Applicable to: C12ST2 (C12ST2, C12ST2/S, KCST2) Starters on A65, A75-12, C85-12, O-200, IO-240 engines

Sky-Tec's ST2 starters are sealed tightly to minimize the likelihood of engine oil penetrating the starter.  In nearly every case where oil has penetrated the starter seals, it most often the result of the engine oil breather tube being clogged or otherwise restricted thus allowing oil pressure in the accessory case to increase beyond allowable limits and permitting oil to be forced into the starter.

Take for example this customer's experience:

At the time of writing I was experiencing many oil leak problems although the engine had just been overhauled by a reputable engine company in Australia. The job included removal and re-fitting of the engine.  Since having the engine back, oil leaks have been a major problem including the starter motor as described to you.

The problem has been traced to a blocked engine breather tube. I sincerely apologize for this problem, although slightly beyond my control it is this blockage that is the cause of your starter motor filling with oil.

You may be interested to know that although three different engineering companies including the engine rebuilder have looked at my aircraft to determine the cause of the oil leak. They have all failed to diagnose the problem and have only replaced seals and gaskets as required. This work always resulted in another oil leak, usually from a different place. Eventually leak repairs went “full circle” as there are only so many places oil can leak from.

It was in frustration that I started to search for a reason myself and a review of your letter lead me to remove and inspect the engine breather tube. Undoubtedly the original fitment, this tube had a 40 year buildup of sludge adhering to its walls. I replaced it and then cut the old tube. The blockage was so complete that there was in fact no hole whatsoever. Since replacing the breather tube the engine has been oil tight.

Although I don’t really feel responsible for the oil leaks I feel responsible to your company somewhat as this replacement starter was needlessly caused by lack of engineering inspections.

- Robert Sharman
Tasmania, Australia

Recommendation:  If you determine oil to have penetrated the starter seals, return the starter to Sky-Tec for repair and prior to reinstalling the starter, ensure there are no restrictions in the engine oil breather or anywhere in the breather line.

Remember, even a momentary build-up in oil pressure beyond allowable limits may cause the starter seals to be compromised.  For example, an engine subjected to occasional negative-G's (less than perfect aerobatic maneuvers - not that we would do any such thing ourselves) can cause this problem.