Engines operated with a lean mixture at full throttle will see abnormally high combustion temps, which can cause detonation. The results of detonation will be visible on the piston crown as a pitted or eroded surface. Piston crown damage due to valvetrain contact will be visible as indentations or cracks near the valve pockets. Valvetrain contact can occur due to valve float caused by excessive RPM or mis-timed valves.
Once the mixture is ignited, the cylinder pressure increases which energizes the compression ring and forces it against the cylinder wall, causing it to slide in its groove. On four-stroke engines, the compression ring will transition from seating on the bottom of the ring groove to the top ring groove at the end of the exhaust stroke due to forces of inertia acting on the ring. Ring and groove wear can occur due to the sliding and reciprocating motion of the rings and can be exasperated by carbon deposits that accumulate in the ring groove.
Ring and groove wear can be qualified by thoroughly cleaning the ring and groove and then measuring each. Most service manuals outline specifications for ring width, groove width, and piston ring to ring groove clearance. Wrist pin bore wear occurs as a result of the loading of the wrist pin joint through inertia and combustion loading. The wrist pin bore will typically wear into an oblong shape. In some engines, wrist pin bore wear will be visible in the top and bottom of the bore.
Usually, a portion of the bore will appear burnished or polished. Alternatively, the wrist pin bore can be measured from top to bottom and from side to side. Both measurements can be compared to one another to determine how much the bore has become out of round and to the diameters specified in the service manual. The importance of replacing the piston at regular intervals in high-performance powersports engines cannot be overstated.
If left unattended, the resulting cumulative wear of the piston will eventually result in a catastrophic and expensive engine failure. Typically, too much time on a piston can lead to gradual and finally complete failure of the skirt in both two and four-stroke engines. Between aftermarket suppliers and OEMs, replacement piston options are plentiful and can be overwhelming.
The most common upgrade and consideration most riders are faced with is whether or not to move to a forged piston. Forged pistons can be a nice upgrade for many riders because they can offer additional strength and wear resistance over cast pistons. Forged pistons achieve greater strength than cast pistons by using different aluminum alloys and manufacturing processes.
The forging process for pistons results in finished components that have a tighter molecular structure and grain flow optimized for strength. Comparatively, cast pistons are not cast under high pressures and have molecular structures that are not as tight or organized, which in severe cases, can lead to voids, inclusions, and air pockets.
On some pistons, this is generally located 0. If the engine builder recorded the original piston diameter, a simple comparison will reveal any changes. A minor change in piston-to-wall clearance can be considered normal, but changes in the piston-to-wall clearance of more than 0.
Perhaps the place where wear can cost the most can be found in the top ring groove. All rings use cylinder pressure behind the top ring to increase sealing load on the ring. This demands a somewhat tight axial or vertical clearance between the ring groove and the piston.
Although axial clearance recommendations will vary with specific pistons and manufacturer , a generic clearance of 0. This can be measured with a feeler gauge between the top of the ring and the groove. Worn ring lands can also exhibit more clearance toward the outboard edge of the groove creating a bell mouth effect, which will negatively affect ring seal.
One of the first things to check on a used piston is that the ring groove clearance is still within spec. Install the ring in the groove and use a feeler gauge to measure the clearance. Beyond worn ring grooves, high output engines and especially supercharged or turbocharged engines tend to load the top ring with far more cylinder pressure. Micro-welding is a term used to describe the transfer of small amounts of aluminum from the ring land to the ring surface.
This material transfer tends to reduce the axial clearance and may in fact contribute to sticking the ring in the groove. Clues that may point to lost ring seal due to mirco-welding include increased blow-by and lost power. Check for extreme bluing on the bottom of the piston. This can be an indicator that the piston has been exposed to too much extreme heat.
This piston is perfectly fine, and on a cooked example, the bluing would fill up the whole area between the struts and be much darker.
Piston rings are designed with vertical clearance so that they can freely move within the groove and are induced to move by the angle of the cross-hatch pattern honed into the cylinder wall. Micro-welding can reduce piston ring movement which also contributes to reduced sealing efficiency. When the rings are removed from the piston, evidence of micro-welding will be pitting in the lower surface of the ring groove and the lower horizontal face of the ring itself.
This will be more prevalent with pistons that place the top ring closer to the piston crown as this increases the temperature the ring must face. Checking wrist pin clearance is a great evaluator of piston condition. A pin-bore gauge can be used for extreme accuracy but sliding the pin in and out on both sides and feeling for binding is also acceptable.
Do you think I am correct or am I just being paranoid? Please answer and help! When your engine is cold, oil is thicker and tolerances inside the engine are different, so the engine may not be lubricated quite as well as it would be while the engine was warm.
If your compression test is less than stellar, a leak down test will tell you exactly where you are losing compression. If you have any real time monitoring software, plug it in while driving around in a safe area away from traffic. If you notice your air fuel ratio going excessively lean or rich, or you notice large knock events, start there.
Consider performing a smoke test to find any and all leaks. Thanks Sean! Well, one of the shops I go to told me they ordered replacement hoses for my intercooler and see if that fixes the issue. I am always running rich…somewhere in the to range for LTFT. I wonder what that could give a clue to? A leak down test will tell you a lot more than I could.
I think it would be best to lean on your shop s for more assistance, since they are already in there helping you diagnose the car. Great info! So, I have a Infiniti M35x with K miles. Car has symptoms Also, have a Infiniti G37s with K miles. It has symptoms Cat conv is failing in this car, too. Are Nissans and Infinities known for bad pistons — or is it just old age? I also had a Nissan Maxima that made it to K.
I tore that car up after having the engine clean — dumbest thing ever. Heyo i know this thread is Old but i have some of these issues on My bmw e90 i from , i have had a compression test and it showed 1 of the cylinders had half the compression as the others, but the power delivery is stil solid, but mabye a bit shower when i tried comparing My acceleration to the same model on YouTube videoes, but anyway, when i turn on the car it bounces the revs alot up and Down for like second and then idles, and then i smell burned oil for the first 5 mins in idle and then it stops, and when its warm and ive come to an idle then some times it Smells and some times not, but the weird thing is i dont lose oil, when it first started occuring i lost oil everytime i drove it very hard for a Long period, but just cruising i could go months without loosing oil, so do guys Think its just the piston ring Thats the problem from all that or is there more to it?
A leak down test is where you remove one spark plug, put compressed air into the cylinder on the compression stroke, and see where air is excessively leaking out. You then repeat this process for each cylinder. I changed my oil and the oil filter yesterday and went on a mile journey to fined out there was no oil in my engine.
Used to see black blue smoke from the pipe. What is my problem? Have you checked under the car to see where the oil went? My car had been consuming more oil than usual. Changed my piston, crankshaft and the engine block. The engine was mounted and sounded pretty good. How do I correct this. I have a Peugeot RCZ, 70, miles. Always been serviced by Peugeot. Burned through an oil change in about 2 months. Took it to mechanic and no pressure from cylinder 2.
Best way for this to be repaired? I find it interesting that poor acceleration could be a sign of bad piston rings.
The car had been in storage for a few years so it probably got worn out there. Table of Contents. Looking for Replacement Parts? I am beyond concerned because my has never done that it is a Toyota Avalon it blows out smoke every few days once or twice throughout the day of me driving , any suggestions Reply. This engine may need to be rebuilt and may cost thousands of dollars to do so. A number of things can go wrong at the oil change location: 1 Oil drained, filter replaced, technician forgets to fill with oil.
Service at dealerships is often more expensive than independent shops. How true is that Reply. Hi so i bought a scooter recently the lad i bought it of claimed a big hole in the exhaust was causing the engine to be loud asf now further on inspection it is blowing alot of white smoke out when running and its spitting fuel out from between the cylinder block and the cylinder head am i right to think my head gasket has blown and its coolant mixed in with the fuel that spits out everytime its kicked over Reply.
I drive c model the auto my car looses power when I drive and reverse the car this car need a new pistons Reply. Hope this helps, and good luck.
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