Rubber Parts of the Rotax 912/914 Engine

The Rotax 912 engine and the related 912S and 914 engines have many parts made of rubber or closely related material. To guard against failure of these parts, Rotax recommends replacement based on hours in service and elapsed calendar time.

If that replacement policy is to work for all possible installations, in particular in very hot Africa and very cold Alaska, then the replacement intervals must be quite small.

As the US Air Force discovered many decades ago, such frequent replacement triggers so many new problems that the cure becomes worse than the effects of the aging process. Since that discovery, the Air Force has used on-condition replacement, where parts are replaced as needed and not due to some time-in-service or calendar-time rule.

The conclusions drawn by the Air Force apply to the Rotax engines as well. My friend Philip tells horror stories about the difficulties created when wholesale replacement of all rubber parts is done every few years on his Rotax 912S, as mandated for his LSA plane.

Since our plane is in the Experimental Category, we have the option to use replacement on-condition. Before going into some details, here are the statistics. Over 17 years, the engine has run for close to 1,500 hrs. Originally, TBO was projected by Rotax to be 1,200 hrs. Oil and filter changes are done every 25 hrs. During those 25 hrs, the engine burns about 1/3 quart of oil.

There has never been an in-flight engine problem, except when the engine with less than 10 hrs total time stalled on take-off. The culprit turned out to be debris left in the carburetors from the manufacturing process.

At this point, the engine shows no sign of aging, as would be evident from compression changes, metal in filter, and so on. The only problems have come from the mechanical fuel pump, which has been replaced several times. Evidently, that design problem has been solved now.

All rubber parts still look and feel good, whether it be the diaphragms inside the carburetors, the carburetor floats and needles, the rubber blocks connecting the carburetor with the intakes, and so on.

But two problems have pointed to aging of some parts and thus have triggered replacement action.

The first problem was a significant harmonic vibration during low rpm, say between 2,200 and 3,800 rpm. We could find absolutely nothing on or around the engine that could explain this vibration. Our friend Jack, an experienced airplane mechanic, suggested that maybe the engine mounts were the culprit, since some of them appeared to be quite hard.

We replaced all engine rubber mounts, an involved process. The replacement also required removal of the lower water hoses going from the water pump to the cylinders. These hoses had to be cut off, but otherwise turned out to be still in excellent condition. Of course, they had to be replaced, too.

For the replacement, we selected Goodyear Hi Miler blue heater hose with 5/8" ID. This is a high quality hose that is far cheaper than the Rotax hose. The hose has a slightly smaller ID than the Rotax hoses, which have 18mm ID. The slight reduction does not pose an unwanted flow restriction since the four 5/8" ID hoses going from the water pump to the cylinders have a combined cross section that is 60% larger than the 1" ID hose going from the radiator to the water pump.
 
Despite the slightly smaller ID, the hose is easily installed using the following steps. First, clean the aluminum tubes with Scotch-Brite or 400+ grit sandpaper. Second, warm up the ends of the hose with a heat gun on low setting, until something like 140 deg F is reached. Use an infrared thermometer gun to test the temperature so that you do not overheat the hose. Third, dab a small amount of Ivory dishwashing liquid inside the hose ends and onto the connecting tubes, then slip the hose ends onto the tubes.

An aside: In 2019, which is seven years after installation of the Goodyear hoses, we had to remove the engine after 1,950 hrs of service to repair a crack of the engine mount. To get to the bolts of the engine mount, we had to remove two of the Goodyear hoses. Cutting them open, the hoses turned out to be in perfect condition, proving that the Goodyear Hi Miler hoses are indeed of terrific quality.

Goodyear Hi Miler hose replacing Rotax hose

Removal of the old hoses must be done very carefully so that the thin-wall aluminum tubes at the water pump and cylinders are not damaged. Use a sharp knife and cut the hose at a shallow angle until it is easily twisted off the tube. 

If any tube is damaged and must be replaced, it is a wise decision to first view the video for Coolant Elbow Servicing at rotax-owner.com for important instructions. We only can add that cleaning of the thread in the cylinder head or water pump housing is easily accomplished with a toothbrush and Toluene. Make sure you do this outdoors so that you do no inhale Toluene vapors.

When we started the engine after the lengthy repair, it ran perfectly. There was no harmonic vibration for the entire rpm range. In fact, the engine ran as smoothly as on day one 17 years ago.

The second problem was a spark plug connector that came off a top spark plug. This was detected during a run-up ignition check. To prevent this problem, Rotax demands a pull test of the connectors during each annual inspection. The connectors of our engine have always passed that test easily. In fact, during each annual inspection, we have a heck of a time getting the connectors off the spark plugs while pulling as hard as possible.

So how could that connector come off? The reason is the aging of the rubber of the connector. Let's see how this can trigger the problem. A spring clip inside the connector grips the threaded end of  the spark plug and thus prevents the connector from coming off. The pull test confirms that the clip tightly grips the spark plug. But when the rubber of the connector hardens, then engine vibration can shake the connector, weaken the clip within a few hours of service, and the plug may come off, as happened with me when the engine was shut off after a cross-country trip. We suspect that the earlier mentioned harmonic vibration contributed to the problem.

Lessons learned:

(1) When a new vibration occurs, track it down until the source is found and eliminated. This may be a complicated and frustrating process. Consider hardened engine mounts to be a possible cause of the problem.

(2) For the spark plug connectors, do not only the pull test, but also check if the connector material has hardened. If that has occurred, replace the connector, regardless of whether the pull test is passed. In fact, at that time, replace all connectors. For the replacement steps, you may want to read the post Rough Running Rotax 912/914 Engine.

The new connectors have a shiny black surface.

New NKG ignition plug connector

Within a few years, the shiny black surface turns into a dull grey. The change does not mean than the connector material has become hardened. To decide that question, the connector must be pulled from the plug. It is then easily checked whether the material that slips over the plug is still flexible or has hardened.

(3) During cross country flights with refueling stops, we used to do the customary ignition check at 3,400 rpm as part of the runup for the first takeoff. But for subsequent takeoffs after refueling stops, we would skip that ignition check. The reason was that such a check puts undesirable thermal stress on an already hot engine, particularly in the Texas summer. We now do an ignition check for subsequent takeoffs, but at 2,000 rpm instead of 3,400 rpm. If a plug connector has come off during the preceding leg, that check will produce a badly shaking engine for one of the two ignition systems.

During the next annual inspection, we will once more test all rubber hoses and parts, inspect the diaphrams of the carburetors, and so on. If we detect anything else, we will add that information to this post.