Gearbox wear: Mobile 1 vs Aeroshell
 

I have an RV12 SLSA with 300+ hours on it. It has run Aeroshell Sport 4 since birth. I bought it at about 165 hours, and it has had quite a bit of vibration at low RPMs. At startup, I have to idle close to 2500 RPM to keep the vibration under control. I've rebuilt the carbs, and balanced them a few times. I've noticed quite a bit of ferrous metal on the magnetic plug and in the filter. I believe that the dog gears have excessive wear, which is very annoying since it has been run according to specs.

I also have an AutoGyro Calidus with a 914 motor. It has over 800 hours. Magnetic plug and filter are always very clean. It idles very smooth. It has used Mobile 1 4T since birth.

I'm wondering if the oil has something to do with the wear. I am curious what oil others have been using, and if they have seen metal in the filter/magnetic plug. I suspect that Mobile 1 has better gearbox additives.

Both engines are almost completely run on 91 octane non-ethanol fuel, although the RV12's fuel before I bought it did have ethanol.

What has been your experience?

Cammie Patch
Glass Cockpit Aviation
A&P/IA/LSRM/CFI

If you have noticed a lot of metal on the magnetic plug and you have vibration problems I would identify their source before this causes premature wear in other parts of the engine. If you suspect the gearbox then I would have that properly serviced by rotax center and I would ask them advice on what else to properly inspect on the engine.

You are asking a lot of people here that believe Aeroshell is the only acceptable oil for use in a Rotax 912ULS with gearbox.

Some people have gearbox failure at low hours. I am not an engineer, nor a lubrication specialist or tribologist, but something is telling me there is lubrication failure in the gearbox, that you start getting metal to metal wear, with no protective oil barrier between the two parts that are wearing.

I'd say, pull your gear box, get it rebuilt and fixed up right, and then run the Mobil 1 4T oil with MOgas only in your 912ULS in the RV-12, and change your oil and filter every 50 hours, as you would with Aeroshell.

Get back to us when another 300 hours are on the motor and gear box in the RV-12, and do a gearbox inspection. If your gearbox is still good, you are but one statistical point that shows what the difference in brands of oil makes on this motor and gearbox. You get to be a guinea pig. You also have the ability to enlighten us as to if the boundary layer lubrication properties are better with Mobil 1 T4 than AeroShell Semi Synthetic.

If you run LL100, your oil options are a lot less, not many will handle all the lead in the fuel. Rotax designed the motor to run on unleaded fuel.

The other theory is that Rotax has a bunch of gearboxes in 912 ULS motors that have soft steel parts, or inadequately hardened steel parts in the gearbox, that won't handle the increased compression going from an 80 HP UL to the 100 HP ULS design, and that is causing the failure mode.

Take your pick, as to cause of failure.

Just for background, I am an A&P/IA Rotax tech, Rotax heavy maintenance qualified, and have been working on Rotax 4 cylinder aircraft engine for about ten years. I've worked on gearboxes quite a bit. I've ordered new dog gears, as I want to have them on hand when I open it up so I have a quick turnaround.

Rotax markets the AeroShell because of the relationship between the two companies. The recent reformulation of the AS made it a better (than it was) oil, but I actually believe that the Mobile 1 has better gearbox additives.

My plan is to run the mobile 1 exclusively once I get the gearbox opened and cleaned.

But, I'm asking for feedback from those who have had the gearbox wear.... What oil were you using? Has anyone had the wear when using the mobile 1?

Hi Cammie, I have been chasing a vibration in my 12 for quite awhile. This vibration I experienced was throughout the rpm range and increased with engine speed. Starting with the usual culprits, I checked the carb synch, found them out of synch and fixed that. It did seem better of course, but they soon fell out of synch again and again. At this point I replaced the throttle cable and cured that. I then replaced the engine mounts which netted no difference. I then balanced the prop. It did feel better, but still not smooth. I worked on the balance a bit more and got the final results to .02 ips at 4900 rpm. It actually shows .00 ips in some rpm ranges, so that ruled out the prop. I will say that all along I have suspected the gearbox, but with 400 hours I felt it was a bit premature, but entirely possible. A couple weeks ago, as my wife walked away, I started the engine and she told me later that ?something didn?t sound normal ?. At this point I was pretty confident in my suspicions of the gearbox. I immediately pulled it and sent to Lockwood?s. As it turned out, the ball bearing in the front of the gear box on the prop shaft was noisy. I?ve never found any metal and have always used Aero shell. It?s smooth now.

That's interesting . I haven't seen a problem with that bearing unless there was a prop strike. When that happens, it can bend the bolts that hold the bearing in place, and you may find cracks radiating from the bolt holes.

When doing a prop balance, you can determine if it's the engine or the prop that's causing the IPS by looking at the frequency. If it's at the engine RPM, it's the engine. If it's more like the engine RPM divided by 2.43 (the gearbox ratio), then it's a prop imbalance. These light props are more forgiving of inbalance, but it will still help to balance them.

I have balanced a few Sensenich props on the 912 and found that it usually doesn’t take much weight at all to bring them way down. My prop is the exception. It took a little more than usual to tame it. I am thinking that could cause premature failure of that front bearing. I think I first balanced it around 275 hours as soon as I felt the vibration. IMO, all of these props should be balanced. There is absolutely nothing to lose. I’m taking appointments.

Btw, Joe at Lockwood’s was surprised to find this bearing in a failed state. They have replaced very few of them.

Cammie,

How confident are you that there wasn?t anything like a prop strike before you acquired the plane? That might explain things.

Rich

Very. Original prop serial number.

You might have just drawn the short straw and ended up with a lemon. My second homebuilt had a 912ULS that developed a huge oil seal leak after 5 hours. Sometimes you just get unlucky.

Perhaps after you pull your gearbox and open it up, you can take some pictures, host them somewhere on the internet and post the links to the hosting site up here, of the parts that wore out or failed? Worn dogs, Bellville washers, clutches, etc.

If you'd like a larger sample size of gearboxes and oils they are run on, you might try posting up the same on the Rotax Bulletin Board, as this is a problem that is unique to Rotax 912 ULS and iS gearboxes in all aircraft, not just RV-12s.

https://www.rotax-owner.com/en/rotax-forum/search

Good ideas.

Any follow up on this yet, to the OP?

No yet, i've been so swamped with broken airplanes...
But, I did annual a Tecnam this week with a 914, and I could hear quite a bit of gearbox chatter around 2000 rpm. He has been using aeroshell sport 4+ exclusively. I just suspect that the gearbox additives are better with the Mobile 1. Apparently from what I have heard from Rotax gurus, the additives can only be put in motorcycle oils. Can't remember the details.

I presume that if you switch to mobile 1 as it?s fully synthetic it would not suspend lead and so you couldn?t running avgas at all. I?m guessing that is part of the trade off that rotax is making, having a blended oil provide benefits of both synthetic and mineral

True about the lead. More frequent oil changes might mitigate that a little.

Not sure I'd roll the dice on that; the whole problem with the old a/c synthetic was that it didn't suspend the lead in the oil. If it's not holding the lead in suspension, then you won't be draining it out with the oil.

As a FWIW to the OP: Completely different system, but the designer/mfgr of the RWS reduction drive for rotaries switched from dyno automotive oil to synthetic automotive oil many years ago for its much better 'shear strength' when lubing the gears in the gearbox. (Most rotary guys run the cheapest auto fuel available; no concerns with octane or alcohol.)

Charlie

True about the avgas lead.
We rarely need to use avgas. Running about 60 hrs/month. I do 50 hour oil changes, and 100 hour inspections, but this is hobbs time. Flight time is actually 70% of hobbs, so technically they are about 35 hour changes, which is perfect. Currently using Aeroshell but once i replace the gearbox dog gears I'm switching to Mobile 1. I have yet to hear of any gearbox issues when running Mobile 1. Not a big sample size though, hence the inquiry.

We just put in an order on a new SLSA, really would like to have a firm idea before we start to operate it. My goal is to have four RV12s and an AutoGyro on the flight line.

Would the Mobil 1 be the type for motorcycles? This would be for the slip clutch that's for prop strikes?

You?re probably right, Charlie. You saw how tentative I was about frequent oil changes helping.

Mobil 1 Racing 4T 10w-40 . Formulated with gearbox additive.

A little update
 

I just finished another 100 hour inspection on our RV12 (SLSA). We are putting about 70 hours a month on it.

About a month ago I ordered a new gear set and dog gear for the gearbox. If you remember, I couldn't idle much below 2500 rpm cold, due to vibration and gearbox chatter. I rebuilt the carbs, balanced them many times etc. But it didn't cure the problem. Aircraft only had about 120 real hours on it when we got it.

So, yesterday I cracked it open. I have to say that Sensenich is a bit barbaric in their prop/hub/bulkhead assembly on the -12. It's at least a two person job.
But, once I got it opened, I did see a lot of where on the dogs. Way too much wear for such a low time engine (about 400 hours now).

I wasn't happy to realize that CPS hadn't sent me the dog hub yet, just the gear set half. Since it's in a flight school, I had to put in what I had, and go with that for now.

But what a difference!! Now it runs smooth all the way down to 1600 RPM, with no chatter above 1800 rpm.

I'll open it up again when I get the other side of the dog gear and put that in.

The previous owner was very meticulous on how he operated it. He took Rotax and LSRI classes, and kept an incredibly well detailed report on each annual. He used AeroShell Sport 4 oil, and almost always used 91 octane fuel.

I have switched over to Mobile 1 4T oil. I have no other smoking gun than oil to blame for the early wear. I understand that Rotax has an obligation to promote the AeroShell oil, but I just don't think it's as good for the gearbox. I run about 95% mogas in the -12.

Another reference source: I just did an annual on a Tecnam SLSA, which has also been running the Aeroshell. It ran rough as well, with excess chatter.

http://www.glasscockpitaviation.com/...crc=4291031688

Gearbox wear
 

That picture looks exactly like mine did after 450 hours. My engine has had Aeroshell Sport 4 Plus used in it since it was new.

I bought another SLSA RV12, and have seen the same wear in the gearbox, but the magnetic plug is pretty clean. I shimmed it up and switched oil. Still has some vibration, so I may crack it open again and replace the Bellville washers.

Tach time on the new to you S-LSA ? And new pics if you open it up inside. Good data base being formed.

I suspect I may have the same problem. 235 hrs TT but it was completed in 2009 and I'm the third owner. Upon a cold start the engine seems smooth but when it starts to warm up I get a growling sound and more vibration. I was thinking it was the prop, but both blades were replaced early on by Sensennich due to a S/B and I just recently replaced the hub clamp due to the most recent S/B, had the carbs inspected, float height adjusted and the carbs sync'd. Anyone have a good idea of the cost to send the gearbox in for inspection/repair?

I now have enough runtime to share my experience with Aeroshell Oil Sport Plus 4 Vs. Mobil 1 Racing 4T 10W-40.

I bought my plane from original builder four years ago with 48TT. Engine had synthetic Mobil 1 Racing 4T 10W-40 in it from new. I changed oil at 50TT using Mobil 4T. I ran Mobil 4T for two oil changes and then switched to Aeroshell Oil Sport Plus 4 based on Rotax recommendations. After three oil changes with Sport Plus I have changed back to Mobil 4T. I use 93E10 Mogas exclusively and change oil at 75 hours.

My experience with engine oils is as follows? First two oil changes with Mobil required very little replenishment oil during 75 hours run. The three subsequent oil changes with Sport Plus each required one liter replenishment during 75 hours run. Now this latest oil change back to Mobil has again shown very little oil replenishment (2-3 oz) during 75 hours run. My plan is to stay with Mobil 4T.

Mobil 4T is full synthetic and formulated for 4-cycle engines with integral gearbox where engine/transmission share common lubrication system.

On a related note... I've owned a lot of various VW turbo diesel motors, and I've run Shell Rotella T6 5w-40 in all of them and Mobil 1 Turbo Diesel Truck 5w-40 in all of them. 10,000 miles between oil changes on all of them. The Rotella T6 oil changes all required make up oil of .75 to 1 qt between oil changes, and the Mobil 1 TDT required none. This was in 5 different model VW TDI's.

My friends RV-12 had 183 hrs on it when having it's 4th oil change, usually every 40-45 hrs with Shell. It goes through 3/8 through 5/8 of a liter of oil between oil changes, summer time has the higher oil consumption levels. It's my opinion that something is evaporating or burning off out of the Shell AeroSport 10w-40 motor oil, due to heat.

Owner likes to run 100LL, thinks it will make the carb floats last longer than E-10 mogas. Mogas and replacment Bing Carb floats are cheap compared to a gearbox overhaul and needing parts from Rotax to R&R.

I am not at all surprised by your findings. The only thing I am wondering is if the Mobil 1 T40 will increase longevity of the gearbox parts on the Rotax. If it does better in that area, it's a win, win.

I will be following Cammie Patch and their posts to see how that plays out. 70 hours a month flying time is a good bench mark to measure wear. 7 months is about 500 hours of run time.

The only limitation of the Mobil 1 T40 synthetic is no 100LL should be run with this motor oil. It doesn't handle lead in suspension.

In making an oil usage choice, it is important to keep in mind that fully synthetic oil should not be used if leaded fuel (100LL) is being used very often.
Fully synthetic oil does not hold lead in suspension.

Sport Plus4 is semisynthetic. I think Mobil 4T is fully synthetic.

Any updates on wear from the fleet from Cammie patch?

This is an interesting discussion for sure but the larger issue seems to be lack of proper lubrication rather than type of oil.

Apparently the gearbox problems were pervasive enough that Rotax decided to modify the gearbox by adding an oil spray nozzle about two years ago.

So now the question is - did that solve the problem?

I guess we won't know until enough owners with the new gearbox (sn 74000 and higher) get to several hundred hours of flying time.

I thought that was a 912 iS gearbox change.... has it also been incorporated into the 912 ULS? How is the gearbox fed oil, and is there a return line now, too?

Yes - see the latest IPC revision.

Sorry not good with Rotax accronyms. What does IPC stand for?

Illustrated Parts Catalog. :)

https://www.rotax-owner.com/en/suppo...series-engines

Do you work on these gearboxes regularly?

Seems far too much to know about them with all the iterations, unless you are in the business of repair, and do this stuff regularly?

No, I just read a lot.

I wonder if an existing gear box housing can be retrofitted with the spray nozzle or if there's no existing oil gallery to feed the nozzle in the legacy gear box housing.

Yes the -12s are running pretty well. We have put about 1000 hours on them in the last 1.5 years or so. The magnetic plugs are very clean.

One of them has apparently vapor locked on the take off roll a few times, and i am attributing that to a winter blend fuel. We use a non-ethanol 91 oct fuel, and even though the supplier swears there is no difference throughout the year, we had problems in the spring, which went away when we began mixing about 10%-20% avgas in.
The other one has a periodic drop in oil pressure that remained when the probes were swapped between planes. No other indications show a problem, and I am replacing the oil regulator spring and ball bearing (to the mushroom style).

Students love these planes. I do too, but I'm tired of all the SBs!
We also have a C172 and a Piper Archer on the line, I much prefer the maintenance on the RVs.

I would eliminate the carbs in a heartbeat if I could.

Can you elaborate on replacing the oil regulator spring and ball bearing (to the mushroom style). Is mushroom style an improvement? First I'm hearing about this. Do you have P/N's?