Snip
From: Chuck Ramsey


How could someone (anyone?)not notice in 2 1/2 years of construction that
something just didn't look right?  But then Frank reminded meof the Mars mission
that crashed a lander into the surface of the planet.  An ounce of prevention is
worth 0.4536 kg of cure.  We humans...ain't we a kick. chuck
Unsnip

So true Chuck. Negotiations sometimes end up with a compromise that suits the
negotiators but is less than satisfactory in the real world.
In a previous life I was the engineer in charge of writing assembly instructions
for heavy equipment components. We bid on a 80/20 project in support of a
supplier who was tooling up for increased production but couldn’t meet the
current demand. We were to be the 20% supplier involving one group of
components.
I went with a crew down to the current suppliers plant to gather information for
assembly instructions and to train the welders on the assembly.
The customer had supplied prints to the Project Manager who ordered parts. One
of the parts was an access port on the side of a tank. After the crew assembled
the first tank they sent it over for leak testing. Following the assembly prints
supplied by the customer and the assembly instructions they tried bolting on the
blank plate made for testing the tanks integrity under 15 psi. The 3/8” fine
thread bolts would only thread in about 1/2” and then hung up. The tester got a
3/8” fine thread tap and chased the threads. The tank passed. It was assembled,
painted, and ready to be shipped.
I stopped out and talked to the crew and the tester walked over to complain
about the screwed up threads in the access port. Knowing that the chances of all
8 threaded holes being damaged at 1/2” deep were pretty slim I stopped the
assembly process and started chasing prints.
The set that the Project Manager had ordered the parts from were a set from the
customers engineering department. The access port had not been converted to
English inches like the rest of the assembly had 5 years previously. When the
supplier was making the tank they just knew that they were supposed to be 3/8”
fine thread and made them that way.
The company had bid and bought materials based on the prints given so the
project had a hermaphrodite tank assembly. After consultation at a much higher
level than I was used to, the first 40 units were shipped with metric bolts.
Doesn't sound like a problem until in the field someone tries to work on one of
the oddball tanks and taps out what appears to be damaged thread. Tapping the
holes to 3/8” will hold 15 psi. I never heard any complaints but I know in the
field the operations that ran this equipment used automatic fill systems to fill
the tanks through an auto shutoff  that stopped when it hit 50 psi of back
pressure.