Stripped head at exhaust stud

by Dennis Urban in 912 / 914 Technical Questions 9 years ago

Does anyone know where I can get an aftermarket Rotax 912 exhaust stud that is oversized on the side that goes into the head? When I used to rebuild VW air cooled engines, we would often find stripped heads where the exhaust stud is inserted. They used to make a repair stud that is a larger diameter at the head side, same size at the exhaust pipe side. I do not remember if it was one size larger standard thread (e.g., M10 instead of M8) or a special, lightly larger thread using a special tap. Perhaps I should simply use use a Time-Sert or helicoil, and wonder if this is what others are doing if they have this problem. Perhaps there is a Rotax approved part or procedure? If I use an insert, so I still use Loctite 648 (and on which threads)?

Thanks,
Dennis

Re: Stripped head at exhaust stud

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

The overhaul manual allows a helicoil repair of the stud hole (or at least it is not on the forbidden list)
That a good question about the 648, I have not had to repair a exhaust stud hole so I cant speak from experience but I would say a tiny, tiny amount of 648 on the helicoil and let it sit for 12hrs before inserting the stud. Maybe Conrad has some advice?

Thank you said by: Dennis Urban

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Re: Stripped head at exhaust stud

by Dennis Urban » 9 years ago

Dennis Urban

17 30 48

Does anyone know the thread pitch on the exhaust stud (M8x1.00, 1.25, or 1.50 – the parts explosion doesn’t say – and it might be different inside the head vs outside)? I am not at the airport and don’t have any studs off the engine. I am working on an order of studs, nuts and other repair parts ASAP because I like to fly.

A Time-sert might be better than a helicoil - I just need to know the thread pitch to order either. Unless someone knows, I must head way back to the airport (hour trip) with a thread gauge.

The loose stud departed, presumably inflight – good thing my prop is up front. I am a firm believer in torque wrenches, but I bought the plane this way, found the bad/stripped hole right away, but in (stupid) denial I just kept flying hoping it would hold. Fortunately nothing bad happened other than loss of the stud. Nor did I read anything in the papers about anyone getting hit in the head by a stud falling from the sky.

And it is my opinion that in most cases it is best to have a steel insert whenever a steel bolt is screwed into an aluminum part. Ok, lots of 912s are flying without problems, I get it.

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Re: Stripped head at exhaust stud

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

M8x1.25. inside and outside.
A timesert may be too thick, remember you have a water jacket in that area.
No need to have every engine come with inserts. I have never heard of anyone pulling the threads on the exhaust stud before and wonder how it happened, it is such a low torque on the nut.

Thank you said by: Dennis Urban

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Stripped head at exhaust stud

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