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Which piston do you guys use?

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It's about time for a new top end on my bike (2005 KX250). I've been using Vertex pistons in my last couple of bikes with great results, but I think that company may no longer be around, I checked a couple of places I have bought them from in the past and they do not carry them. So my question is do you guys prefer something like a stock/ProX piston or Wiseco, and why? I have heard enough love/hate with Wiseco to be hesitant to put one in my bike. I have heard, noisier than stock, too lose fit, cold seize if not warmed up thoroughly, does not last as long as stock. But yet, other people love them. So anyway let me know what you guys/girls suggest. Thanks!

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Cast pistons can develop cracks and eventually come apart if not replaced in timely intervals. I have never had one seize though.

Forged pistons are inherently stronger and less prone to or unlikely to develop cracks in the skirt. They can however seize if not allowed proper warmup.

With that said I have been using the cast oem pistons for the last 5 or so years without any issues.

I have also found out that the oem head gasket, which is pre-treated with sealant, is much easier than using a cometic or athena or such. I have found that the KX's are prone to head coolant leaks if you don't use some type of sealant on the head gasket.

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I like the round ones .

just kidding my Monday is Friday.

how many hours on the KX? let us know what it looks like torn down.

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Probably about 30+ hours on it. Getting hard to start when warm, starts easy when cold.

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I've always heard mixed reports about the Wiseco pistons. Here in NZ most of the bike shops recommend Prox - They say over 50% of the siezes they get in with Dirtbikes have Wiseco pistons. HOWEVER there are very many folk on US forums who swear by the Wiseco pistons and with proper warm up time they perform GREAT!!

I spoke to the Team Green Tech manager here in NZ and he had some interesting input. He said that the OEM pistons are specifically matched to the OEM porting which is why so many of them are year specific, whilst the Proxy and Wiseco are 'one size fits all' '92 - '02'. He says with the race ported bikes that they use they use aftermarket pistons but they are customised to the porting of the race bikes. He recommends the OEM pistons for std bikes.

The OEM pistons are often available at very good prices on ebay and if your bike has standard porting they would certainly be worth considering.

Saying that my '97 has just done 37hrs on a Proxy 'one size fits all' piston and it came out looking very nice. I was about to change it for a Wiseco std bore piston cos I picked up one very cheap but have decided on the Eric Gorr 285 option, and that has a Wiseco piston too.

I'm pretty sure that OEM, Wiseco, Vertex and Proxy would all give you good service if warmed up properly and your bike is properly set up and jetted. I'd be fery nervous about using periperal brands that I knew little about - Just not worth the risk!

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I swear by Wisecos. Cast pistons do have their place, but forged pistons are superior in every way that matters. Forged pistons are more durable than cast pistons will ever be. Cast pistons are very brittle, and are prone to skirt breakage as a result.

For riders that will take the time to do a proper break-in, and will discipline themselves enough to fully warm their engines before each and every ride, Wiseco pistons are a superior product to OEM. They are more durable under high-heat high-load conditions, less prone to skirt breakage from piston slap if the bore has a bit too much wear on it, and longer wearing than any cast piston will ever be.

Cast pistons don't require much in the way of special break-in, that's part of the reason they come as OEM equipment in two-strokes. Forged pistons, on the other hand, are a different story. A forged piston should be heat-cycled before it is run hard, or you're playing roulette with your engine.

A new forged piston should be heat-cycled several times to relieve the internal stresses left over in the metal from the manufacturing process. If this isn't done properly, the piston can distort and become out-of-round, causing it to wedge itself into your cylinder, more commonly known as a "seizure". A seizure with a two-stroke isn't a very dramatic occurance, usually the engine just stops running. You replate the cylinder, rebuild the top-end again, and you're back in action. On a four-stroke, however, a seizure is often catastrophic. The four-stroke has more rotating mass, which creates additional inertia. So, when the piston seizes, the rest of the engine tries to keep spinning. The result is, more often than not, a snapped connecting rod, broken crank, and destroyed cases. A big bill usually follows.

To better understand why forged pistons are more seizure prone, you need to understand what makes a forged piston different than a cast piston.

A cast piston is manufactured by pouring molten metal into a mold. The final shape is machined to it's final exacting tolerances.

A forged piston is made by taking a chunk of metal, and beating it into shape with a die-press under enormous pressure. Like casting, the final shape is achieved through precise machine work.

The main difference between a cast and forged piston is the grain structure. A forged piston is beaten into shape, and as a result the metal stretches and compresses as the piston takes shape. The varied, branching, elongated grain structure is like fiber reinforcing, and it makes for a very strong piston. Microscopic cracks don't readily propogate through the structure of a forged piston due to the high density and the irregularly spaced and sized grains. A cast piston, on the other hand, is made up of grains that are all the same size, because it starts out as a liquid that, after being poured into a mold, undergoes a controlled cool-down process that allows the metal to reach a near-perfect equilibrium right out of the mold. The highly regular grain size and distribution makes them more prone to crack propogation and failure.

The break-in of the two types is very different, because the metal properties are very different. The forging process produces a lot of internal stress from beating the metal into the intended shape. The stress is trapped in the metal of the finished part. A cast piston has lower internal stress, because it was able to seek it’s own internal equilibrium as the liquid metal flowed around inside the mold and then underwent a controlled cool-down. Since a cast piston has lower internal stress, it won’t distort nearly as much as a forged piston will when heated to a high temperature. The forged piston's propensity to distort when heated is the reason they require an elaborate break-in procedure.

To relieve the internal stress, and maintain it's correct shape, the forged piston has to go through a series of heating and cooling cycles. As it heats up, the grain structure will re-distribute itself into alignment to relieve any trapped stress. As it cools, the cylinder will contain and restrain the piston, maintaining it's shape. After a few heat/cool cycles, the internal stress reaches equilibrium, and the piston will no longer distort when heated in the engine. It will maintain it's shape for the rest of it's life.

Forging produces a higher grain density than casting, making the part much more durable under high-heat, high-load conditions. As long as you are patient enough to break a forged piston in correctly, you will have a piston that is more durable under extreme conditions.

Problems with Wiseco pistons are almost always mechanic and/or rider mistakes, caused by one of these three issues:

1. Improper break-in

2. Improper warm-up-A great way to cold-seize the engine. This also kills crank and rod bearings, not just forged pistons.

3. Cylinder-to-piston tolerances incorrect-loose tolerances are just as bad as too tight.

These problems are caused by the mechanic that does the work, or the rider that's too lazy to perform a proper break-in or proper warm-up. You can't blame the product for your own incompetence (although that's usually what people do).

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Chokey, I just KNEW you'd be a Wiseco man! LOL :thumbsup:

Really interesting post that .. I knew the general gist of forged v coast but not all that detail.

Could you tell me what your recommended break in procedure is for a Wiseco piston?

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Probably about 30+ hours on it. Getting hard to start when warm, starts easy when cold.

THATS IT ! I have 76 hrs on mine starts first kick,runs perfect.

I ride mine off road, no MX track riding. I am curious whats the norm for top end

hour wize.

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I think the manual recommends something like 15hrs!! Most folk I know change over at 30hrs. I lot depends on how hard you push it. I ride reasonabley hard but not super hard and my pistons always look good at 30hrs but I change them anyway because I'm thinking that at those hours problems could be round the corner and I don't want to take the risk.

I'd have thought 76 hours is VERY high. Be interested on Chokeys view on this one.

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Could you tell me what your recommended break in procedure is for a Wiseco piston?

From Moto814

1) Assemble the engine properly and torque all fasteners to specs.

2) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), shut it off.

3) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

4) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), shut it off.

5) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

6) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), take the bike off the stand and put it in gear. Take it for a ride. During this ride you want to keep the engine under a load at all times. Do not coast. Do not let the bike idle. Do not allow the engine to stay at one RPM. Riding on a mild slope is fine for this, as is slightly dragging the rear brake the entire time. Do this for about 15-20 minutes. Then shut the bike off.

7) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

8 ) Re-torque the head and base nuts.

9) Go ride.

The cool-down steps are crucial to this operation. You must let the engine cool completely for the break in process to work properly.

Also, do the warm up procedure I outline here before EVERY ride. Your top ends will last much longer if you do.

-Steve

I think the manual recommends something like 15hrs!! Most folk I know change over at 30hrs. I lot depends on how hard you push it. I ride reasonabley hard but not super hard and my pistons always look good at 30hrs but I change them anyway because I'm thinking that at those hours problems could be round the corner and I don't want to take the risk.

I'd have thought 76 hours is VERY high. Be interested on Chokeys view on this one.

There is no cut&dry interval for a top-end rebuild. It depends on the bike, how hard the rider pushes the engine, how close the tolerances are inside the engine, and how well the engine is maintained.

One thing you can use as an indicator is a compression test. Immediately after break-in, check your compression, and write down your reading for future reference. Re-check every few hours. Once you begin to detect a measurable drop in compression, it's time to open things up and measure parts.

The problem with that method is that if you are performing compression tests under different conditions (cooler/warmer weather, engine not at the same temperature, etc.), your readings aren't really directly comparable. And always use the same guage for every test.

The best thing to do is to tear the engine down the first time at a reasonable interval based on your useage, say 20-25 hours for starters, and measure everything. This will help you to determine the wear rate in your engine, and you can adjust your tear-down intervals accordingly. For my riding style (I push the engine hard but can't hold it on as long as I used to) I have found 40-45 hours to be about right, for me. The rings will be just out of spec, and the piston will be worn but still in spec. If I were riding a 125, I have no doubts that my rebuilds would have to be more frequent.

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How about Athena? Has anbody tried them?

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I have heard that Pro Circuit use Athena Pistons. made in Germany but I can't realy find anything about them :ride:

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I to agree on the wiseco with to piston ring grooves..right now on my 05 kx100 I have the stock piston and I just did a re ring on the top end which made a big difference. Also the stock had the usual wear towards the bottom of the piston. So i guess next time I replace the rings I'll to the piston also..maybe a forged wiseco with again two piston ring grooves.

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I've always heard mixed reports about the Wiseco pistons. Here in NZ most of the bike shops recommend Prox - They say over 50% of the siezes they get in with Dirtbikes have Wiseco pistons. HOWEVER there are very many folk on US forums who swear by the Wiseco pistons and with proper warm up time they perform GREAT!!

I spoke to the Team Green Tech manager here in NZ and he had some interesting input. He said that the OEM pistons are specifically matched to the OEM porting which is why so many of them are year specific, whilst the Proxy and Wiseco are 'one size fits all' '92 - '02'. He says with the race ported bikes that they use they use aftermarket pistons but they are customised to the porting of the race bikes. He recommends the OEM pistons for std bikes.

The OEM pistons are often available at very good prices on ebay and if your bike has standard porting they would certainly be worth considering.

Saying that my '97 has just done 37hrs on a Proxy 'one size fits all' piston and it came out looking very nice. I was about to change it for a Wiseco std bore piston cos I picked up one very cheap but have decided on the Eric Gorr 285 option, and that has a Wiseco piston too.

I'm pretty sure that OEM, Wiseco, Vertex and Proxy would all give you good service if warmed up properly and your bike is properly set up and jetted. I'd be fery nervous about using periperal brands that I knew little about - Just not worth the risk!

Just so you know, wisecos are set up to the same tolerences, if not better in most cases, than stock. Look at the oem piston clearance and then check Wisecos, it will be the same if not tighter. Porting changes and piston design have little effect in stock engines. I seriously doubt you could even find differences in many oem pistons. since some are used for multiple brands.

I can also say with certainty that most race teams that run wiseco are running the same pistons that are available to the public.

I swear by Wisecos. Cast pistons do have their place, but forged pistons are superior in every way that matters. Forged pistons are more durable than cast pistons will ever be. Cast pistons are very brittle, and are prone to skirt breakage as a result.

For riders that will take the time to do a proper break-in, and will discipline themselves enough to fully warm their engines before each and every ride, Wiseco pistons are a superior product to OEM. They are more durable under high-heat high-load conditions, less prone to skirt breakage from piston slap if the bore has a bit too much wear on it, and longer wearing than any cast piston will ever be.

Cast pistons don't require much in the way of special break-in, that's part of the reason they come as OEM equipment in two-strokes. Forged pistons, on the other hand, are a different story. A forged piston should be heat-cycled before it is run hard, or you're playing roulette with your engine.

A new forged piston should be heat-cycled several times to relieve the internal stresses left over in the metal from the manufacturing process. If this isn't done properly, the piston can distort and become out-of-round, causing it to wedge itself into your cylinder, more commonly known as a "seizure". A seizure with a two-stroke isn't a very dramatic occurance, usually the engine just stops running. You replate the cylinder, rebuild the top-end again, and you're back in action. On a four-stroke, however, a seizure is often catastrophic. The four-stroke has more rotating mass, which creates additional inertia. So, when the piston seizes, the rest of the engine tries to keep spinning. The result is, more often than not, a snapped connecting rod, broken crank, and destroyed cases. A big bill usually follows.

To better understand why forged pistons are more seizure prone, you need to understand what makes a forged piston different than a cast piston.

A cast piston is manufactured by pouring molten metal into a mold. The final shape is machined to it's final exacting tolerances.

A forged piston is made by taking a chunk of metal, and beating it into shape with a die-press under enormous pressure. Like casting, the final shape is achieved through precise machine work.

The main difference between a cast and forged piston is the grain structure. A forged piston is beaten into shape, and as a result the metal stretches and compresses as the piston takes shape. The varied, branching, elongated grain structure is like fiber reinforcing, and it makes for a very strong piston. Microscopic cracks don't readily propogate through the structure of a forged piston due to the high density and the irregularly spaced and sized grains. A cast piston, on the other hand, is made up of grains that are all the same size, because it starts out as a liquid that, after being poured into a mold, undergoes a controlled cool-down process that allows the metal to reach a near-perfect equilibrium right out of the mold. The highly regular grain size and distribution makes them more prone to crack propogation and failure.

The break-in of the two types is very different, because the metal properties are very different. The forging process produces a lot of internal stress from beating the metal into the intended shape. The stress is trapped in the metal of the finished part. A cast piston has lower internal stress, because it was able to seek it’s own internal equilibrium as the liquid metal flowed around inside the mold and then underwent a controlled cool-down. Since a cast piston has lower internal stress, it won’t distort nearly as much as a forged piston will when heated to a high temperature. The forged piston's propensity to distort when heated is the reason they require an elaborate break-in procedure.

To relieve the internal stress, and maintain it's correct shape, the forged piston has to go through a series of heating and cooling cycles. As it heats up, the grain structure will re-distribute itself into alignment to relieve any trapped stress. As it cools, the cylinder will contain and restrain the piston, maintaining it's shape. After a few heat/cool cycles, the internal stress reaches equilibrium, and the piston will no longer distort when heated in the engine. It will maintain it's shape for the rest of it's life.

Forging produces a higher grain density than casting, making the part much more durable under high-heat, high-load conditions. As long as you are patient enough to break a forged piston in correctly, you will have a piston that is more durable under extreme conditions.

Problems with Wiseco pistons are almost always mechanic and/or rider mistakes, caused by one of these three issues:

1. Improper break-in

2. Improper warm-up-A great way to cold-seize the engine. This also kills crank and rod bearings, not just forged pistons.

3. Cylinder-to-piston tolerances incorrect-loose tolerances are just as bad as too tight.

These problems are caused by the mechanic that does the work, or the rider that's too lazy to perform a proper break-in or proper warm-up. You can't blame the product for your own incompetence (although that's usually what people do).

I email tt member Wiseone who is a big cheese at wiseco a lot asking for advice and one thing he told me that stuck out was the heat cycling was a thing of the past. He told me just warm up the bike and let er rip. I have done this in my kx250 and my rm125. No seizures:excuseme: You do not need to heat cycle wiseco pistons.

From experience, cast is junk, and forged is the only way to go. They are not noisier, they are not heavier, they do not seize. No matter what, a piston going up and down 8000 times a minute will put a hell of a lot of stress on the skirt causing it to wear out, causing piston slap. A wiseco will not crack and trash my jug and bottom end like the OEM I had. And with the Wises being the same price if not cheaper in most cases why in the world would someone buy OEM?:ride:

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I email tt member Wiseone who is a big cheese at wiseco a lot asking for advice and one thing he told me that stuck out was the heat cycling was a thing of the past. He told me just warm up the bike and let er rip. I have done this in my kx250 and my rm125. No seizures:excuseme: You do not need to heat cycle wiseco pistons.

:

Interested to Chokeys take on that? I was assuming that the failures that I hear about re Wiseco were due to 'improper run in'.

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Interested to Chokeys take on that? I was assuming that the failures that I hear about re Wiseco were due to 'improper run in'.

This was from the guy who designs the pistons... Chokeys opinion is a little outdated I think, nothing wrong with what he says its just not necessary anymore:excuseme:

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Chokeys opinion is a little outdated I think, nothing wrong with what he says its just not necessary anymore:excuseme:
There are many that would argue that. And I would prefer to err to the side of caution.

By the way, this was not written by me, it was written by Moto814 (Steve), a very reputable engine builder:

From Moto814

1) Assemble the engine properly and torque all fasteners to specs.

2) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), shut it off.

3) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

4) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), shut it off.

5) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

6) Start the engine with the bike on a stand and allow the engine to come up to operating temperature (top of the raidator hot to the touch). Do not allow the engine to run at one RPM at all. Constantly vary the RPM and do not allow the engine to idle. When then engine reaches operating temperature (about 3 to 5 minutes of running time), take the bike off the stand and put it in gear. Take it for a ride. During this ride you want to keep the engine under a load at all times. Do not coast. Do not let the bike idle. Do not allow the engine to stay at one RPM. Riding on a mild slope is fine for this, as is slightly dragging the rear brake the entire time. Do this for about 15-20 minutes. Then shut the bike off.

7) Let the engine cool completely (at LEAST one hour). You want the engine to be dead-stone cold. Longer is better.

8 ) Re-torque the head and base nuts.

9) Go ride.

The cool-down steps are crucial to this operation. You must let the engine cool completely for the break in process to work properly.

Also, do the warm up procedure I outline here before EVERY ride. Your top ends will last much longer if you do.

-Steve

It seems he also believes in heat cycles.

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There are many that would argue that. And I would prefer to err to the side of caution.

The only seizures I've ever seen on Wiseco's are from peopel who just put them in and run without heat-cycling. And yes, I've seen it within the last year. I'll keep doing it my way. My way works.

By the way, this was not written by me, it was written by Moto814 (Steve), a very reputable engine builder: <snip>

Man, that little article I wrote a long time ago for motocross.com has gotten alot of mileage.

It seems he also believes in heat cycles.

Why yes, yes I do.

-Steve

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