longdan

These are pics of con rod and piston out of my Super Tigre .45. I was flying it this morning in a new WM Super Stunts 40. I was just getting ready for a flight, with the engine idleing, when it suddenly stopped with what sounded like a backfire.. I put the starter on thinking that I'd need to tweak the needles a bit. Straight away, I could feel that something was not right. Turning the prop by hand, it only had about 180 degrees of movement before coming up against something inside. I knew this was the end of my flying day (I only took 1 plane with me).
I pulled it apart when I got home and this is what I found. As you can see, the gudgeon pin has been sheared into 3 pieces. There is also a piece missing from the piston skirt. This is what was jammed up in the crankcase. And there is metal debris all through the case and head and grey sludge everywhere. The liner is fine without a scratch on it.
Looking closely at the broken ends of the gudgeon pin, it looks like they have rubbed against each other a bit.

The landing just before, I had hit a rabbit hole and the plane nosed over, causing the prop to hit, and stop the engine from idle. This may have been a contributing factor. But I restarted it after fueling and away it went. I ran it up to full revs for a few seconds, then back to idle. This is when it stopped.

proptop

My Feedback: (8)

Just a WAG...but mine is that the gudgeon/wrist pin broke or fractured when you had the prop strike...probably only on one side, and that allowed the engine to still run, albeit briefly, until it broke (again ) on the other side of the rod...

I've never seen a wrist/gudgeon pin break like that but it appears that it is almost too hard, and therefore brittle?

One would think that the pin bosses of the piston or the rod, or crank-pin would have broken first...or at least that's what I have seen before, after prop strikes.

Then again maybe the piston skirt broke and locked things up and that's what broke the wrist pin...

Who's next?[)]

downunder

How old is your engine? Not how much running so much as when was it made? I got an ST G51 back in '95 and when I put a prop on it and gave it a few flicks (as we all do ) it felt strange so I pulled it to pieces. The wrist pin was broken in halves. I sent it back (yes, they still honoured the warranty) and it turned out that when ST sent off their wrist pins for case hardening they were left too long so were hardened (brittle as glass) all the way through.

It's hard to judge from a photo but it seems yours has snapped without showing any bending from what's supposed to be a softer interior

longdan

I'm not sure how old it is. I've had it for a couple of years and I bought it second hand. It's made in italy and has the older style round head. I've done plenty of hard running with it. I had thought it was going to be one of those engines that just went and went.
I too have a feeling it was the prop strike that started it. Although it was only idling and the ground was pretty wet and soft. The prop was fine. I've had engines hit hard packed ground at full revs with no damage except for a carb full of dirt or bent NV.

pe reivers

I think, part of the piston skirt broke off first, probably during the start procedure after the prop hit. (coincidence??)
the wrist pin seems to have failed all at once due to engine blockage, hard to see well from the picture. The break mode seems to indicate too hard a pin, like Downunder noticed. The break angle seems to suggest piston locking in top dead centre, maybe due to a skirt part entering the exhaust port. Need to examine the engine closer for full analysis.
This damage is regardless of rabbit hole strike. A prop hit at idle will never cause internal engine harm like a broken gudgeon (wrist) pin or piston skirt damage. Over revving maybe?

Use of the starter caused the later damage like wrist pin failure, head indentations and more piston, damage.

rainedave

My Feedback: (1)

In your first photo there is damage to the head/combustion chamber surface so bits of metal were traveling around while it was still running. I had the same impression as pe, that a large piece of the skirt jammed the piston from moving. I think the snap happened on the upstroke because of the angle of the wristpin break and the damage to the conrod. I think pe is right that a piece of the skirt got sucked up into an intake port, locking the piston in place, and the up-force of the conrod punched through the wristpin. What caused the bottom of the piston skirt to break off I have no idea.

David

Campgems

Take a close look at the fracture surface on the wrist pin. If it is all grainy, it broke because something stressed it beyond it limits, IE piston skirt. However if there is shinny spots on one edge of the fractur and grainy appearance oth the rest, it is an indication of stress failuer. It had a spot that started with ffine crack and progressivly got deeper untill it finaly failed. THe shiny areas is where it was flexing and the sides were rubbing togethere. THe movement was in microns, at first.

If you see the stress fracture indications, it is more likely that the pin failure started the break down where the full grainy fracture would indicate it was the result of the breakdown.

Don