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I took the day off on Friday to fly to St. Thomas, ON (CYQS) to have the prop dynamically balanced. RV–7 builder Gary Wilcox has a RPX Dynavibe prop balancer and he offered to do his magic after reading about my problem with recurring exhaust system cracks. Two other RV builders had emailed me to report that they had recurring exhaust system cracks until they had their prop balanced.

After arriving at St. Thomas we removed my cowling and plenum chamber cover, then Gary bolted the accelerometer to the crank case.


The optical pickup was bolted to a rocker box cover.


A piece of reflective tape was applied to the back of a prop blade, lined up with the optical pickup. This allows the system to know the prop angle when the vibration is at its peak amplitude.


This is the brains of the Dynavibe system.


The aircraft was tied to the front bumper of Gary’s truck, as I would be running the engine at my normal cruise power setting of 2400 rpm and about 24 in HG manifold pressure. I was concerned about possibly having the prop suck some debris from the ground into the prop at high power, so Gary swept the area, then used a leaf blower to blow any grit away.


I was hoping that the first reading would show a high vibration level, as that could explain the exhaust crack issue. But, the initial reading was only 0.12 IPS, which is fairly low, which means that excessive vibration is not likely the root cause of my problem.

The Dynavibe system told Gary what prop angle was too heavy, so he put a 1/4" bolt, washer and nut in one of the holes in the flywheel. The next run showed that too much weight had been added, so the shortest possible bolt was tried. Still too much weight.


Next he tried an AN970 washer under the head of one of the screws that secured the spinner (the originally spinner screw was replaced with longer structural screw). The next run showed that this had brought the vibration down to 0.06 IPS, which is considered excellent. The washer under the spinner screw isn’t an acceptable permanent installation, so I’ll need to put an equivalent weight in a hole in the spinner backplate. I’ll acquire a suitable small scale to determine how much heavier the washer + structural screw are than the original spinner screw. I’ll drill a hole in the spinner backplate, and install a suitable combination of hardware that will be equivalent to the temporary weight. I’ll need a bit more weight than the temporary one, as the distance from the prop shaft will be less, but this’ll be a simple calculation.


I couldn’t feel any difference in the vibration levels when I flew home, as they were pretty low to begin with. But, it was still a worthwhile thing to do, as it at least rules out one cause of my exhaust problems. I’ll probably order a new exhaust system, on the assumption that perhaps my current system has some sort of quality issue.

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