rickburnout666

MrMikeG the weedy is also an old 2 port model I removed the base gasket, raised the exhaust port and widened and polished the transfer ports and(also lightened the piston) it now revs to just above 18,000 RPM and now it is ready for putting in a boat at anytime not sure when that will be as i have now got 2 zenoah 260 pums , still waiting for a grey coil for one of them . I just wanted to let Strong know that things can be done to these engines by yourself if you know a little bit about tuning, I wouldn`t want to hold the old 2 stroke motor flat out for 5 minutes as this would deffinitlyblow it up with that sort of punishment.

rickburnout666

Simpley less load gets higher RPM

patriktegelberg

Seems everybody know that a lighter piston gives increased rpm to the engine. But when I thought about why the rpm increases I found no answer that was obvious or trivial, at least not to me []. After some thinking I did find one physically feasible answer I was prepared to belive in. But better than doing my own guessing I will be very glad to hear from someone here why a lighter piston gives higher rpm. Guesses are welcome to. Thanks

rickburnout666

Simplify it this way, if you have a 1lb piston i can bet you wont get very high revs , but a 20 gram piston it will rev faster and higher, like a car out of gear revs high and easy , but when in gear and under more load it does not rev as high and as quick.

wayneuk01

iam all for self doing but is there nowhere u can purchase a modded piston already done ? I am sure it is not going to be much more expensive purchasing a modded one if u can and will save u time u could then compare it to your stock one and then maybe have a go at your own...or just do it and see good luck either way & keep us posted

patriktegelberg

The quicker reving I agree is quite easy to understand. But the higher reving would recuire more power or less load. The mass of the piston is not a load in my oponion. A lighter piston would disipate less power trough the bearings and thus lowering the load but I imagine this to be a marginal effekt.

rickburnout666

my weedy is only a 23cc and twin ring

http://cgi.ebay.com/Zenoah-260-Modde...QQcmdZViewItem

http://cgi.ebay.co.uk/ws/eBayISAPI.d...MEWA:IT&ih=008

Wayne not sure what engine you want the piston for but ebay looks like the place to get a modded piston at a good price

rickburnout666

less weight is less load, the moving parts are now at a lesser weight so there is less load on the engine, i am unsure if a lightened piston alone would give a much higher RPM but i have never only added a lighter piston ,ive always had port work done too, Sorry i cant give a 100% answer to that one point, maybe start a thread in the engine forums ??

OLD_SLOW and in the WAY

Don't try this at home....this one is 22 grams

MrMikeG

It has to do with Reciprocating mass, you are trying to throw a weight back and forth. Lightnening the flywheel is less rotational mass so that will give you less load but when you start to think about less load being thrown back and forth it makes much more sense. One warning, less mass means weaker piston, a 20 gram piston won't survive a gulp of water, one small flip could rip the pin right out of the bottom of the piston. I get them down to 22-24 grams and that is good enough, never had one fail yet.
The sikk piston is heavier than the Zenoahs so they respond best to the piston mod, there ports are also the biggest. Zenoah 260's have a little lighter piston but small transfers so transfer work is best there as well as bigger intake and exhaust. 231 have better transfers so just piston mod with exhaust and intake widening is best. There is no one best mod for all engines yet. [8D]

45deepvee

heres an atricle on pistons


http://www.stockcarracing.com/techar...on_technology/

that should help, same basic principle for all pistons, but thats about v8 pistons. same principle for recipricatiogn mass and weight. pistons are totally different though

patriktegelberg

A lighter piston will lessen the load on the bearings and thus slightly reduce drag and also decrease wear. An imbalanced engine dissipate energy due to vibrations, a lighter piston makes possible a better-balanced piston. But as for increasing rpm I think these are marginal effects.

Reducing internal friction in an engine I believe could yield substantial power gains. A lighter piston does most likely reduce piston-liner friction noticeably.

The most interesting effect a lighter piston will have is that it will increase the time the transfers are open and also increase the time available for proper combustion. This does not reduce frictional load but increases the rpm by increasing the power produced. How this extra time comes about might not be all trivial to see. A heavier piston accelerates the flywheel at top and bottom and so makes the time spent there smaller.

A light piston will have a sinusoidal temporal velocity dependence i.e. constant angular velocity of the engine and propeller. For a heavier piston the velocity curve will tend to a triangular shaped curve and non-constant rpm of the propeller, thus also impairing the effectiveness of the propeller. But this, I suggest, is again a small effect.

This is my two cents theory on the subject. Needless to say I do not buy any of the “mass is load� arguments. Sounds plausible to me.

45deepvee

look at piston weight like this

FOR EXAMPLE, because this is the only example i can think of.

in a chevy 350 race motor, the stock cast pistons most likely always get tossed for the more lightweight forged aluminum pistons. i have held both of these, the weight difference is noticable. now if you tried putting stock chevy cast pistons in a nascar motor, do you think the motor would still rev out to 9500rpm?

main thing with the materials is the stregnth, but weight will free up horsepower and allow everythign to work smoother. once you get the weight of the piston reduced, the rod, and the crank will not have to try as hard to force it up, even bringing it back down will be easier.

once you have your compression in the motor the piston is forced downward in the cylinder, now the crank and the rod have to overcome that force, push the exhaust gases up, fight compression, and move the weight of the piston all at the same time. think of it that way.

strong

Any best way to get the clips out...............[sm=confused.gif]

MrMikeG

Patric, take a yoyo and fling it back and forth over your head, now take a ski rope and do the same with a 200 lb guy????? That is the difference, you want a smaller difference, get a punching bag and do a work out with thin sparing gloves, now put on 16 oz regulation gloves, which one tires you out quickest? Rotational mass, take a 80lb kid and spin him around, now do the same with an 120 lb adult, which could you spin the quickest?
A stock Sikk piston is about 30 grams, getting it down to 20 is a huge difference to a 25cc engine.

45deepvee

nicely put

rickburnout666

Strong the things i use are needle nosed pliers and/or a small flat ended screwdriver , old clips out, and put new ones in, i don`t use old ones again.

patriktegelberg

Yes, the flinging an 80lb kid argument is really convincing. I stand corrected.

strong

Well I got the piston off......................Now all I have to do is start grinding..................What is a good bit to grind with using a dremel.[sm=49_49.gif]

MrMikeG

They have a carbide bit that is straight all the way with a rounded nose, that is what I use. [8D]

rickburnout666

I use Tungsten/Carbide bits for porting, and for taking the piston down i use a diamond disc ,but most dremel discs would do the job on the soft aluminium piston.
I use a dentist compressed air drill as it doesn,t get hot unlike my old dremel did.

DaveMarles

Patrik, You say that you disagree with the mass is load principle. In the case of piston weight, mass is load
because that mass has to be accelerated and decelerated twice per revolution which is going to use energy.
Because it takes energy to accelerate the piston, this will take power at all rpms but will have an
increasing effect as rpm goes higher and will be a major factor in limiting the ability of the engine to
produce power at higher rpms.
Reducing piston weight (part of the reciprocating mass) will always be of benefit to power production in
part by reducing the energy use as stated before but mainly because it allows the engine to produce power at higher rpms safely and that means more power. It also has the VERY important side effect of improving
the reliability of the other engine components because they will have less stress placed upon them (
bearings, conrod, crankshaft, wrist pin). This is most important because it allows the engine to run at higher
rpm with safety. On my own engines I reduce all reciprocating mass as much as possible which includes
piston weight, wrist pin weight, weight of the upper part of the conrod and the piston spacer washers. Piston
weight reductions in the Zenoah of 30% are not too difficult to achieve and additional weight reductions in
the other components mentioned can result in a further weight reduction of ca 10% . These are very
important reductions. The question of piston reliability has been mentioned but the only problem that can
occur if the piston is lightened correctly is damage caused by hydraulicing after the boat turns over. You just have to be quick on the throttle or build some kind of airbox to cut down on that possibility. I try to run pistons of 19 grams in my own engines and never suffered that kind of failure in many hours of use. The gain from reduction in
friction loss with lighter pistons must be minimal and may be offset anyway by friction loss increase due to
the change in balance factor. I would be interested to hear how the lighter piston would reduce piston
friction. I‘m not disputing that but had never thought that it could be an issue.
Regarding the velocity curve of the engine, I think this will be affected in a far greater way by increases in
compression ratio etc and the propeller will always be accelerating and decelerating on each revolution
although on a boat with a flexishaft maybe those differences are damped out a little.
To others. I fail to see what difference a balanced piston makes in a single cylinder engine. What is it
being compared with? The important thing is to reduce weight. The lower weight piston will change the
balance of the engine overall but these single cylinder engines can only be balanced to a compromise in the vertical and horizontal axis and have a balance factor probably designed in by Zenoah for
much lower rpm than we use and so I would think that is not a problem at all with vibration. In practice I see no problem. As a matter of interest the main flywheel is out of balance from the factory and improving that would surely help if someone had a vibration problem with their engine.

patriktegelberg

Dave, I saw you reading this thread and hoped for you to contribute. The improved reliability and safety due to a lighter piston is important and easy to understand. Lighter pistons allow you to design the engine to operate at higher rpm and thus increase power output. However important, that is not what I am interested in here. I am searching for the physics behind why there is a power increase by just lightening the piston. Or is it so that all such effects, although existent, are small, and that the real reason we run light pistons is to be able to maintain structural integrity during high revs.

Regarding the velocity curves, yes, I to think the variations in angular velocity in a revolution are affected far more by compression and combustion. I was just on a roll and listed effects of piston weight regardless of significance. I did state the effect to be small with regard to prop efficiency. The timing benefits for transfer and combustion might also be small. But I did hope for someone to say that that the effect is there and is non-negligible.

On the subject of reduced friction. In the lower half of the stroke, piston-liner friction from accelerating the piston when the conrod is at an angle would be semi-linearly reduced with piston weight, would it not? In the upper half the compression force dominates. I calculated ballpark figures and found that at 16 000 rpm an 18 gram piston has the same kinetic energy as the adiabaticly compressed (10:1) gas energy at TDC. So an 18-gram piston has in that sense approximate minimum friction loss in the compression phase.

I still disagree firmly with the mass is load principle, and I am surprised that you do not. Could it be that we just misunderstand each other due to terminology differences? Or am I going to learn something today that makes me have to adjust in a way I thought to be extremely unlikely. I do agree that during acceleration (rev up) the piston mass is inertial load. But in steady state it is not. If it was, how would the power dissipation depend on the mass, linearly? Would a crank-piston assembly spun in rigid frictionless constraints lose energy and stop. If so where did the energy go?

Humble regards

45deepvee

ok here we go. heres an article found here, copy and pasted.

http://www.screamandfly.com/home/eng..._balance/2.htm














Lightening and balancing the engine’s rotating assembly components involves removing metal in strategic locations using a variety of methods and special tools, and therefore should only be attempted by a qualified technician. Removing too much substrate in the wrong areas could weaken the components being modified, resulting in engine failure.

For this article, we used our high performance Mercury 2.5-litre 260 horsepower engine. We had Jay Smith Racing Engines modify our flywheel, connecting rods, and wrist pins, while Marles US1 Racing lightened and balanced our Mercury high performance pistons. All components were factory stock prior to modifications, and weights were measured before and after the process.


Lightening Pistons and Top-Pinning

Traditional 2-stroke Mercury V6 pistons utilize a pressed steel pin in the ring groove to prevent the piston rings from rotating. In high performance applications, this has proven to be a less than ideal approach for securing the piston ring, since these locating pins have a tendency to work themselves loose during high RPM use. When that happens, the results are usually catastrophic and very expensive to repair – damaged sleeves, pistons, and heads. On the flip side, sometimes the factory locating pin holes were drilled too deep, and the pins simply sink below the piston rings, allowing the edges of the rings to enter the ports, resulting in damage similar to that mentioned above.







Locating pin failure will result in very costly damage to your engine. Note broken piston ring and piston scoring.

John Marles of US1 Racing has developed a unique process for installing locating pins into the top of the piston, rather than the sides. The process starts with removing the old locating pins and carefully inspecting the piston under a microscope for any damage. A new locating pin hole is drilled into the piston crown in a location that will allow the boost charge to cool the pin. Remember that the pin is steel, and the piston is aluminum, so proper pin placement is crucial for engine reliability. The new pin is pressed into the piston using a six ton press, milled, and then countersunk into the piston crown. Finally, the pin is welded into place. Marles uses a special high-carbon steel locating pin that is more dense than the piston ring. This will prevent the piston ring from wearing the locating pin down over time.

The actual welding process and machine are unique to US1 Racing, and are critical components of the process. Marles’ custom welding process allows the pin to be welded while not allowing the piston’s temperature to go above 120 degrees. A standard welding process would overheat and warp the piston, rendering a close tolerance fit impossible.

Our test boat is equipped with Marles’ top pinned pistons, and with many hours of hard testing under its belt, the pistons show no signs of impending failure.

Marles US1 Racing and Jay Smith Racing also perform the delicate and precise task of lightening, balancing, and angle cutting pistons. The benefits of lightening pistons in any combustion engine have long been known, and this modification becomes even more beneficial with today’s high-RPM V6 engines. Less weight on your rotating assembly means less bearing loads, which translates into faster acceleration and greater reliability. The key to successfully lightening pistons is knowing where and how much metal to remove.


Lightening and Balancing Connecting Rods and Wrist Pins







Close-up detail of lightened and polished connecting rod.
Some may ask why factory stock engines – even high performance engines – are not internally balanced from the factory. The answer is simple: balancing and lightening engine components is very labor and time intensive. This extra layer in the manufacturing process would add to the cost of the engines considerably. The benefits of this process become worth more as an engine’s potential to make power and develop more RPM is increased.

We had Jay Smith Racing Engines lighten and balance our Mercury V6 connecting rods and wrist pins. Balancing these components will not only make life on your crankshaft and bearings a little easier, but it will add to the running quality and acceleration potential of your engine. JSRE uses a seven-step polishing process to lighten the connecting rods from bare casting to a polished finish. During this process, a 120-grit flapper wheel is used to produce a lateral grain on the sides of the connecting rod. This metal ‘grain’ helps reduce the chances of failure by stress propagation to vital areas.

Below are some typical weight measurements before and after lightening:

Mercury V6 Connecting Rods
Connecting rods lightened and balanced by JSRE

Rod Type Stock Modified
"41" Series 350-370 grams 320-330 grams
"50" Series 475-525 grams 375-390 grams





Mercury 2.5 Litre V6 Pistons
Pistons lightened and balanced by Marles US1 Racing

Stock Modified*
470-490 grams 440-460 grams
*For race applications, lighter piston weights are available.





Mercury 2.5 Litre V6 Wrist Pins
Wrist pins lightened and balanced by JSRE

Stock Modified
106-111 grams 70-82 grams





Lightened wrist pins start at 55 thousandths of an inch at the end of the pin, and then taper to stock thickness toward the center. This is the same technology used to lighten wrist pins in Pro Stock cars. JSRE recommends the use of full-length wrist pins to avoid them pulling out. Additionally, new custom wrist pins are now used that are made to a weight of 82 grams, and require no cores.

Our connecting rods were all weighed on a calibrated scale - a crucial piece of equipment for the success of this process. The lightest rod was then taken first and lightened by removing material from the outside edges – but the process is not as simple as it sounds. The rods must be lightened while maintaining their strength. Substrate is removed in a lateral direction, creating a ‘grain’ in the metal that will reduce longitudinal stress transitions. Wrist pins are progressively honed to reduce weight in specific areas, and then balanced as a set.

What are the overall benefits of this precise machine work to your engine? First and foremost, engine reliability - especially during constant high-RPM use. We also observed slightly improved idling quality, and since unbalanced and rotational weight has been greatly reduced, the engine has more potential to accelerate more quickly. If you're serious about your high performance outboard, and achieving greater performance and reliability is important to you, then the above described modifications make sense. After all, who wouldn't want more performance and reliability? It's a win-win situation for any high performance boater.

45deepvee

here is a link to more articles abotu piston design and such


http://www.aa1car.com/library/2005/eb60522.htm