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Any ideas on what would cause 5 cracks on this piston?
Too much HP?
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Either too many hours on it or excessive piston to bore clearance.
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Would that be because the 500,s collapse the piston skirts over time?
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That engine should have been torn down at least 10 to 20 hours ago and the piston replaced You are lucky the skirts did not break off and destroy your cases and cylinder.
The piston had too much time on it for the way the piston has been modified. When the engines are highly modified and the pistons are modified, piston life is always shortened. The power output, the air filter efficiency, premix oil ratio and how hard the engines are ridden determines the piston life.
The following table is a typical ring and piston change schedule recommended by Yamaha, Kawasaki, Honda , Suzuki etc for the stock two stroke motocross bikes. Keeping fresh pistons and rings is essential for the engines to perform very near their optimum level and prevent piston failures due to fatigue.
125cc single ring pistons: Install new ring every 2 to 3 hours and a new piston every 4 to 6 hours. These stock engine make 28 to 35 HP.
250cc dual ring pistons: Install new rings every 4 to 6 hours and a new piston every 8 to 12 hours. These stock engine make 41 to 46 HP
500cc dual ring pistons:: Install new rings every to 4 to 6 hours and a new piston every 8 to 12 hours. These stock engine make 45 to 55 HP.
Following the above rebuild schedule will keep the engines producing power within about 98% to 99% of what they produce when the piston and rings are new.
Some guys think this is short piston life but is really in line with most High Performance two stroke engines. Some of the highly modified 125 and 250 shifter Kart engines get new pistons every 1 to 2 hours.
Most guys with Lt 500S do not care if the engines are loosing a few percent off of it's peak power potential but all of the large displacement single cylinder two stroke Wiseco and Wossner pistons collapse because the skirts are too thin. The big bore forged pistons will usually collapse .003" to .005" in 4 to 10 hours of hard running. Thin skirts flex and crack. The modified big bore engines will usually loose 2 to 4 HP if their rings are not replaced every 4 to 10 hours.
The LT 500s that have good pipes and porting should be torn down at around 10 to 20 hours the first time after the engines are modified to inspect the piston. If the piston does not have any cracks and the clearances are still ok put some new rings in it and run it another 10 hours or so. Riding styles affect piston life. Some guys get more time out of a piston than others.
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Tear the engine apart after every ride? That's not happening.
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Tear the engine apart after every ride? That's not happening.
Manufactures do a lot of testing. Testing reveals when engines have enough ware for there to be a noticeable drop in power. Testing reveals how many hours on average an engine will go until there is a catastrophic failure of one of the engine components. With the high power output of a two stroke engine the piston is usually the component that fails first.
Pistons are a consumable component in a two stroke engine just like tires, spark plugs, fuel, oil etc. Every time any engine runs the time clock on all components is ticking.
We see the 4 stroke engines like the 450 quads and MX bikes also have high consumable parts. These high consumable parts are pistons, rings and valves. The manufactures recommend off the show room engines (stock 450 quads) top ends be torn down, inspected and the necessary parts be replaced about every 50 hours (600 km or 350 miles).
The manufactures recommend the MX 450s be torn down every 15 hours or so to inspect and replace parts that are out of service limit.
When components like a aftermarket cam and the air box lid is removed (something like Honda's HRC kit is installed) the 450 quad engines now needs the 15 hour top end maintenance schedule like the 450 MX engines to keep the engine producing max power. Stock 450 quads produce power in the 36 to 40 hp range. The stock 450 MX bikes produce power in the 45 to 50 hp range.
I have a customer that works in the Hendrixs engine shop that builds NASCAR engines. He told me that these engines will show a very slight drop in power at the 500 to 700 mile mark. They recommend the teams rebuild them at 500 to 700 mile mark. He said that they will usually run about 3000 miles or more before something gets loose enough to cause a catastrophic failure.
In summary, high performance engines need to to have certain components replaced frequently to keep their power level high and prevent failures. When you purchase an engine package you have to make a choice as to what level of power you want and the related expense of maintaining such an engine. When a particular engine package is purchased you have to decide which maintenance schedule you fill follow. Do I want to keep the engine producing max power, do I run the engine and tear it down just before a component is about to fail or wait until I have a catastrophic failure.
Do we want to ride CR 500s, KX 500s, LT 500s or something like 350 Warriors, 350 EX. XR 600s? There is huge difference in the performance level as well as the rebuild schedule and the associated cost.
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This happens if you run it too long. Then you get to tear down the bottom end and hope it didn't knock holes in the cases. This was the condition of the piston in my 500 when I got it in 2012. It was the original stock piston. The engine had never been apart since 1989.
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Thank you for this very interesting topic! I was going to ask Jerry the next time I talked to him how often my engine needed to be inspected and now I know.
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So by looking at the 2 posted pictures it looks like these pistons crack on the intake side most of the time? Is this a safe assumption? And if it is, is it advisable to just pull the reed cage and look in at the intake side of the piston every once in a while to look for cracks? I know you can't see the whole piston or measure cylinder wall clearance, but it has to be better than nothing....
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Thanks for the replies guys.
One thing I need to add is that the power valve was stuck in the open position when I **** it open.
MG would that cause the piston to wear out prematurely?
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Thanks for the replies guys.
One thing I need to add is that the power valve was stuck in the open position when I **** it open.
MG would that cause the piston to wear out prematurely?
The power valve being stuck open will not cause the piston to wear out prematurely. Dirt getting past the air filter and not putting enough oil in the gas for your riding style are the two things that accelerate piston wear.
The power valve being stuck open will cause a loss of mid range and high RPM power. Running the engine at high rpm with the power valve stuck open can force the engine into detonation and result in burned pistons and or piston seizures.
The piston pictured in the 1st post probably had cracks in the corner radius of the transfer cut outs before it had 20 hours on it. The cracks in the middle of the intake skirt is very common when the skirt is modified. -
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So by looking at the 2 posted pictures it looks like these pistons crack on the intake side most of the time? Is this a safe assumption? And if it is, is it advisable to just pull the reed cage and look in at the intake side of the piston every once in a while to look for cracks? I know you can't see the whole piston or measure cylinder wall clearance, but it has to be better than nothing....
Removing the reeds and looking at the piston has to be better that no inspection. If you find a crack it will be definately worth you time.
The cracks may be very difficult to see. The incoming fuel and air washing away the fine powered aluminum that emerges from the cracks may make it difficult to see on the side of the piston that is rubbing the bore. Using a small dentist type of mirror to look at the inside surface of the piston could make it easier to see cracks. -
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LTZilla also forgot to add that this piston has been in his bike since 2007...
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Thank you for this very interesting topic! I was going to ask Jerry the next time I talked to him how often my engine needed to be inspected and now I know.
Only 1 or 2 of these cracks went all the way through. Meaning, if you were just looking through the intake would may have missed them. -
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Thanks for the replies guys.
One thing I need to add is that the power valve was stuck in the open position when I **** it open.
MG would that cause the piston to wear out prematurely?
I forgot to ask you what your definition of "wear out prematurely" is or what is your expectations of how long (how many hours) should a piston last? -
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MG - Just wondered if having the PV stuck open would wear out the piston and rings sooner due to having an full operational PV. Like having pressure excessive pressure in the cases and detonating issues. Just wondering.
Thanks for your help on getting me on track. Sooner maintenance will be in my future.
Q2W those cracks were completely through the piston walls(both sides) you saw it first hand.
One last thing I need to add....My right rear cylinder stud came loose and was causing my cylinder to move during operation. Thankfully there was no sand in my cases.
A rebuild is now underway. -
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....................MG - Just wondered if having the PV stuck open would wear out the piston and rings sooner due to having an full operational PV. Like having pressure excessive pressure in the cases and detonating issues. Just wondering.................
The power valve being stuck open had nothing to do with the piston cracking. A poorly designed piston is causing the cracking..............(not enough of the right type of alloy in the right places is making these pistons flex and crack). A solution to these types of failures is not rocket science. I have talked to Wiseco's engineers about this problem numerous times and they agree. The engineers said they are often handcuffed by their marketing department. Wiseco's marking department often instructs their engineering and production departments to make their pistons lighter and lighter as per the request of enthusiast and amature engine builders in the drag racing and ATV racing community. I think this is the the motto of most marketing departments..........Give them what the want ............not necessarily what is best? ........ The customer is always right!
If the public and marketing department is controlling piston designs, it seem obvious to me by the pistons designs we are getting, that the majority of the enthusiast and the drag racing community does not understand basic metallurgical and engineering principles, yet Wisecos marketing department seems to listens to customers more than their engineers. When a customer reports a problem they want to offer you a free hat, Tee shirt or sell you a replacement piston at dealer cost to pacify you.
Drag racing is a sport where the majority of the participants and engine builders have a full time day job doing something unrelated to the sport and is just a hobby as well as a hobby and or secondary income for the engine builders. As a result of their limited understanding of the science that is occurring inside of their engines, they tend to drive thumb tacks with sledgehammers. This aggressive approach to engine building misses obvious problems that quickly become apparent for engines that have to run many full throttle blasts, that last for more than 10 seconds at a time after the whole engine is up to full operating temperature. Drag engines and many of their components typically do not have to be designed very well to take the heat and stress for 3 to 10 seconds of high-RPM full throttle and then take a rest and cool down.
A large number of engine builders now have dynos that make a sweep through the usable RPM range in 1 to 5 seconds. These short dyno runs will not allow many of the poor engine designs or components with heat and or tuning related problems to show their ugly head.
There must not be enough experienced engine builders that are building engines for road racing, circle track racing, salt flat racing and shifter karts reporting heat and stress related failures for Wiseco to make changes to their pro lite series of pistons. When I talked to Wiseco they said that they do not have any forging molds in their inventory that would produce pistons with my requested design changes. It would require 4 new forging molds to be made to incorporate the required design changes for bores 86mm to 89.5mm at a cost of over $7500.00 for each mold.
Wiseco will build custom pistons for customers (minimum of 4 pistons) using forging molds from their existing inventory of mold. The custom designed pistons are the typical cookie cutter type conventional piston with the bore size, piston pin size and piston pin location, ring pin location and skirt shape of your choice. These custom pistons usually experience the flex and collapsing problem associated with the majority of forged pistons that have bores that are larger than about 55 mm or so.
In the meantime it looks like we have keep changing pistons about as often as we change our underware. Some guys go longer between changes.
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......we have keep changing pistons about as often as we change our underware. Some guys go longer between changes.
And that just depends on who did your motor work, needless to say I always bring at least 2 extra pairs!!!
On a more serious note, I have a full JH built motor and pipe and I usually get 30-35 hours out of a piston. When I pull it it's usually got a crack in it, sometimes pretty small though.
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What's the status on our piston inventory? I remember someone saying our days are numbered and will come when you can't buy one. How many do you guys think are left? I'm not being fecisous, I'm actually wondering!
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I don't know if anyone asked yet, what's the best piston to use for the 500 , 87.5 bore.
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