Paul Lamar wrote:
Ah ha!!! I told you so!!! You did not take those bolts out one at a time
and coat them with RTV, like I suggested, did you? :-)> Do I need to dig
up that message? You know I can. Right after it happened to Perry.
All you new guys. Buy a new set of tension bolts. It is impossible to
inspect used bolts for cracks at the root of the threads. Take your old
tension bolts out one at time. Coat the new ones with RTV and replace
the one removed. Re-torque the bolt. Then take the next one out in
the order recommended by Mazda in the shop manual. This is done to damp
a resonant condition at certain high power continuos RPMs causing the
bolts to fail from fatigue. We do not yet know what that RPM is but
we do know it is less than 5500 RPM as that is Perry's direct drive max
RPM.
Since Finn cruises at low RPM is could be as low as 4000 RPM.
Tom Kendall asks:
OK, I consider myself one of the "new guys". Has this same problem been
reported on the 20Bs? I ask because with longer tension bolts, their
resonant frequency should be quite a bit lower... like maybe even idle RPM.
That's assuming they are torqued to the same values, etc. I'm not enamored
with the idea of buying all new bolts!
As far as inspecting for cracks at the thread roots, it's actually quite
easy, but most people won't have the tooling available. Some shops will
though, and it may be a lot cheaper to have a specialty house inspect them
than trying to replace the whole set. The technique that should be used is
Magnetic Particle inspection.
The bolt is either sprayed with or immersed in a liquid suspension of
fluorescent magnetic particles (the original line of products developed
under the name of "Magna-Flux", hence the name!) and an external magnetic
field is applied to the bolt by a DC coil. If there is a discontinuity
(crack) in the bolt, the particles will bunch up there and under a UV
("black") light the cracks show up as a bright band. This is one of the
slickest tests I've seen. Very sensitive and fairly goof-free.
Finn, if you want to set up shop doing this kind of testing, you may
actually get some business. The coils, black light, and mag particles
aren't very expensive, but certification could be a pain.
Don't confuse this with Penetrant Dye testing (the stuff most people think
of when they hear the words "Magna Flux"). Penetrant dye tests don't work
worth a darn on threaded parts because every thread root looks like a crack.
There is one other trick that can work on larger, fatter bolts (but not so
good on the really long thin ones like our tension bolts): It uses an
ultrasonic probe placed on the head of the bolt to measure it's "length" by
ringing the pulse down the bolt shaft and measuring the time it takes to
travel to the far end and back up again. Cracks show up as smaller
reflected sound peaks returned earlier. An O-scope kind of display is
usually used. This is pretty high tech stuff compared to mag particle
testing and takes some skill to do and interpret correctly. Not recommended
for home builders.
I just talked to a friend of mine who owns an engine shop, he said he could
magna-Flux my bolts with no problem. He first said $17.00 each, but then he
grinned and said lunch would be fine
Royce
If you have an O-scope my method would be much cheaper than $17 each.
Paul Lamar
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