That's true. More than once I've wished the 351 (250) had 2 more head bolts and cylinder studs, tying it all together...

 

Consider this.
At 10,000 rpm the piston moves, on average, 20.4 metres or 66.9 feet per second.
The pistons peak speed at 10,000 rpm is 32 metres or 105 feet per second. (this is an assumption as I don't know how to calculate it)(I have assumed the movement of the piston is like a sine wave and so I multiplied it by 1.57)
It accelerates and decelerates from 0 to peak speed and back 2 times per revolution or 333.3 times per second or once every 1/333.3 of a second.
The piston accelerates to peak spead in 1/2 a stroke.(not sure about this either)(in fact I'm pretty sure its not correct)(rod length will be a factor I'm sure)
!/2 a stroke at 10000 rpm takes 1/666.7 of a second.
So this represents an acceleration rate of 21334m/s squared.
Earths gravity is 9.8m/s squared so the force on the piston is 2177 times more than it's weight on earth.
So 10 extra grams as weighed on a scale would be equivalant to 21770 grams or 21.77 kgs or 48 lbs at 10000 rpm.

Its amasing the thing works at all.

Now I could be totally wrong here and I bow to a smarter person if it turns out that this is all wrong.

 

Can some one tell me the con rod length.
That way I can correct the above calculations.
But whats above i think is quite close.

 

I think the math is little off, but you are on the right track. The constant velocity of the pistons going up or down is not a problem. Its the sudden turning point when mass has to reverse direction. Just like Newton's 3rd law.

 

Sorry to bump an old thread, but I am just installing my Bill Blue 351 big bore kit and weighed my stock KLX250s piston, rings, pin, and clips together and compared to the new JE 351 piston that came in the kit

Stock - 237g for the entire assembly (rings, clips, pin, and piston)
JE 351 - 314g for the entire assembly (rings, clips, pin, and piston)

That's a 32% increase in weight, but as others have mentioned, the difference in weight between the stock 300 piston and the JE 351 is likely much less.

 

This is the first time the wt of the stock piston was posted. That is quite a difference. When I was doing mine, I was trying to find out the info, but I didn't have a scale & never figured it out.
I don't notice any additional vibration on my 351 though.
With this wt difference, I would limit the time spent @ red line. But I got the 351 so I could run lower rpm's anyway, so I don't see that as a problem.
Note: I don't think the rod length has any impact on piston speed or acceleration calculation.

 

If anyone has a KLX300 piston with rings, pin, and locks around it'd be great to have the weight of that for reference. I'd imagine it would likely split the difference between the 250 and 351 pistons.

I'm not too worried about the redline either, given that the KLX300 had a heavier piston than the 250 already, and had the same or higher redline.

Bill also didn't cut any corners from a reliability perspective with the piston either - it could have been made much lighter than it is, with a thinner top ring land, reduced skirt profile, moving to 2 rings instead of 3, etc., so I'm comfortable with the weight.

The rod length has a very minor effect on piston acceleration and peak forces on the rod, but you need a huge change in rod/stroke ratio to even make a small change. The piston weight and RPM dominate the forces.

(Example):

 

Weight of the 78mm piston (including pin and piston rings) from AHL kit is 272.7g. OEM 72mm piston (also with pin and rings) is 235.5g.

 

86mm piston assembly (piston, rings, pin) from Keng 355 kit is 340g. Quite heavier than AHL 78mm piston assembly. I'm worried about vibrations now.

 

I put the Keng engine 402cc big bore kit on my latest klx project bike. Unfortunately I didn't have a scale to weigh the piston before I built the engine. As far as vibration goes there's no difference in the handlebars and seat between the 402 and 351 big bore engines vibration. The klx402 just has more power.