lifespan of the part
There are certain properties of materials that you cannot change. For instance, All aluminum and titanium alloys will eventually fail! Depending upon the stress/strain loading vs the ultimate strength of the part, it could be on the first cycle or the ten millionth cycle, but it will fail. Steel based alloys are different, if your load is below a certain percentage of the ultimate load (10% for example) the part will continue to have an "infinite" life. These loads do not have to be large enough to cause a crack that will be found on magnaflux inspection, by that time the part is ALREADY failing. (in effect, the part remembers the punishment it has received in its life).
That's why nobody buys used aluminum rods for instance, and everybody throws them out after a certain number of runs (or risks grenading an engine). That's also why the NASCAR crowd routinely sells off used parts such as blocks, cranks, valves, etc. When you buy those parts, they do have some life left, but you don't really know just how much! Do you really know the history of the used crank you're buying? Given how cheap the new ones are ($100 to $200 for stock or $600 to $700 for Eagle 4340) I wouldn't gamble and would just get a new one. Howards Cams was selling the stock ones for about $100, I bet it would be tough to even get a crank ground and polished for that price!
Good luck,
Mark
There are certain properties of materials that you cannot change. For instance, All aluminum and titanium alloys will eventually fail! Depending upon the stress/strain loading vs the ultimate strength of the part, it could be on the first cycle or the ten millionth cycle, but it will fail. Steel based alloys are different, if your load is below a certain percentage of the ultimate load (10% for example) the part will continue to have an "infinite" life. These loads do not have to be large enough to cause a crack that will be found on magnaflux inspection, by that time the part is ALREADY failing. (in effect, the part remembers the punishment it has received in its life).
That's why nobody buys used aluminum rods for instance, and everybody throws them out after a certain number of runs (or risks grenading an engine). That's also why the NASCAR crowd routinely sells off used parts such as blocks, cranks, valves, etc. When you buy those parts, they do have some life left, but you don't really know just how much! Do you really know the history of the used crank you're buying? Given how cheap the new ones are ($100 to $200 for stock or $600 to $700 for Eagle 4340) I wouldn't gamble and would just get a new one. Howards Cams was selling the stock ones for about $100, I bet it would be tough to even get a crank ground and polished for that price!
Good luck,
Mark
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