New Gearbox Failure on 912IS - Causes?

I would first confirm that it is the Gearbox and not something else.
Gearbox vibration is a symptom of improper backlash (Freeplay) in the gears.
Backlash should be minimal but NEVER Zero (Binding).
Rotate the prop slowly by hand and feel for any damaged gear teeth.
At least 1 rev of the prop will be needed to engage all gear teeth at least once, but 2 revs or move revs would be better.
At all prop positions, the backlash of the gears as measured at the propeller tip should be about 1/16 Inch.
1/8" or more of backlash is considered a cause for concern. Zero (Binding) is even worse.

You might also want to test this with a spark plug removed from each cylinder for a lighter feel.

Also, check your Magnetic plug for metal chips.
It is there to collect any gear bits that may go astray.
Magnetic "Hair" on the plug that feels like grease (no grit) between your fingers is normal.

- - -
This is something for everyone to consider while "Burping" the engine to check the Oil level.

Bill Hertzel wrote:

I would first confirm that it is the Gearbox and not something else.
Gearbox vibration is a symptom of improper backlash (Freeplay) in the gears.
Backlash should be minimal but NEVER Zero (Binding).
Rotate the prop slowly by hand and feel for any damaged gear teeth.
At least 1 rev of the prop will be needed to engage all gear teeth at least once, but 2 revs or move revs would be better.
At all prop positions, the backlash of the gears as measured at the propeller tip should be about 1/16 Inch.
1/8" or more of backlash is considered a cause for concern. Zero (Binding) is even worse.

You might also want to test this with a spark plug removed from each cylinder for a lighter feel.

Also, check your Magnetic plug for metal chips.
It is there to collect any gear bits that may go astray.
Magnetic "Hair" on the plug that feels like grease (no grit) between your fingers is normal.

- - -
This is something for everyone to consider while "Burping" the engine to check the Oil level.

The vibration occurs at altitude, after the aircraft has been trimmed out in cruise. It's not instant, but once I settle into cruise it comes on and is definitely noticeable. If I start to pull back on the stick and initiate a climb, it goes away very suddenly. Would anything other than the gearbox be causing an issue like that?

I can fly the traffic pattern down here at 1200' and not feel anything. But after settling in for a X-Country, it definitely happens.

Sounds more like airframe resonance.

What propellor type/diameter are you using?  CSU?

Sean Griffin wrote:

Sounds more like airframe resonance.

Wouldn't I have been experiencing that before though? This problem just started occurring within the last couple flights. Also, wouldn't I be noticing something outside of just cruise flight?