Hirth aircraft engine, diagnosing piston damage to your engine.

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These pistons are normal in appearance

The deposits on the piston dome are from the oil and ash, fuel contaminants and unburned carbon. As the thickness increases with operating time, the colour gets brighter because of higher surface temperatures.

Depending on fuel and oil used, normal deposits may also be brown, from light tan colour to almost black dark brown.

Heavy black deposits on the piston dome are unburned carbon accumulated because of too low temperatures resulting from light load operation or too rich a mixture.

Brown or black varnish on piston side below rings is baked oil caused by combustion gases blow-by. Improper oil or poor ring sealing is the cause.

Very light scoring of the piston skirt is likely from foreign material passing through the engine. It does not suggest a problem in the engine and piston replacement is not required.

Always check piston to cylinder wall clearance to make certain the wear limit has not been exceeded.

Deposits should be removed from the piston dome and cylinder head using a wood or plastic scraper. Excessive deposits collection will increase the compression ration and reduce heat transfer.

Seized Pistons

The piston shows yellow spots on the dome and seizure marks on the skirt but there is no sign of metal aluminium.

If you inspect an engine and find yellow or yellow-orange deposits on the piston dome, that engine has experienced an abnormal combustion condition known as detonation. In this situation, combustion begins normally with ignition from the spark plug but as the flame front moves across the combustion chamber, rather than burning smoothly, the unburned portion of the charge spontaneously ignites. This results in extremely high temperatures and a shock wave heard as "pinging." What has happened is combustion has been completed in about 29 degrees or crankshaft rotation instead of the normal 50.

The oxide of calcium, an element found in 2 - stroke oil additives, is normally near white in colour. At temperature very near that of the melting point of the piston, the oxide will change colour from white to yellow-orange and is an indicator that the engine was overheated. The excessive heat results in rapid piston expansion and possible failure of the lubrication film.

Possible causes:

too low an octane fuel used or fuel with too much alcohol used

jetting too lean or failure of the fuel system, i.e., clogged fuel line or filter, fuel tank not venting, problem with fuel pump, carburettor air leak into crankcase

spark plug heat range too hot

ignition timing too far advanced, possible failure of the c.d.i. amplifier box

restricted exhaust system: back pressure too high

overheated, loosely-installed spark plug

Piston dome damage

The piston shows melted areas on the dome and seizure marks on the skirt.

Detonation results in extremely high combustion chamber temperatures. That rapid heat rise, if allowed to continue, will heat combustion chamber deposits and spark plug electrodes to the point that they will ignite the charge before the spark at the plug does.

This condition is called runaway surface ignition or pre-ignition. Once pre-ignition occurs, the temperature rise is so rapid that melting point of the piston is reached while the engine is still running. Metal will melt away beneath the spark plug or in areas of high heat retention such as at the ring positioning pin.

Piston damage by foreign material

Piston shows imprints of small cylindrical parts on the dome

needles of broken wrist pin bearing trapped in the squish area between the piston and head, destroy the dome, ring land and rings.

make certain to check the condition of the big end rod bearing and cage

Piston damage by foreign material

 Small ball shaped imprints on piston dome

Possible Cause

foreign material, e.g. a pop rivet head, has entered the engine and was trapped in the squish area, before leaving through the exhaust port. In some cases the engine continues to run normally until the ring fails

Piston damage by foreign material

Piston shows a vertical groove on the skirt connected to the wrist pin hole.

Possible cause:

A wrist pin circlip has come loose and was trapped in the transfer port window.

Note always use NEW circlips when rebuilding an ultralight aircraft engine.

Piston shows scoring marks below the rings on the intake side.

Possible Cause

Snow or water has entered the engine and destroyed the oil film

if the piston has scoring on the intake and exhaust sides but the dome shows no signs of abnormal combustion, the engine has been run with too little or no oil. Check the operation of the oil injection system or the amount of oil in the premixed fuel. Also, insure proper piston clearance.

if the piston has scoring on the intake and exhaust sides, no signs of abnormal combustion on the dome and is very dark in colour, there has been a failure of the cooling system. Check the fan belt, or coolant level etc.

Piston with lubrication failure

Piston shows dark brown deposits all over the skirt.

Possible Cause

wrong oil quality, e.g. gear oil, automotive oil

unnecessary additives have been used in the fuel, i.e. octane boosters, high performance additives, upper cylinder lubricants, etc.

Piston with lubrication failure

Piston shows scuffing marks on the exhaust side of the skirt without corresponding marks on the intake side. The lubrication film failed under thermal load on the exhaust side of the piston

Possible cause:

Poor quality of oil.

Piston ring scuffing 

Cylinder liner shows scoring areas, which are exactly limited to the ring travel zone. Beside the blue coloured vertical stripes, the surface is in good condition. The rings show corresponding dark coloured marks. The reason is a breakdown of the lubrication.

Possible Cause

improper break in period

poor oil quality

not enough oil in premixed fuel

low delivery rate from oil injection system

Piston fracture 

Material fatigue fracture can sometimes be observed on pistons of high speed engines, however, skirt fracture is usually the result of excessive piston to cylinder wall clearance.

Other possible causes:

connecting rod breakage

damage due to bad handling,

crankshaft locking via piston blocking

engine has ingested foreign material

Underside Black Spot

The underside of this piston has a black spot. The black spot is a carbon deposit that resulted from pre-mix oil burning on to the piston because the piston's crown was too hot. The main reasons for this problem are overheating due to too lean carburettor jetting or coolant system failure.

Chipped Crown Drowned

This piston crown chipped at the top ring groove because of a head gasket leak. The coolant is drawn into the combustion chamber on the down-stroke of the piston. When the coolant hits the piston crown it makes the aluminium brittle and it eventually cracks. In extreme cases the head gasket leak can cause erosion at the top edge of the cylinder and the corresponding area of the head. Minor leaks of the gasket or o-ring appear as black spots across the gasket surface. An engine that suffers from coolant being pressurized and forced out of the radiator cap's vent tube, is a strong indication of a head gasket leak. In most cases the top of the cylinder and the face of the cylinder head must be resurfaced when a leak occurs. It's important to check for warpage of the head every time you rebuild the top end.

Shattered Skirt

The skirts of this piston shattered because the piston to cylinder clearance was too great. When the piston is allowed to rattle in the cylinder bore, it develops stress cracks and eventually shatters.

Snapped Connecting Rod

The connecting rod of this engine snapped in half because the clearance between the rod and the thrust washers of the big end was too great. When the big end bearing wears out, the radial deflection of the rod becomes excessive and the rod suffers from torsion vibration. This leads to connecting rod breakage and catastrophic engine damage. The big end clearance should be checked every time you rebuild the top end. To check the side clearance of the connecting rod, insert a feeler gauge between the rod and a thrust washer. Check the maximum wear limits in your engine's factory service manual

Four-Corner Seizure

This piston has vertical seizure marks at four equally spaced points around the circumference. A four-corner seizure is caused when the piston expands faster than the cylinder and the clearance between the piston and cylinder is reduced. Another common problem of this type is a single point seizure on the centre of the exhaust side of the piston. However this occurs only on cylinders with bridged exhaust ports. The main causes for this problem are too quick warm-up, too lean carburettor jetting (main jet), or too hot of a spark plug range.

Multi-point Seizure

This piston has many vertical seizure marks around the circumference. This cylinder was bored to a diameter that was too small for the piston. As soon as the engine started and the piston started its thermal expansion, the piston pressed up against the cylinder walls and seized. The optimum piston to cylinder wall clearances for different types of cylinders vary greatly. For example a 50cc composite plated cylinder can use a piston to cylinder wall clearance of .0015 inches, whereas a 1200cc steel-sleeved cylinder snowmobile set-up for grass drags will need between .0055 to .0075 inches. For the best recommendation on the optimum piston to cylinder clearance for your engine, look to the specs that come packaged with the piston or consult your Hirth distributor.

Intake Side Seizure

This piston was seized on the intake side. This is very uncommon and is caused by only one thing, loss of lubrication. There are three possible causes for loss of lubrication, no pre-mix oil, separation of the fuel and pre-mix oil in the fuel tank, water passed through the air-filter and washed the oil film off the piston skirt.

Blow-by

This piston didn't fail in operation but it does show the most common problem, blow-by. The rings were worn past the maximum ring end gap spec, allowing combustion pressure to seep past the rings and down the piston skirt causing a distinct carbon pattern. Its possible that the cylinder walls cross-hatched honing pattern is partly to blame. If the cylinder walls are glazed or worn too far, even new rings won't seal properly to prevent a blow-by problem.