It could be undertorqued too. In that case there would be a lot of
fretting under the base flange of the cylinder, and probably on the
main bearing webs too.

Flyers,

Here is a post I found about a Cardinal RG with a similar problem.

This was an interesting situation FWIW. I have pictures if you want
to see them. Can they be posted here?

Kobra
snip
You can tell from the type of cylinders, in this case the angle valve
cylinders, that this is a Cardinal RG. I'll confess that I'm not certain
that you can tell this from the base of the cylinders.. I think I cropped
the picture some since I first asked that question.

Visibly wrong in the picture are the two missing hold-down nuts and
sheared stud and through-bolt. There are two more you can't see on the other
side. Also visible is a long crack running down from the center of the
cylinder base. This can be more clearly seen in the picture to the right.
So, two items visibly wrong.

What could cause this? Usually detonation causes broken pistons. This
is more about looseness in the cylinder base. How could it get loose after a
reputable shop worked on it?

The best guess at this point is that the cylinder base or case may
have been painted, or had a little bit of paint slide into the mating
surface somehow. Over time this gets beaten down or squeezed out and a
little looseness develops.


Once loose, the power pulses at 50 HP per pulse work on the hold-down
studs and bolts, eventually fatiguing them to failure. Then it's just the
case and lower bolts doing their best to keep the cylinder on.

This problem was first observed as a substantial oil leak enough to
make oil visible outside the cowl. There was no perceptible difference in
engine operation or smoothness. The pilot simply arrived at his destination
and the people he was meeting commented on the oil on the cowl.

With the cowl top off the pilot and a mechanic stared at the engine
for about 15 minutes before one of them noticed the missing nuts. In fact
one of the nuts (seen to the right) lying between the cylinders, was found
and a search for where it came from ultimately led to the discovery.


Before you judge these people harshly let me assure you that it was
not obvious. The studs sheared off so smoothly that it looked like it was
suppose to be that way. And the crack where the oil was coming from was
buried behind the air filter baffling.

What's the lesson here? When you have a cylinder off make sure it is
reinstalled properly, with no paint in bad places, and consider talking to
the shop about retourqing after a while. We're not sure what the ideal
answer is, or a single thing that resolves this, but it's a possibility that
should be kept in mind during cylinder work. An interesting point for
discussion with your mechanic.

The final result for this pilot was a series of logistics: first, a
call to a friend (who happened to be me in this case) for a ride home. Next
research on engine rebuilders and the selection of the best one. A fair
amount of research on case half options and purchase of suitable parts to
make a compete case, major overhaul, re-installation of the engine and an
uneventful flight home.

In all, 4 weeks and about $20,000. At least he got a freshly
overhauled engine for that price.