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    Exclamation Three types of pistons...?

    I've come across several posts that mostly have to do with the 4-TEC engines and SBT using the wrong pistons when it comes to the OEM remanufacturer of these engines.

    In short, I don't see why their cast pistons are the wrong choice for their engines that they sale at a greatly reduced price than most others, who want upwards of $5000 dollars.

    Each on these piston types have their own place in the world of efficient engine combustion and from what I read, the forged pistons are best used in modified engines. But the cast pistons seem to have a better life because their ability to disapate their heat over the entire surface is a lot better than a forged. (In most applications {drag racing is not one of them}).

    Below is some research I put together and thought I'd pass it on. The article was wrote and researched by Mike Varse but worth keeping in it's original content rather than summarizing it.

    This is a hot topic and I expect some very informed argument will come from this but do your homework because the cast pistons of 25 years ago aren't what they are todayl.

    Read on:.............



    Which is better, the cast piston or the forged piston? This argument doesn't come up as often as it used to. During the 1970s it was a frequently debated topic: The answer used to be simple, because one type of piston was in fact better than the other. Today however, things are not so simple. A person can't be as dogmatic as they would like, and anyone who is insistent just doesn't know the facts. Here are those facts.
    Piston Basics
    The piston is under-appreciated, for sure. It may be the hardest-working part in the internal combustion engine. Following is just a brief outline of the piston's function and construction.
    Three Jobs
    The piston does three important jobs. It is a bearing, receiving loads from combustion and transferring them straight and true through the connecting rod to the crankshaft. The piston is also a seal, as it seals combustion's forces and compression's trapped air. Finally, the piston is a heat conductor, transferring some of the cylinder's heat to the outside. In fact, nearly 80 percent of the cylinder's excess heat is drawn away by just the piston's rings.
    Piston Parts
    The piston is made of essentially seven parts. The piston's top or crown takes the brunt of combustion's forces and heat. Consequently, the crown is the hottest part of the engine after the spark plug. It must therefore be quite thick so as to not collapse, though it is not always the thickest part of the piston. Moving down the piston, the next thing is the ring groove. The closely manufactured groove accepts the third part, the precisely made piston ring. In the four-stroke engine, natural harmonics cause the ring to rotate as the piston goes up and down in the cylinder. This helps the groove stay clean of carbon. The solid pieces between the grooves are called ring lands. They are similar to the lands in a gun barrel. They support the shock loads the rings receive during combustion. The next part is the piston pin hole. This hole accepts a pin that connects the piston to the connecting rod. The hole is offset from the piston's center slightly so that when the piston and rod reach TDC, they do so at slightly different times. This spreads the shock loads at high rpm, easing stresses on the connecting rod and eliminating a noise called "piston slap." Surrounding the hole inside the piston are pin bosses, thick masses of metal that support the pin when it is inserted in the hole. The pin bosses are sometimes the thickest part of the piston. In some cases, they are not as thick as the crown. In either case however, the thickness of these two parts is important, as it determines much about how the piston deals with heat. Lastly, we come to the piston skirt. The skirt is the bearing portion of the piston. It slides against the cylinder wall, bearing the force of combustion on the power stroke, and the loads of compression on the compression stroke. There are also stresses involved with rpm that the piston and cylinder are designed together to deal with. The skirt is the part of the piston most in need of lubrication. Thus most lubrication problems show up on the piston skirt first.
    Piston Shapes
    There are two important ways in which pistons are shaped. First, the piston is not round, but elliptical in shape. The reason is the afore-meentioned pin bosses. The bosses' mass makes them absorb a lot of heat, which makes them expand more than any part of the piston. If the piston was instead made round, it would not be when fully warmed up. That would be a problem. Therefore, the width of the piston at the area of the bosses is narrower than it is elsewhere. The resulting shape (looking downward onto the piston crown) is an ellipse (an oval). Marine pistons are sometimes called "cam ground," which refers to the same thing (however, it isn't the shape that is being referred to in that case, but rather the machine that produces it). The other (second) shape all pistons have is taper. That is, the diameter of the piston at its crown is considerably smaller than its diameter at the skirt. The reason is the same as for the piston's ellipse. Only this time it is the crown, not the pin bosses, that necessitates the shape. The crown absorbs so much heat that it must be made smaller so that when fully heated, the piston will be straight.
    Piston Manufacturing Methods
    Pistons are manufactured in one of two ways. Those two ways are the cast piston and the forged piston. This brings us back to our question, which is better, cast or forged? But not so quickly. The cast piston is made of molten aluminum. The alloy is flowed into a mold having the shape of the finished product, in much the same way that many other cast parts are made. However, don't imagine wooden boxes full of coarse sand, into which melted aluminun is poured. Piston molds are actually permanent dies, intricately made multiple-piece steel shapes. The molten aluminum is vacuum drawn into the mold. So accurate is the process that the resulting casting requires minimal machining. The forged piston is made very differently. The metal is not molten, but heated somewhat. A blob of this hot aluminun alloy called an ingot is placed in a female mold, and a male ram is pounded into it. The result is not a piston, but a piston blank, which must then undergo many machining operations before it resembles a piston. These two methods of making pistons continue today, and there are interesting reasons for each of them. Let's examine those reasons by looking at the history and applications of each piston type.
    The Cast Piston
    The cast aluminum alloy piston has perhaps the longer history. It took over for the original steel part during the internal combustion engine's early development. The cast piston is the most familiar piston type.
    Casting Alloys
    Early cast aluminum pistons were made with inferior alloys. The piston expanded dramatically, requiring a loose fit in the cylinder and resulting in noisy operation when cold. Harley-Davidson pistons once had steel ribs inside them to control this expansion. Since about the 1960s however, most cast pistons have been unstrutted. Their alloys have gained silicon, a material that gives the pistons natural lubricity and limits heat expansion. All modern pistons have silicon in them. However, cast pistons have historically had the most. Some of them have as much as 25 percent silicon by volume. Silicon does bring a disadvantage however. It makes the piston brittle. Dropping a modern cast piston will usually crack it, so the piston must be handled carefully.
    Mass Efficiency
    Probably the greatest benefit of the cast piston is the efficiency of its mass. The multiple-piece molds allow intricate contours inside and out, resulting in light weight, good expansion control, and predictable heat flow through the part. That is, the piston designer can plan in the specific thickness in each place in the part that is desired, to result in expansion at those places that is warranted. So predictable is the cast piston's heat in fact that race tuners view the undersides of the piston to gauge the combustion efficiency of the engine. In much the same way others read spark plug, they read the dark splotches under the crown.
    Applications
    The cast piston is however expensive to manufacture. Die casting is costly, because it requires huge machines that do very specific jobs, and can't be easily adapted to do more than one kind of job. The result is that the casting process for pistons is relegated to the large piston supplier. The downside is that the cast piston is often found only in OEM specified sizes and types. There aren't a lot of different cast pistons to chose from if you are modifying an engine. The upside of this situation is that since only large piston manufacturers can afford to make cast pistons, they are usually competently made. In fact, the cast piston generally typifies the best technology that the piston industry has to offer. However, this doesn't mean it's the best piston for every application.
    The Forged Piston
    The forged piston is a more recent development. It appeared first on high-powered two-stroke engines. These engines were made in low production numbers, and their performance and use resulted in frequent detonation. Both of these traits, as we'll see, made the forged piston a pretty good match for this application.
    Forging Alloys
    The earliest forged pistons were also made with poor alloys. In many cases however they were even worse than the alloys the cast pistons used, because when the cast piston finally got silicon, the forged piston did not. The same brittleness that makes the cast piston crack when bumped hard would have resulted in even larger defects had it been used in a forging. Consequently, during the time that the cast piston defined a piston's normal expansion rate, the forged piston was far behind the technology. The forged piston had to be fitted loose, which made it noisy and wasted power. Recently however, silicon has been introduced to the forged piston. A mixture of alloys has been found that together with silicon do not result in defective forgings. For example, nickel has been found to offset the silicon's tendency toward brittleness. However, not very much nickel can be used, as it is a heavy metal, and it affects the mixture in other ways. The result is that the modern forged piston is much more dimensionally stable than was true in the past.
    Mass Efficiency
    However, once again, the forged piston's mass does more to define its characteristics than does even its materials. The forged piston has historically had a crude interior shape. The forging ram is straight, which results in a rectangular rather than an inticate interior. There is too much mass there. Consequently, the forged piston has poor dimensional stability. Its expansion is not very controllable. Many engine builders overcome these two problems (too much weight, unpredictable expansion) at least partly by removing by hand the extra material inside the forged piston. This allows them to fit them tighter and rev them higher. However, many forged pistons also have overly thick skirts as well as unsophisticated interiors. This is because the forging produces a piston blank, remember, and not a finished piston. The piston wholesaler takes this blank, and from it, carves out several different sizes and shapes of pistons. If the piston being made happens to be the largest the blank supports, it ends up with the thickest skirt. While hand reworking (or CNC milling, as many do now) the forged piston can lighten it and make it behave more like an intricately made cast piston, there is still excess weight due to the thick skirt.
    Applications
    Unlike the cast piston, the forged piston is easy to manufacture. Smaller piston manufacturers therefore specialize in this piston type, even if some of them may not be as competent at making pistons as are the larger cast piston makers. Forged pistons have quickly become the choice of custom engine builders because they can be had very quickly, and in virtually any configuration desired. Moreover, the forged piston's added thickness is used by these builders to custom configure the piston even further. For example, flycuts on the piston's crown for high performance valve relief is an easy process with the forged piston. There's a lot of material there in which to do it, much more than there is in the cast piston. The forged piston was also the first piston type to adopt the modern ultra-thin piston ring, for the same reason. It could be done easily and immediately. There were no molds for such a piston among the cast piston manufacturers for at least a year afterward. This situation has resulted in the forged piston acquiring something of a high performance personna, even though its overall technology is less current than the cast piston's. Most of that reputation is unearned, but in at least one way it is in fact a reality. The forged piston is inherently stronger than the cast piston. Lower silicon content of course would result in this, making the forged piston less brittle. However, there is another reason as well. The forging process compresses the alloy's molecules, making the material more dense than a casting. The result is a piston that withstands the pounding of detonation better. This is why OEMs use the forged piston in two-strokes and turbocharged engines. Forged pistons are also included in many OEM high performance options kits for their street models.
    Summary
    To sum up, the cast piston is light and very dimensionally stable. It is found in high-rpm mass-produced engines that are not subject to modification or prone to detonation. The piston is however fairly brittle, and the cost of its manufacture has limited its availablity outside the OEM sources and applications. On the other hand, the forged piston is inherently heavy and less dimensionally stable. It is a good choice for engines in which detonation is probable, and its wide availability has made it the choice of engine modifiers. The special demands of these end users has given the forged piston its own niche in the powersports market. The next time someone tells you how superior one piston type is over another, tell them the truth. Because, as Paul Harvey likes to say, "Now you know the rest of the story." Hold the fries, please.
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  3. #2
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    Default 3 types..

    somewhat did an Evilyn Woods, on the thread, but cant seem to find the 3rd piston write up, Chief...

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    I've never seen an elliptical piston in any 2 stroke. I actually caught myself pulling out one of the piston I have handy and measured it and it is indeed perfectly round with no elliptical shape.

    I'll measure one of my Wiseco forged pistons later but I don't see all the info as being correct for a modern day piston.

    "This is why OEMs use the forged piston in two-strokes"

    They do? I have yet to see a forged piston in any OEM supplied powerplant with the exception of PJS and their Sea Jet model which utilized Wiseco forged pistons.

    I might do a little research myself on the benefits of a forged piston versus a cast piston as whoever wrote the information you got this from snipe, seems to have some incorrect information.
    Last edited by PWCdoc; 04-12-09 at 09:19 AM.

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    Rotax designed and build these motor and for some reason with the 4tecs they choose to put forged pistions in the 215hp & 255hp motors my 155hp has cast pistions. Im guessing they choose to put forged one in because of the higher boost pressure. This was all started by the SBT debate, I truly think there 2 strokes are fine. Ive talked to alot of people that have had a rebuilt 4tec from them and were not happy. SBT does stand behind there motors but they make all there own part so rebuiling for them is cheap. If you blow a motor they take it back give you a new one then rebuild the blown motor again and sell it. the mamber on her said that by the time sbt replaced the motor that his warranty was almost up and was crossing his fingers he didnt have another failure. The way i look at it is if a motor from the factory blew up then why would a motor built with inferior parts to oem standarts have a long life span?
    Last edited by sporsterjoel; 04-12-09 at 10:15 AM.

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    What do you consider a "long life span"? And a rebuild that uses inferior parts can't be an OEM level rebuild. That's a contradiction in terms.

    A rebuild that uses inferior parts can last long if you remachine certain parts that will net you less HP and torque. Hence less strain on those inferior parts allowing it to last longer.

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    Default Got Beer?

    I hope you got the beer too Robin! I knew this would be right up your alley.....

    Because I use SBT quite a bit and expect reputation means a lot to people and companies like them, I figure they will use what they think is best by default. Not just because it's cheap. I used Weisco pistons in my 350 cu.in. Chevy back when it was set up with 2:02 heads and ran solid lifters and a 486 total duration cam. But, working with the Seadoo's, I'm only looking at what is stock in replacement and what SBT has to offer.

    Are the 4-TEC engines pistons forged from the factory or are they stock cast?

    I saw that one of the pistons got omitted somehow from the reading I was doing last night, my fault timmyboy. I never heard of it before but it started with an "H"......like helic ..something or other. Not helical ...but a weird name I never heard of.

    It's time for me to go eat lunch, I will be back on this topic soon.

    Robin, finish the popcorn and put a couple cents in on this one. I know you got an opinion just waiting to be brought to light!.....

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    Quote Originally Posted by seadoosnipe View Post
    I hope you got the beer too Robin! I knew this would be right up your alley.....

    Because I use SBT quite a bit and expect reputation means a lot to people and companies like them, I figure they will use what they think is best by default. Not just because it's cheap. I used Weisco pistons in my 350 cu.in. Chevy back when it was set up with 2:02 heads and ran solid lifters and a 486 total duration cam. But, working with the Seadoo's, I'm only looking at what is stock in replacement and what SBT has to offer.

    Are the 4-TEC engines pistons forged from the factory or are they stock cast?

    I saw that one of the pistons got omitted somehow from the reading I was doing last night, my fault timmyboy. I never heard of it before but it started with an "H"......like helic ..something or other. Not helical ...but a weird name I never heard of.

    It's time for me to go eat lunch, I will be back on this topic soon.

    Robin, finish the popcorn and put a couple cents in on this one. I know you got an opinion just waiting to be brought to light!.....
    I sure have nothing bad about sbt. I agrea with what you think about sbt. I had a engine failure damed sbt pistons still looked brand new eccept where the problem hapened and it had nothing to do with the sbt pistons. I hope that I do not regret the wsm pistons that I am replaceing them with.
    Last edited by robin savell lloyd; 04-12-09 at 03:11 PM.

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    seadoosnipe's Avatar
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    Default Researching...?

    I'm actually on my other computer, at the same time, reading up on their pistons. They do sell two different pistons for the 4-TEC engines, one for the supercharged version and the other for the naturally aspirated.

    I can't seem to find any info as to their grade though. I'll probably have to wait till Monday and give them a call. But I did read a statement where they say their pistons carry a lifetime warranty against defect.

    The page link is http://www.sbtontheweb.com/Merchant2...tegory_Code=47 if anyone wants to check it out.


    I do want to make it clear though, in my experience with building Chevy 350 engines from scratch (in my late teen's through my 30's) that I experimented with several piston/ring combinations and for the money, I never found a better set than the Weisco forged, dome top pistons.

    I'm not saying at all, that forged pistons are inferior, I'm only looking into the idea that SBT is selling an inferior or problematic remanafactured engine to it's customers. Like PWCdoc says, there is a contradiction in the statement by SBT that states their rebuilds are done to OEM specifications but, if they are not done with OEM pistons, then I want to know about this.

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    I could be wrong but I do not think that you have to use oem parts to meet OEM specifications. I'm actually on my other computer, at the same time, reading up on their pistons. They do sell two different pistons for the 4-TEC engines, one for the supercharged version and the other for the naturally aspirated.

    I can't seem to find any info as to their grade though. I'll probably have to wait till Monday and give them a call. But I did read a statement where they say their pistons carry a lifetime warranty against defect.

    The page link is http://www.sbtontheweb.com/Merchant2...tegory_Code=47 if anyone wants to check it out.


    I do want to make it clear though, in my experience with building Chevy 350 engines from scratch (in my late teen's through my 30's) that I experimented with several piston/ring combinations and for the money, I never found a better set than the Weisco forged, dome top pistons.

    I'm not saying at all, that forged pistons are inferior, I'm only looking into the idea that SBT is selling an inferior or problematic remanafactured engine to it's customers. Like PWCdoc says, there is a contradiction in the statement by SBT that states their rebuilds are done to OEM specifications but, if they are not done with OEM pistons, then I want to know about this.
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    Last edited by robin savell lloyd; 04-12-09 at 03:15 PM.

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    Default My last response....

    O.K....I believe this will bring to a close (for me anyway) the debate on not recommending SBT remanafactured 4-TEC engines.

    SBT has always offered the best in OEM products that I have been able to find on the internet, anywhere.

    When I need a part, like this past week, I needed the valve body for a Mikuni carb, I couldn't find a better place to order one for a better price than www.seadoowarehouse.com. But, when it comes to complete engines, our seadoowarehouse does not offer them, so I recommend SBT.

    After reading threads here in the forum on the problems with the pistons that are stock with the 4-TEC remanafactured engines, I decided it was time for a little "look see". The threads state that they are using cast pistons which are unreliable and should be avoided, that forged was the best choice.

    I have looked everywhere possible, even at the Weisco web site and can not find a "forged" piston designed for the 1503 engines. I"m not going to say they are not available because it's possible another manafacturer does make them, but not Weisco.

    So, I started looking for the composition of the 1503, 4-TEC pistons offered by the SBT remanafactured rebuild or replacements.

    SBT carries a lifetime warranty against defect of their pistons and they are cast using a high-silicone-content aluminum, and anodized on the top surface to increase the operating temperature range and retard normal wear. The sides of the pistons are graphite coated for minimal wear during break-in. The rings are made of chromium-plated ductile iron.

    From following some issues I read about in the break-down of these engines, it seems the failures reported by SBT are from improper break-in procedures.

    If anyone has found anything that would help in identifying another source of piston that has a better composition or warranty than SBT with a user friendly price, please post here and let us all know.

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    I'm not aware of Wiseco providing pistons for them yet either. I can't see them "not" getting into a line of pistons for them sometime in the future.

    As for pistons and warranty against defect. I rarely see damage caused by a piston defect. Well, except for when the 951 powerplants were having their piston skirt problems. Beyond that, it is rare for the piston to be the problem.

    On the SBT topic, take a look at these two heads in this photo and tell me what differences you guys see. Also, tell me what symptoms one will exhibit over the other. This is a Kawi 750 head. Got some good examples of a 657 head here too. Since the subject of inferior came up.
    Attached Images Attached Images

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    Default Wish I could!...

    Man, what a challenge! I wish I could. I wear bi-focals and have to use a lighted magnifier when working on parts. My eyes aren't what they used to be.

    Just from a glance, it seems the head on the left is clean, maybe fairly new and the one on the left looks like it has some white ash color to it, like it's been heated up. It also looks like the chamber volume is a bit more shallow at the dome. But for me, that could be an optical illusion. If the file sizes from the pix were large enough for me to open and enlarge without them distorting, I'd probably have a chance!...........

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    The one on the left has had a significant amount of material machined away. That would bring up the volume of the head by a large percentage, which in turn is going to drop your compression. On an engine with all stock internals you are going to end up with a noticeable drop in power.

    The more you squeeze the fuel/air charge the bigger the bang. The only time you should see that kind of machine work is if you are going to add some sort of forced induction system to an engine that was originally naturally aspirated.

    Aaron

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    Quote Originally Posted by rookie101 View Post
    The one on the left has had a significant amount of material machined away. That would bring up the volume of the head by a large percentage, which in turn is going to drop your compression. On an engine with all stock internals you are going to end up with a noticeable drop in power.

    The more you squeeze the fuel/air charge the bigger the bang. The only time you should see that kind of machine work is if you are going to add some sort of forced induction system to an engine that was originally naturally aspirated.

    Aaron
    Very good Aaron. The initial clearance is significantly larger and the angles are also much steeper than the stock head and the squish band width has been drastically reduced. The head on the left is what you get in an SBT rebuild. To my knowledge this same sort of increase is performed on all of the 2 stroke heads. At least I've seen this on every single motor I've ended up taking apart from SBT. Anyone care to see more pics?

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    Hypereutectic? Is that the third one you were thinking of?

    Not sure if they use them in Seadoos but we use them a bit in car engines.
    I just wanna go fast, get wet, and have fun...

    And if you get bored check out my Maverick website http://scooper77515.fordmaverick.net/index.htm

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