The International Cessna^® 170 Association

For the last 12 months I have a minor vibration in the engine. it was dependent on engine rpm. I checked around and found nothing out of the ordinary. ok prop balance coming up at the next 100hrly. I was putting the spinner back on and checking the screws for condition. if a screw looks real used its replaced.it makes getting it off next time a lot easier. All the screws were lying on the bench and I was spraying them with WD40 when I noticed some were longer than the others . It dawned on me the shorter ones were on one side of the spinner and the longer ones were on the other side . so I spat the dummy and replaced all the screws on the spinner [all the same length]. Vibration gone . so the moral of the story is check your screw length.

Before I finished reading your post and not knowing you'd already found and solved the problem, my first guess was that the prop got installed at the last annual not clocked in the proper position.

It's an Aussie expression.
https://en.m.wiktionary.org/wiki/spit_the_dummy

If you can get a hold of Chadwick-Helmuths pamphlet "The Smooth Propeller" (google found it for me last year) it has all the calculations for determining the effect of bolt and screw length on balance. It's pretty interesting, and after reading it your results will not be surprising.

DEM

I tired to attached a copy of "The Smooth Propeller," but at just 2.8 MB it exceeded the file limit of the forum. You can download a copy here:
http://www.expaircraft.com/PDF/SmoothPropeller.pdf