"Fretting" on a Sky Tec starter

[g zero]

Just spoke with SkyTec, they received my starter , they scrapped it .
Said the starter was defective and have sent me a new one .
If you have an issue with your starter ,might consider sending it in or send some photos to them as it looks like they acknowledge a problem

[scsmith]

As a result of this thread, I went right out to the airport, pulled the cowl, and inspected my NL style SkyTec starter. Its fine. Screws are tight, case is tight.

My starter is 6 years old, with 350 hrs.

Looking at the screws that hold the case together, they are threaded into the mating case, with quite a deep thread engagement. Enough so that even though normal custom would be to use a coarse thread, there is so much thread engagement that I can not imagine that thread failing. The screws look like 8-32, but I didn't measure. That is a moderately coarse thread. So it looks like the design is sound to me -- I wonder if they have changed the design on newer versions, or if the problem is in torque retention.

[g zero]

recieved this from SkyTec , I asked what was different about my new starter ,

We changed the type of thread lock compound used on the bolts that hold the starter together and also our torque and locking tab procedures.
You may periodically spray LPS on the pinion gear to prevent corrosion, but we don?t recommend any other maintenance as the starter is sealed.

[g zero]

I'm now sending in the starter for warranty again , same issue .

[BillL]

Typically, in highly loaded fasteners a tab makes the joint force retention worse. The required material to allow bending the tab, without failure, renders it soft, then it yields in loading. This is worse for shorter fasteners. It also reduces the applied torque to the fastener relative to the installation torque as it smears under shear load. Surely ST should know all of this, but maybe not. I learned it from 30 yrs of knowledge X 4000 engineers of history. Top fastener companies know this stuff very very well.

Factoid: Large diesel engines use hydraulic cylinders to stretch the head studs (all at once), then take a 1/4 dowel rod and turn the nut/sleeve down snug before releasing the stretch force.

[CharlieWaffles]

Just a data point, I was performing my annual and noticed I had some small fretting/grease on the bottom of the starter, so I bent back the tabs and tightened the case bolts - well, I should say I tried to tighten. Once took a couple of partial swings of the socket wrench. The other, well, apparently took too many.

These bolts are pretty small #8 bolts and apparently they don't have much sheer strength as I wound up snapping the bold head off. So I had to remove my starter and am working to get ahold of SkyTec to return it for repair.

So diligence with inspecting is key as is keeping the housing snug. But be VERY careful of any torque you are putting on these bolts.

[BillL]

What grade fastener is that? Your finding might be a clue to the issue, the bolt is yielding. I have had this happen, loose bolt, thought it was for another reason, retorqued, then happened again. Third time during retorque the bolt failed at the top thread. Replaced the bolt, never occurred again. It happens.

Not claiming this is the root cause, but is among the possibilities.

[CharlieWaffles]

I was able to get my starter sent in and even though it was first installed over 3 years ago, they replaced it under warranty at no cost. I noticed they have made a few changes - see photos below. The bent tabs appear to have been replaced by internal teeth style lock washers. In addition, there appears to be a rubber gasket between the housing sections - where the fretting has been reported.