OS 91 broken ring!

[Hendo67]

Here is one for you. I tried to start my OS 91 after filling the 3rd tank of the day ( about 6 weeks ago...I've been busy). It made a "snapping noise, and the pitch of the engine changed while cranking. I tried a couple more times but it did not sound right, so I de-fueled and gave up for the day. I tried to start it again on Sunday, and it would not even draw fuel from the tank (it is pumped). Things that make you go hmmmm. I pulled the motor, and turned it over by hand, and it felt like it was making compression. It was real tight at the top of the stroke, but there was not the usually compression hiss. The ring looked good from the exhaust port, but I decided to pull it apart and check it out. As it turns out, it was missing about 1/4" of ring on the other side of the piston, and little pieces of it were on top, compressing against the head. The ring land has very little damage, and there is not a mark in the cylinder. The pieces of ring that were stuck to the head were easy to remove, and did not leave too much in the way of marks either. The piston and cylinder are not scored at all and the rest of the ring looked like normal wear.
Has anyone had this happen to them before (during a start)?
Since the cylinder still looks good, do you think I can get away with a new piston and ring? Should I hone the cylinder with a brake hone to give a fresh finish for a new ring to seat into?
This thing only had maybe about a gallon of fuel through it before this happened.

[BrokenRotor]

I would hone the cylinder for the new ring break in. Your luck with engine does not sound that good from what I have read. I would not take the chance.

You must have got a Monday or Friday engine. I have not heard of a ring breaking like that. Strange bad luck. That stinks.

Maybe it sat too long. I think you should do more flying and less being so busy.:) It's better for you anyways.:D

[mflander]

I've heard of it Ron, but it is not common. I would hone it also, although i wonder the cost difference between just piston and piston/sleeve? It might not be much....

[MikeB]

If it's at all scored then hone or replace. If it did not mark the sleeve then I doubt the honing is needed. Since it never ran with the broken ring you may be ok in that respect. Honing it without the right tools may actually put a worse finish on it than what you have now.

Like the guys said better safe than sorry.

Edit: If the pistons damaged even a little you might want to just replace it. Damage to the top of the piston head can cause weak/hot spots not to mention the stress to it and the rod caused by compressing the fragments. Maybe new sleve, rod, piston and ring >.<

[Hendo67]

I did not consider the rod being tweaked. It never really compressed that hard against the head though. If it would hit hard enough to bend the rod, I would think that it would stall the starter or spin the hex off the shaft. I might have a couple of hot spots on the head as there are a couple of "divets" in it. If I need a new head and rod, I might as well get a whole new engine. I wonder if the sell an engine without the carb and pump?

[Hendo67]

Well it's official.....I'm an idiot! All the engine troubles I have had has all been my fault. At least the ones that I have rebuilt. I have always had trouble getting the rings in the cylinder during a rebuild and I just found out why. There is a small (very small) indexing pin in the ring land on the pistons. The gap of the ring is supposed to fit over this pin so that the ring will be indexed properly in the liner and not go over a port during operation. I assume I had a cracked ring the whole time, and it finally worked itself into a position to "fall out" through a port. What a dumb A$$. Doug I will need a new ring for an OS 50 as I am sure the last one I put together is going to go at any time. The thumping sound you are now hearing is me beating my head against the wall. :mad: :confused: :eek: :doh: :arrrggghhh:

[Pete]

Don't feel bad, I do stuff like that all the time. At least you've solved your motor problem.

[BrokenRotor]

If you don't make mistakes sometimes, then you are either not trying to learn, or you are just sitting watching TV.

Great job on figuring it out. You probably will have saved others from doing the same thing now.:D