RPM:

[KymcoKid]

I put a big bore 50 mm.Now my manual states around @7900 for my agility 50.I running now @ 8400 with 45 mph top.I know that these engine a made for high rev.But if I can get more safely I would like too.

[90GTVert]

The 50mm+ bores may be more likely to experience premature failure when operated at high RPM.

[GrumpyUnk]

I would suggest that the larger the bore, the larger the piston diameter, and logically, the more mass being accelerated back and forth in the cylinder. More mass leads to larger forces on the connecting rod, and the crankshaft. The forces go up with the square of the mass, due to acceleration(I think).
The increased forces will be more likely to find the faults in the piston casting, the connecting rod, the bearings, and the crankshaft, if there are any. All will have 'faults', as nothing is perfect, but with the added mass being accelerated, finding them will also be accelerated.
I think a 49cc/50cc is 'de-rated', and will last decaades. The 47mm, ~70cc, called '80cc', will likely last a bit less, but still have a good life. The sizes beyond are going to stress the 'system of parts', and put loads that may accelerate wear and finding that faulty piece.
My MY MY, to emphasize, is that if you run at high rpm most of the time, and have one of the larger BBK's installed, you failure rate will increase, re-stated, your useful lifetime will be decreased. Use of the mid-range torque increase rather than high-rpm use should result in longer life.
Evidence? I have none. I made it all up from physics and experience. I remember reading somewhere about the force exerted on the connecting rod/pin/piston at BDC and TDC as the piston slows to zero, and is then accelerated in the opposite direction. I am not going to compute, but it stands to reason increased mass subject to acceleration adds load to the reciprocating parts, and the rotating parts.
If you are going to run a large BBK at high rpm, make sure you use good oil, and make sure the engine cooling system works, and the fuel:air ratio is tipped a bit towards 'rich' to get optimum lifetime. Leaner mixes run hotter, and air-cooled plane engines are run "Rich" to help cool them. Not a lot, but a bit. Actually, YMMV.
tom

[jackrides]

Energy = mass X velocity squared. The big question is: Why isn't it the other way around?

[GrumpyUnk]

I think more apropo to the verbiage is: F=ma

Force = mass X acceleration

The numbers can be determined from available information.
mass= that of the piston, rings, and wrist pin, plus the connecting rod
acceleration = the time from zero v (velocity) to max v(partway up the stroke) back to zero v (at TDC)
and thus the force...
The higher the rpm, the quicker the zero-max-zero happens, thus, with fixed mass, the higher the force to be able to cause the acceleration/deceleration.
Putting it in short words, if you accelerate a greater mass(BBK piston) at a higher rate(higher rpm), the force on the writs pin, connecting rod bearing and the crankshaft rod journal is significantly higher. I think increasing at a 'square'.
Read here for simple, down the page to Newton's Second Law as a guide to thnking:
www.physicsclassroom.com/class/newtlaws/Lesson-3/Newton-s-Second-Law

The force will increase if acceleration is held constant, just by adding mass.
The force will increase even more if the rpms(acceleration) is increased...
Adding accelerated mass adds a LOT to the force.
Read. Doubling... etc. I can't teach it.
tom
The energy in e=mc^2 refers to things approaching the speed of light, c. We are not that close, I think.

[jackrides]

An answer! At last! Thanks!