It happens to the very best stuff. I used to work a nuclear plants. You
really don't get parts more carefully made than in nuke plants, even though
the stuff is by definition obsolete by the time it gets installed (the
qualification process is so long that virtually no new tech can be
installed). Bolts broke that had been made with certified processes and
radiographed prior to installation. I asked a mechanical engineer why, and
he said there is no method other than failure testing every part that can
predict how and when a part will fail, and if you failure test every part
all you have is broken stuff. Care in manufacture, under stressing, NDT, all
that stuff is just predictive of a statistical norm. You can still have a
bolt that passes everything that happened to have all the microscopic
impurities and flaws concentrated in just the right place to break.
In other words, even a good part can be bad.
Of course we also had the problem of counterfeit parts being sold to dumb
procurement people who know the price of everything and the value of
nothing, but that's another story...perhaps.
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Barr, Scott
Sent: Thursday, July 29, 2004 11:43 AM
To: FOT@autox.team.net
Subject: ARP Bolt Failure?
I pass along the following for whatever interest it may generate. A couple
of weekends ago at Road America, my new Spitfire motor blew up in a pretty
impre$$ive manner. My first thought was that a rod bolt on #2 had broken --
the #2 rod cap was seriously damaged and one of the bolt holes in the cap is
now D-shaped -- clearly, the ARP rod-bolt was not in its hole when that
damage occurred. But, these being ARP rod-bolts, I concluded that there
must be some other explanation -- that it had been starved of oil or
something.
On disassembly, however, none of the other bearings show any real wear. My
dad (with 40-some years of experience in the engine building business) took
a look at the mess and concluded an ARP rod bolt broke. He showed the mess
to a Caterpillar Certified Failure Analysis Instructor, who concluded an ARP
rod bolt broke. See below for that discussion.
Anyone else have experience of ARP rod bolts failing?
Scott
-----Original Message-----
From: Jerry Barr [mailto:jerrybarr@charter.net]
Sent: Thursday, July 29, 2004 11:25 AM
To: Barr, Scott
I sat down with a Caterpillar Certified Failure Analysis Instructor (who now
teaches for Northeast Wisconsin Technical College). His thought is that the
rod bolt broke first then the catastrophic failure. The bolt shows signs of
fatigue fracture with small beach marks then brittle fracture when it
separated and fell out of the rod. The rod bolt showed no signs of over
torque but does have small amounts of fretting under the bolt head which
leads him to believe it started to break, loosened slightly, allowed the rod
bearing to move, seize the bearing, bolt broke off and fell out, cap
ratcheted open, and all hell broke loose. So the ultimate failure was a flaw
in the bolt which allowed the bolt to break. We should look at it under a
microscope to prove that though.
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