Let’s Talk Gearbox…

[Piper J3]

My friend has a 5-year-old RV-12 with 912ULS @ 490TT, that he has flown since new, running 93E10 Mogas. On a recent flight we noticed a slight engine roughness just below cruise RPM of 5500. A speed reduction of just 150 RPM shows the vibration. Prop is dynamically balanced and 5500 RPM is very smooth. We called Lockwood and they said it’s a classic gearbox problem and most likely wear on the drive dogs, a symptom they call “pocketing”. Their advice is to stop flying and send gearbox in for repair.

Carbs have always been in good synch and slow engine speeds avoided to eliminate gear chatter. The gearbox feels and sounds completely normal when burping the engine. No excessive play in gear backlash or clunking sounds from the drive dogs. Slip clutch torque is within published specs. Oil has been changed every 50 hours with AeroShell Sport Plus 4 and magnetic plug is very clean.

For comparison, I have an early SN RV-12 with 900TT running 93E10 Mogas and no noticeable vibration problem. My engine is the original design and doesn’t have the new oil spray lubrication in the gearbox. I have been using Mobil 1 Racing 4T 10W-40 synthetic oil since new.

So, the question is…. Has anyone experienced a vibration at near cruise RPM that is attributable to a problem with the gearbox?

 

[rvbuilder2002]

Yes
Dean knows what he is talking about.

 

[rcarsey]

Yes, sounds like you need to send the gearbox out for servicing. The turnaround time is very very quick with Lockwood for this.. You can keep flying -just stay out of the vibration range. you aren't going to be falling out of the air or anything (Lockwood's words), but the problem will continue to get worse.

The gearbox is removable with basic tools - gentle taps with a soft blow mallet without getting it cocked. The Heavy Maintenance manual details the removal and replacement of the gearbox procedure pretty well.

 

[bbaggerman]

Pulling the gearbox requires a special slide hammer that bolts on to the side. It's not hard. Call Lockwood and they will rent you one for a nominal fee. Beats having to buy one.

 

[Piper J3]

Update…. We flew last night to confirm the problem that happened on both legs of last XC flight. We ran engine on the ground and listened from outside the airplane and didn’t detect any abnormality. We then flew the airplane in all flight regimes with landings at two outlaying airports and smooth as a baby’s butt. So, no gearbox problem. Everything hunky-dory... I can’t explain engine roughness from previous flight. Last night’s flight was still with same 93E10 Mogas that was remaining in the tank. Maybe partial clogged mid-range or main jet? We will closely monitor going forward…

 

[Roku]

Update…. We flew last night to confirm the problem that happened on both legs of last XC flight. We ran engine on the ground and listened from outside the airplane and didn’t detect any abnormality. We then flew the airplane in all flight regimes with landings at two outlaying airports and smooth as a baby’s butt. So, no gearbox problem. Everything hunky-dory... I can’t explain engine roughness from previous flight. Last night’s flight was still with same 93E10 Mogas that was remaining in the tank. Maybe partial clogged mid-range or main jet? We will closely monitor going forward…

That's great news!
How many hrs on the motor?
I'd check/clean fuel lines.
Clean carb bowls.
Run 100LL
You may have had water/other in the fuel and it cleared up. Id still clean the fuel system and clean/check carb bowls.
Also order 2 carb rebuild kits for the shelf and future winter project.

Erich LSRM-A