carbon deposits on piston or cylinder head, causing excessive compression.
• Wrong cylinder head or milling of head increasing compression ratio.
Pre-ignition is the igniting of the fuel-air mixture prior to regular ignition spark. Pre-ignition causes internal shock, resulting in pings, vibration, detonation and power loss.
Severe damage to piston (A), rings and valves results from pre-ignition.
Check the following for causes of pre-ignition:
• Internal carbon deposits.
• Incorrect spark plug (high heat range).
• Broken ceramic in spark plug.
• Sharp edges on valves.
Check rod and piston alignment when piston shows a diagonal wear pattern (A) extending across the skirt of the piston. Contact with the cylinder wall shows on bottom of skirt at left and ring lands on the right.
A cylinder bored at an angle to the crankshaft can also cause improper ring contact with the cylinder.
This condition causes:
• Rapid piston wear.
• Uneven piston wear.
• Excessive oil consumption.
A broken spring clip caused the damage (A) shown.
Spring clips loosen or break due to:
• Rod misalignment.
• Excessive crankshaft end play.
• Crankshaft journal taper.
• Weak spring clips.
• Incorrectly installed spring clips.
Inertia can cause a broken spring clip to gouge out the piston and cylinder, causing extensive damage.