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250 stroked to 263= amazing!

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I stroked the crank of my ’05 YZ250F with an eccentric pin by 3mm. Installed at the bottom of the cylinder a spacer 2,3mm and also one for the chain tensioner. The result is amazing it brought to live the bike. Now I can lift the front without pulling the bar even in 3-th at high speed or with light pulling in 4-th. I finally can steer with the back with ease (this is very important for me, I’m coming from a yz250), because of the improved torque. All of this with 14-50 gearing! Till now I was thinking to sell the bike it was not very good for trail riding. Sure, it is a very efficient weapon, but no fun. This mod brought the fun factor! Now is somehow like a 200exc.

I definitely recommend this mod!!!

I have a question. I read very carefully the Thumperfaq on the chain timing but I couldn’t set it to 12 pins from punch to punch. Now is 13, the intake mark is a little high and the exhaust mark is half a tooth lower. The bike starts and revs like before, it is very responsive (JD kit and AP set with camera) and runs great. If I set it to 12(the exhaust will go higher by almost a tooth, I think is too much….or not?), is it possible to obtain even better results?

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you can't ,it is made here in Europe, but the hole thing is very simple as i described, so i'm sure that it can be made in US. Must only analise the material and hardness of the crank pin and made it eccentric by 1,5mm. Finaly you must take 1,5 mm from the crank on the side that come near the piston

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I have a question. I read very carefully the Thumperfaq on the chain timing but I couldn’t set it to 12 pins from punch to punch. Now is 13, the intake mark is a little high and the exhaust mark is half a tooth lower. The bike starts and revs like before, it is very responsive (JD kit and AP set with camera) and runs great. If I set it to 12(the exhaust will go higher by almost a tooth, I think is too much….or not?), is it possible to obtain even better results?

Your cams should have 13 pins between the 12:00 punch marks because they are OEM YZF cams the 12/13 changeover is only used to convert the stock WR cam, which is marked differently, to YZ timing, or vice-versa.

Because you compensated for the extra stroke by raising the cylinder, you have slightly thrown off the cam timing by changing the distance from crank to cam. This will not change the cam-to-cam distance, of course, and I would suggest that you select the closest position to the correct timing you can get for the intake cam, and the rotate the engine slightly until the marks on the intake line up with the head as they were intended to. At that point, correct the position of the exhaust cam if necessary until it also aligns with the head.

If the cams cannot be brought closer than one half tooth to the right position, you might consider indexable cam sprockets. But, if you are happy with the way it runs, it may not be that important. 🙂

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Grayracer, thank you very much for your answer, now i'm sure i already found the best position for the cams. Your answer help me not to lose time in searching something that i thought it could be better. You spared me a lot of time! thank you again!

RiderX, this mod improves the leaverage, because the distance from the rotation center now is bigger by 1,5mm. The rotation moment (or how is called) Rm=F*L, F=force, L=the arm of the force, so the bigger the arm the bigger the moment which lead to an improved torque(sorry for the terms but are direct translation from my language). So you don't have more hp but more torque. Indeed it is exactly what i feel when riding. The bike is pulling way better from low to a healthy punch in mid and then fade away the more the rpm are rising. I think it also raise the compresion, the total stroke now is bigger by 3mm but the cylinder is raised only by 2.3mm. It doesn't require special gas, it runs just fine with 95 pump gas. So i guess because of this improved compresion the power (hp) is bigger too. I think this mod is like 262 Gorr mod but better because of the torque, but i can only suppose that because i didn't have the chance to try out the Gorr mod.

Hope i was clear, sorry for my english

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Sunruh to be honest i'm also a bit concerned about the reliability, only the time will prove it. A thing that convinced me to do it is the fact that there are engines modified like that and raced that have 1.5 years now and have no problem. I calculated for myself the piston velocity and it is raised only by little, so it should be ok. The mod is not that radical, the masses are practically the same, so…. it should be ok. The only thing I whish I could have is a stronger rod from Falicon. There are 450F with this mod that are raced in super moto (I think that puts a bigger load on engine on acceleration and revs) and after 2 years are just fine.

All in all I take this small risk because now the bike is what it should be, a fun bike with temperament

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I'm a bit confused about this 263 stroker mod. How does this eccentric pin work?
It works exactly lie it sounds like it would; the crank pin is made with the bearing surface off-center (eccentric) from the part that presses into the crank by an amount equal to half the desired stroke increase, which in this case would be about 1.6mm. One of the first most obvious limitations of this method is that if the offset is too great, the rod bearing cannot be assembled over the step and onto the pin. As it is, it has to be worked into place before the rod goes over it. Too much, and you need a special rod with a bigger bearing.

The pin also has to be very precisely indexed, so that not only the oil hole aligns, but that the offset is exactly along the line drawn radially from the crank axle through the pin bore.

Offset pins might also require tack welding to prevent them from being rotated out of place by the force of combustion.

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So basically you're lengthening the rod by offsetting it. How is the cylinder raised? Is there some sort of plate? One thing that would make me lean more towards the 262 big bore kit is the fact that it only requires a top end tear down and you're not playing with the geometry of a motor that spins a lot of rpms. If the kit is any good ut will be picked up by one of the US marketers and whored out all over every dirtbike magazine. Until that happens I wouldn't even consider it.

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Keep in mind that a longer stroke means more low-end, but also a lower redline due to the increased piston velocity.

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The piston velocity at 13500 rpm is only with 0.7m/s bigger then standard ( standard=12.06 m/s, 3mm starked=12.7m/s) so this is nothing for the materials used in today engines.

As i said I'm a little concerned about the reliability but I'll find out till the end of the season, one way or the other. The kit is raced with 0 problems so i hope that my kit will follow this path.

The most important thing for me is that now the bike is alive, no more flat character, which almost got me back to 2 stroke.

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The price for the kit is 160 euro (almost 200$), the kit is from Austria (the kit is made by a super moto race team) and is imported here by a friend of mine, which also has replaced the pin in the crank for 12$. The rest of the work was done by me. I'm sure that for serious quantity of orders the price can drop. Here were made like 5-6 bikes, so.... .Sorry but this is all i know.

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OK thanks. It would help to clarify what is in the kit as it is not 100% clear because you then go on to say "The rest of the work was done by me". Do you mean you did the assembly?

Also there may be an error in your piston speed - this site comes up with different numbers - approximately double, but I do agree that the increase is relativley small and does not exceed what is regarded as a threshold of 5000ft/min for a race engine.

http://www.csgnetwork.com/pistonspeedcalc.html

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there is NO threashold for 5000ft/min!!! what was a mid 80's and earlier theory.

that was broke YEARS ago by the 1.0L bikes. now most of the 600cc bikes break that as well. heck a lot of the litre bikes are over 1 mile per minute now due to their high redlines. then go look at an f1 motor. at over 20k rpm, they blow by the 5k ft/min mark with ease.

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So basically you're lengthening the rod by offsetting it. How is the cylinder raised? Is there some sort of plate? One thing that would make me lean more towards the 262 big bore kit is the fact that it only requires a top end tear down and you're not playing with the geometry of a motor that spins a lot of rpms. If the kit is any good ut will be picked up by one of the US marketers and whored out all over every dirtbike magazine. Until that happens I wouldn't even consider it.
The rod length is unchanged. The distance traveled by the piston is increased by the pin offset because the pin center is farther from the shaft center. I believe floting stated he raised the cylinder with a base shim.

Anything that you do to increase the size and/or output of an engine is going to load it more heavily. Increasing the stroke increases the power loading on the crankshaft. It also raises the piston speed at any given RPM, which increases the importance of proper piston lubrication, and increases the tension load on the rod as it goes over TDC on the exhaust stroke, or at high revs with the throttle off. Interestingly, because the piston travels faster, and because burning fuel will ultimately only push the piston just so fast, stroker kits tend to reduce an engine's peak revs slightly (in this case probably very slightly).

But remember that big bore kits are not without penalties. Bigger pistons means more power loading, and the increased weight stresses the rod more at high revs. One manufacturer pulled his 302 kit off the market because the frequency of rod failures was too high for his liking.

On the question of piston speed, it can't really be calculated without knowing the rod angularity, but the crankpin speed of a stock YZ250F at 13500 is 7400 fpm, so I'd be surprised to find out that peak piston speed wasn't well over 5000 already. It's also interesting that the red line of the little YZF is 500 rpm higher than that of an R1. Amazing machinery.

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@…family, yes you are right, I have calculated the speed wrong, entering in the formula revolution, not frequency. At a complete rotation the piston is doing 2 trips so at 13500 rpm the frequency of the piston trips is 27000/min = 450 Hz. It means that correct is 24,16m/s (4748 ft/min) standard and 25,4 m/s (5013,8 ft/min) stroked. This last speed is a little over that threshold. I do remember reading in a prestigious Italian magazine about this maximum speed, which is becoming bigger and bigger along with the improving of the materials used.

The kit is composed of the pin and 2 spacers, one for the cylinder base, one for the chain tensioner. Yes I assembly the engine.

@ grayracer- I think you are right about engine peak revs, I didn’t verified it but on the same little hill I could go up with standard at 104 km/h, now I can go 98 km/h, so I suppose that the rpm are not that high.

You said that the piston speed can’t be calculated without knowing the rod angularity, I don’t understand why? The piston travel is a line regardless of the rod angle, also in 1 rotation the piston will do a trip up and down regardless of the rod angle.

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okay, call me a knave, but what exactly is the definition of rod angularity? is it like the angle of the dangle (in reverse, of course) 🙂

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okay, call me a knave, but what exactly is the definition of rod angularity? is it like the angle of the dangle (in reverse, of course) 🙂
The length of the connecting rod affects the range of angles the rod swings through relative to the line running up the center of the cylinder from the crank, and the cranking angle between the rod and the crankshaft "arm". Shorter rods run at greater angles than longer ones, and the change in length and angle has an affect on a number of aspects of the engine's power characteristics.

Visualize the crank at any position besides TDC or BDC, and a triangle formed by a line through the cylinder center from the wrist pin center to the crankshaft center on the long side, a line drawn from shaft center to the crankpin center on the short side, and a line drawn between the two centers on the connecting rod. The piston is bound to run only up and down the cylinder center with the wrist pin, and the rod big end swings in a circle defined by the crankpin.

Now imagine this assembly in motion (or draw a diagram of it in several positions), and you will see that the piston moves very little in the first 23 degrees of downstroke, and in the last 45 degrees near the bottom. It moves the fastest, depending on the length of the rod, when the angle on the short side (crank radius) gets close to 90 degrees to the connecting rod, around 50-60 degrees after TDC, usually.

The formula posted on the web site referenced is flawed to the low side because it fails to take into account the 135 or so degrees of crank rotation (38% of the full rotation) during which the piston is barely moving, and instead calculates a simple average. Basically, the piston covers 90% of a single stroke in about 65% of the crankshaft stroke, which means the peak actual piston speed is much higher than the average.

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