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Scoring inside Cylinders 2 & 4

I just had a pre-buy inspection done on an RV-12. I am pretty new to the aviation world and just want some feedback on this. Here are some things I'd love some opinions on:

1. scoring on upper cylinder walls of 2 and 4
2. engine driven fuel pump replacement past due
3. will need all rubber replaced soon

Avgas has been used primarily and the plane has about 400 hrs TT on it. Any feedback is appreciated!

The primary purpose of getting an airplane is to build hours. I want to get 1200 hours asap!
 

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Personally, I would pass on any Rotax engine, that has scored bores.

The bores on these engines are silica ceramic on aluminum bores. The question then becomes how did they end up getting scored and abused, and what else is possibly wrong with the motor. A possible prop strike? Was gear box, engine and clutch thoroughly torn apart and gone thru?


I would just walk away from it. Not worth the down time, money or headaches for the potential surprises.

YMMV, I don't have the time for such nonsense.
 
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I just had a pre-buy inspection done on an RV-12. I am pretty new to the aviation world and just want some feedback on this. Here are some things I'd love some opinions on:

1. scoring on upper cylinder walls of 2 and 4
2. engine driven fuel pump replacement past due
3. will need all rubber replaced soon

Avgas has been used primarily and the plane has about 400 hrs TT on it. Any feedback is appreciated!

The primary purpose of getting an airplane is to build hours. I want to get 1200 hours asap!

If your primary mission is to build hours, I would not exclude certified aircraft, specifically Cessna 150, 172, or Piper Cherokee. Not as new as Van's, but in many cases, much less expensive. Cost per hour may be a bit higher (in the case of the 172/Cherokee), but worth looking at. I know of several folks who have bought one of the above and sold them when they got the hours.

I've seen Cessna 150 sell for 25k, sometimes less.
 
A 150 is a tough aircraft in Utah summers. It’s pretty marginal and basically a single seater. 172’s are more money than a 12 unless really trashed. A 12 probably is a good choice.
 
What is the compression test results on a warm engine? Oil consumption? Back in the day before there was good enough optics this is what we went by, you can always remove the cylinders for further inspection or replace them, there are Rotax 912 ULS flying around with 5K hours I've been told, the cylinders are pretty robust, look up the allowable limits or contact Lockwood Aviation.
 
I know nothing about rotax, but I do know Nicasil (silcone impregnated aluminum alloy) if that is the type of cyl barrel used in the rotax. Either way, scuffing is not usually bad. Problem with cameras in a bore like this is that you cannot ascertain the difference between a scuff (no depth, just surface change) and a scratch/scoring (material removal in the base metal). Scuffing is not really a sign of imminent, serious problems. I would be more focused on reliable oil consumption data and compression testing to identify issues. Sure, we want things as pristine as they can be, but perfection often eludes us and you need to make compromises in your decisions, especially if on a budget. Might be wise to ask a Rotax expert how they feel about scuffing and the potential for future problems. I would call Rotax directly for an opinion.

Larry
 
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The Predator Drone used a Rotax 914 in pusher configuration. 914 is a glorified 912 with turbocharger to allow operation at altitude. My guess is the military put 1000’s of hours on those engines with acceptable reliability.

I attended a Rotax seminar at KOSH two years ago and factory rep said that most 912’s disassembled at 2000 TT show very little wear. Statement was “You need a very good set of micrometers to measure any appreciable wear”.

I’ve seen lots of photos of torn-up Lyc and Cont cylinders, but don’t recall seeing borescope images for 912 family of engines prior to this post…
 
A 150 is a tough aircraft in Utah summers. It’s pretty marginal and basically a single seater. 172’s are more money than a 12 unless really trashed. A 12 probably is a good choice.

A 12 is a LSA and a good choice if the air is calm, when you want to go fly. It's a really bumpy ride if you're flying by yourself at 1150 # total, no passenger, and Windy.com is showing much over a turquoise blue color on their Winds Aloft map at the elevation you plan of flying at. While "Green" means go in most airplanes, in a RV-12 LSA, that's getting into turbulent air. You can forget completely about it if Windy.com shows hints of yellow with the turbulence levels aloft. 10 # per square ft of wing load is pretty darn light.

It's been 6 weeks now, here, where I live, and it's pretty doggone rough trying to get from 1500 ft to 5500 or 6500 feet MSL, where the air isn't whacking your head against the canopy from time to time, from gusts of wind.
 
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The Predator Drone used a Rotax 914 in pusher configuration. 914 is a glorified 912 with turbocharger to allow operation at altitude. My guess is the military put 1000’s of hours on those engines with acceptable reliability.

The 914 is a glorified low compression 912 UL 80 HP with the 1200 cc's of displacement, that now makes 115 HP for up to 5 minutes, I think it's max 107 HP the rest of the time, continuous.

The 912 ULS is already a high compression motor at about 10.8 : 1 that makes 100 HP and is 1365 cc's, bigger bore. Rotax went and put turbo on it and EFI, and calls it the 915IS, and that's rated 141 HP for 5 minutes, 135 HP, the rest of the time.

Having owned, built and modified quite a few VW Beetle engines from the 1960's and 70's, from my youth, into early adulthood, before work got in the way, the HP being made by these low displacement Rotax engines, reliably, normally aspirated, on regular old car premium pump gas, is really an engineering marvel. Of course available materials and alloys strength and heat handling, have played a big part in the improvements, along with engineering experience gained over 30-50 years.
 
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The piston wall clearance on the 912 is 0 to .02mm. That is tight. Tighter than almost any other engine that I know of. The cylinders are coated with an extremely hard material. With such tight tolerances there is bound to be some scuffing. If it is at the top of the cylinder I would not be too concerned, AS LONG AS, the compression tests are on the high end of the acceptable range. That is really the only way to tell if the scuffing is deep or just superficial.

Or you could remove the cylinder and take a look; which I would not recommend. To quote a great aircraft professional Mike Busch "Cylinder removal is a risky business".
 
The 9-series Rotax uses heavy cast cylinders and heavy (thick) cooling fins as opposed to Lyc or Cont. Piston diameter is small compared to conventional aircraft engines. As a consequence, the cylinders stay round when running at temperature. Water-cooled heads control temps and help maintain close geometry.

Lyc & Cont. use a choked-bore cylinder that becomes a true cylinder when heated. 300F+ head temps expand the top of the tapered bore. Suffice to say… one quart of oil consumption in 2 hours is within specification and acceptable.
 
Pages 5-7 of interest... what fails, and where it first fails... galling, aluminum showing up first at the bottom of the bore. Usual cause is overheating of the motor oil above 249F, and failure of the Zinc plating aspects in ZDDP motor oil additive.

Students, this summer, perhaps not paying attention to oil temperatures while climbing out, another problem being the oil cooler location is too close to the muffler location, restricting air flow. Van's has addressed this with a newer exhaust system and muffler design, about $2000, relocating the muffler further aft of the oil cooler mounted in the lower front cowling.

As I said before, I wouldn't touch that plane, or the motor in it.

http://lnengineering.com/files/2019...ng-Bore-Scoring-in-AL-Si-Cylinder-Systems.pdf
 
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I would not be afraid of that engine. Some scuffing is nearly always apparent. We ran a 912 ULS 1000 hours (to a total of 1300) when we sold the plane. It never ever used oil. (You only check oil after it has been burped....)

Differential checks are only reliable if you use 87 psi (what Rotax recommends).
If you use 80psi it will always read 80 over 80...

Use 4T synthetic motorcycle oil and avoid using 100LL as much as possible. The engine and gear box run best on premium auto fuel. (The gear box and engine share the same oil. Lead is especially hard on the gear box and clutches).
 
If you think bore scuffing in an aluminum silica bore is a minor issue, do a search on Google under aluminum silica bore scuffing and Porsche. Similar technologies, large bore short stroke motors at higher rpms and catastrophic failures, that often sound like " a valve ticking" and soot and oil deposits in one side of the engine bank.

That both rear cylinders display this wear pattern, pull the exhaust systems on the rear and look inside at the coating of exhaust inside those headers. Sooty or oily?

To me, scuffing deposits are a sign of not if, but when there will be imminent bore failure. Scuffing is never normal bore wear, it signifies triboligy oil film lubrication failures between bores and pistons, due to excessive load, excessive heat, over revving the engine, creating huge side load on the piston at the top and bottom of the bore stroke, or poor maintenance and timeliness of oil and filter service changes, perhaps more common in rental and flight school fleet airplanes.

It's very easy to over rev a 912ULS in a RV-12 if you start descending by moving the stick forward, before you pull the throttle back. Pull the throttle back first, then let the plane descend. Doing power on stall work is another potential problem for over revving, when you release the back pressure on the stick, or you push the nose forward, then down.
 
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That both rear cylinders display this wear pattern, pull the exhaust systems on the rear and look inside at the coating of exhaust inside those headers. Sooty or oily?

It is not both rear cylinders.
It is both left side cylinders, which share an intake manifold and carb.

That in itself points to possible causes that should make this issue not be considered no big deal (though it might be).
 
It is not both rear cylinders.
It is both left side cylinders, which share an intake manifold and carb.

That in itself points to possible causes that should make this issue not be considered no big deal (though it might be).

That would make me consider leaky intake manifold problems, or carburetor problems, or failed or leaky air filter problems, and dirt getting into the motor.

I have seen K&N air filters that have rubbed a hole through the air filter element where it either vibrated and contacted a portion of hose or engine mount, on other ultra lites. Clearly in need of replacement, whole new air filter.
 
The 912 is not a large-bore engine and it is oversquare at 3.3” bore x 2.4” stroke. Compared to Continental 0200, at similar horsepower, with 4.0” bore x 4.4 stroke. Larger (heavier) pistons with longer stroke need to be operated at slower speeds. The 912 operating speed is not particularly fast at 5500 RPM. Rotax allows 5800 RPM overspeed for 5 minutes duration.
 
The 912 is not a large-bore engine and it is oversquare at 3.3” bore x 2.4” stroke. Compared to Continental 0200, at similar horsepower, with 4.0” bore x 4.4 stroke. Larger (heavier) pistons with longer stroke need to be operated at slower speeds. The 912 operating speed is not particularly fast at 5500 RPM. Rotax allows 5800 RPM overspeed for 5 minutes duration.

It's a short stroke overbore engine, very desirable for the engine rpm range it operates at. It also has some pretty stout angles of load force from the crank at BDC and TDC on the load bearing portion of rotation side of the piston, transmitted from skirt to the cylinder wall.

As I see it, cylinder wear scuffing is not normal wear and tear in a Rotax motor. Feel free to contact someone like Hal Stockman who offers the Zipper Big Bore kits that make low compression 104 HP on regular gas or high compression 114 HP on premium mogas or 100LL what his take is on bore scuffing in a Rotax stock engine. He rarely has seen it, I believe.
 
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