Since I am a Molnar Technologies dealer I thought it would be interesting to hear what Tom Molnar had to say about his thread.
Here was his reply:
"I am eating an early lunch and when I read " Disappointing to say the least on a well known brand. ", I almost had it coming out of my nose. Eagle is quite well known and they sell a lot of parts but mostly to the end user who thinks they are a quality product. Not many real engine shops use them as most of the cranks need to be ground .010 - .010 before they can be used. I cannot tell you how many people say they measure them out of the box and find .002" of taper in the rod pins. The other guy talks about "a big hole for a cross drilled rod main". That's a lightening hole, not a cross drilled crank.
In reality, there can be several things that caused this.
1- What damper was used?
2- It was a used crank with a lot of time on it.
3- It was ground undersize. If the grind shop did not have the radius on the wheel right, it could have left a stress riser.
4- It could also be a cast crank. Even the OEMs use a forged crank in engines that make much power.
5- If the crank was made for a certain bob weight and it was balanced for something else, it can cause problems. As an example, lets say the crank was designed with counterweights for 1700 gram bob and it was balanced for 1900 by adding heavy metal. The middle of the crank is still too light to cover the forces in that area and since the balance machine only reads the forces on the #1 and #5 mains, it does not know what is going on in the middle. Since the middle effects what the balance machine tells you it is light on the ends. When heavy metal is added to the ends to fix the middle, the ends end up being too heavy but the balance machine tells you everything is fine and at 700 RPM, it probably is. At high RPM, the heavy ends pull in one direction and the middle pulls in a different direction. This bends the crank. Rev it up, it bends, go back to idle and it relaxes. This back and forth is jut like straightening out a paper clip and bending it back and forth. After a while it breaks."
