Redrive mounting stud failure on the Miniplane Top 80
by Had Robinson
A broken stud
For some reason, Miniplane decided to use stainless steel rather than class 45H black steel studs to hold the re-drive onto the engine casing. SS hardware, though rust resistant, does not have the strength of tempered black steel – which should have been used. Consequently, the stock re-drive studs have a tendency to shear. A propeller strike, for example, can put a enough stress on the SS stud to weaken it by means of the slightest crack at the base of threads. It will not be noticeable at first but may eventually sever entirely.
A failure of this type can be catastrophic. Under certain conditions, the re-drive can become detached from the engine at high speeds, sending the propeller into the cage while permitting the engine to free-wheel at extreme speeds, destroying it. The least that usually happens is that the broken pieces wind up in the propeller, damaging it.
The pilot of this motor was lucky and only felt a slight vibration while in flight. However, the broken stud and nut made a slight graze on the propeller as it went through.
Pilots should replace these weaker studs with high strength studs, especially if they have engines with over a few hundred hours. For a modest cost, Miniplane-USA can supply the heavy duty class 45H black steel studs which will never shear/break. Whether using the OEM or the heavy duty replacements, high strength red threadlock must always be used when re-installing the studs in the engine case. Vibration of the engine is extreme and it will loosen the studs or destroy the threaded holes if this type of threadlock is not used.
As the OEM SS stud had been installed with high strength threadlock, it took a torch and screw extraction tools to remove the broken stud. It took over an hour of tedious work to finally get the stud out without damaging the engine.
A stripped thread in the engine casing
Another type of mounting stud failure is when a pilot torques down the mounting nuts excessively. This can cause the steel stud to be pulled right out of the engine casing. The stripped threads in the engine case can be repaired with a HeliCoil only on the right side. On the left side it is much easier to install a longer stud with a flanged nut or a nut with a flat washer securing it on the backside as in the photo below. With this repair the original stripped threads do not need any attention. Be certain to apply red threadlock to the threads where the nut is attached. This is a permanent fix that is better than the original.