Don't forget that resonant components can respond to harmonics as well.  My
 > cruise rpm is in the range of 4500 to 5200 rpm.  This equates to a
frequency
 > range of 75 - 86Hz. The second harmonic would be 150 - 173 Hz. This isn't

 > close, it's dead on!
 >
 > Having said that, I am fairly confident that damping the bolt near the
 > center will eliminate the problem.  Tune in again in a few years when I
 > again hit the hour mark where my tension bolt failed.
 >
 > Interestingly, my old engine had the early bolts which had a shank necked
 > down to a smaller diameter than the threads.  These bolts had almost

twice

 > as many hours (not even counting the hours in the car) as the brand new
full
 > diameter shank bolts in my current engine.  The old engine never had a
bolt
 > failure in 856 hours of flight.
 >
 > Tracy Crook
 > tcrook@rotaryaviation.com
 > www.rotaryaviation.com

 Perry Mick's failed tension bolt was the necked down version.

 Paul Lamar

    The racing bolts for a 12A have a molded on plastic or rubber like sleeve
in the
    center. Some seem to hang up a bit on installation. Not a press fit at
all, just a bit
    of resistance. Before the "racing engine" came along we used the old
necked
    down bolts from junk yard motors. I have yet to break a bolt, but we tend
to pass
    through the critical RPM only twice per race. I know it seems impossible
to
    believe that the thick bolts could "vibrate" enough when torqued to 26
pounds to
    suffer any damage, but here are those nasty failures to prove the
pudding. I think
    some of the plastic sleeves don't touch anything when installed. So it
must be
    just a bit of extra mass that damps out the frequency to below the danger
zone.
    The skinny bolts had a threaded length that was much longer than
required. So I
    think they did that to get the critical frequency above the "zone" at
street RPM.
    Racing Beat had a sheet metal disc that was held on by the stationary
gear bolts
    so that if a case bolt broke, the head end couldn't slide out and collect
the
    counter weight. A big goober of silicone near the head end would do the
same.
    Lynn E. Hanover

 
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