> This is when he discovered that the rod throws on the crank were ground almost 1/8″ too wide.
In precision machining where tolerances are typically measured in thousandths of an inch that's way beyond unacceptable. At this point trying to make it work is throwing good money after bad. The fact that he didn't even realize it until he was trying to fit it into an engine means he did no QC on his own too. This whole endeavor was destined to fail from the start. He seriously needed a business partner with actual manufacturing experience.
It's even worse than that--a bearing surface on a crank journal will be specced in ten-thousandths if an inch (~2 \u m). Width of the journal isn't typically as precisely specced, but for an aircraft motor journal width is extremely important, as the crankshaft is placed under high thrust loadings. While there will be a thrust bearing, crank endplay on an aircraft motor is even more critical than on an automotive engine.
I have no sympathy for the businessman who had the cranks made, something that far out of spec should have never made it into a customer's hands. If you are serious enough to invest 30k in tooling to get parts made, you should also be consulting with an experienced engineer and lawyer who both have experience dealing with Chinese suppliers.
Even more dangerous was that he was selling cast cranks as billet. Without doing x-ray good luck finding occlusions in a Chinese part. It could look fine and even measure out fine (these did not thank God) but have bubbles within the casting.
There is a reason forged and billet American performance car parts are expensive!
There is a huge amount of testing, engineering, and quality control that goes into a crank made by Bryant or Callies.
I can only imagine he fell deep into the sunk cost hole. An 1/8” says, “yes, this bears a slight resemblance to what I ordered, but it is not what I ordered. Not even close. You might have well sent me a random camshaft.”
Why he didn’t abort at that point is beyond me, other than ignorance (in which case he shouldn’t be build avaiation engines) or the above mentioned sunk cost. But even if the latter, he would have ended up selling sub-par parts whether he knew it or not (in which case he shouldn’t be selling aviation engines).
The fact that I've seen the Chinese produce parts with much better tolerances in far cheaper products suggests there was likely some sort of massive miscommunication (language barrier?) --- being off by a few hundredths could be ascribed to lack of quality, but to be so massively out of spec even for a bottom-of-the-barrel manufacturer is so unusual that it would more likely be that they were given a wrong spec to start with. I suspect something related to metric/imperial conversion...
In precision machining where tolerances are typically measured in thousandths of an inch that's way beyond unacceptable. At this point trying to make it work is throwing good money after bad. The fact that he didn't even realize it until he was trying to fit it into an engine means he did no QC on his own too. This whole endeavor was destined to fail from the start. He seriously needed a business partner with actual manufacturing experience.