Originally Posted by
Roelof
You would be amazed about the flex. Indeed some play etc is also hapening. I can tell you that an engine compleetly builded up with fresh bearings and almost no play on the rod with a 0.35mm head clearance at full rpm the piston of our engine is hitting the head. Making 40.000 or 45.000 rpm can make a difference of hitting the head and/or backplate or not.
With wrong hardened crankshafts we have seen that they will snap within an half our, hardened in the right way is keeping some flex damping the peak forces. We also had made conrods from a very strong piston material, because it had no flex it shattered in pieces in 15 minutes.
Flex is always needed, maybe you can not see it but it is there!
Maybe you are confused on what flex is?? I dont dis agree with the fact that you need a certain amount of flex but not on a crank, you will get more play from the bearings than you will from the crank. Exactly where do you think the flex will come from? think about it, your not going to get any flex between the bearings ( if you did the crank would bind)meaning it can only happen on the largest area of the crank! secondly with a properly hardened crank at most you would see maybe .0002" and once again thats not enough to do any damage. now on top of that you have to think of what would cause the flex and that would be the force of the piston and rod moving the crank and both of those two parts are made of soft aluminum with bronze bushings! both of which will bend/flex before the crank ever would. As stated before i have been building real engines for most of my adult(I am 42 now
) life ranging from 700hp methanol rotary engines to twin 100mm turbo charged 2500hp v8's!! Granted i dont have that kind of experience with these engines , the principle is still the same metal is metal! If you are breaking cranks i tend to believe it is more of a frequency issue than flex,40k RPM's is alot no matter what scale!
I am just giving my 20+ years of machine shop/engine building experience, i could be wrong as i have been before. But i dont see how the laws of physics change with nitro engines
Z, i think over revving and too lean is right and probably what caused the damage. Hoes is probably correct.