O-320 3.5 crank speed vibration

Hi,

I tried to balance a prop on a O-320 on a RV-6 the other day and could
not because other large vibrations were present.

Im summary, I got:

0.7 IPS at 0.5 crank RPM

0.2 IPS at crank RPM

0.5 IPS at 3.5 crank RPM

The vibration levels at the other multiples of 1/2 crank were all
relatively small.

I can find lots of info on the web re 1/2 crank vibration and its
causes but I can find nothing on 3.5 crank and I am puzzled as to why
it is so high.

Is it normal for this kind of engine?

Thanks,

Mark