OS 91HZ ring rotates

[Sirolf]

I bought an Align 91H last year for my 600 build. Rebuild the engine with new bearings,
piston and ring.
Flew great for 8 months till last june it flamed out in the air and had almost no compression so obviously it wouldn't start again explaining the flame out.

Opened it up and found that the ring rotated because the locking pin in the piston ring
groove was missing. Found some little remains down in the crankcase.


Ordered a new piston, ring and sleeve this time. Problem solved i guess.
So last thursday i had a nasty crash due to a set of front torque tube gears give up
when it was i a knife edge position. Switched throttle hold a tried to save as best i could. First 600 crash since i bought it new 3 years ago.

Long story short, muffler broke loose and as i looked into the engines exhaustport
i could see the piston ring rotated yet again... pin is missing just like the first one..!!!!


Luckily no sleeve damage... but what the h..ll?

What could be causing this..? It never runned lean and both time i took extra care on
a good break-in.

Thanks guys

[marc8090]

I had it happen to me once in 18 years. Can't explain why it happened but the only thing I could think of is it just wore out. Happened to me on a 4 year old engine with dozens of gallons on it. As far as it happening twice in a row to you is it possible you are damaging the pin or ring during installation? It is very easy to not get the ends of the ring aligned with the pin when you push the sleeve in.

[Sirolf]

Before this one i've changed bearings in about ten other engines like the OS 50 and 55HZ.
Always took extra care when refitting the cilinder and pistin with ring accordingly.
Never had any problem. So maybe i'm just having bad luck with em.

Any possibility to fit a alternative locking pin in there?

[marc8090]

I tend to think it's maybe a bad batch of pistons since the only time it happened to me was also on a 91HZ. I would just put another piston in it and hope for the best.

[Sirolf]

Let's hope so.. first one was OS brand, second one Align. Gonna get a completly different 91HZ

Sent from my GT-I9070 using Tapatalk 4 Beta

[Sirolf]

Well.....

same thing happened again with a brand new engine!
Bought a 91HZ-R last august from someone who never used it.

I never had it run lean or what, i never had this issue with either a 50 or 55H.

So what the h...

[mat081278]

Same think here around me.
I checked my OS105, no pin
My recently bought second hand 0S91HZR same thing, no pin.
A friend, bought a 91HZ-R 2 weeks ago, same think no pin.
A friend at my field, do not like the power of his bird. I asked him to check his piston.
And guess What ? No pin.

What's happ !!!!

[Sirolf]

Your kidding, right?
Check to see if the pin ia really missing or buried deeper inside the piston. Take a magnet, lay the piston on it's side on on a flat surface with the location of the pin facing up.
See if you can move the piston with the magnet without touching it...

I think that or it got buried deeper inside the piston groove or the pin just broke in half..

Right now i'm finishing my 3rd piston and ring break in and going easy on it. But is suprising to see the 105 has this problem also..

Any of the engines had a lean run in the past?

[mat081278]

I will have a look to see If the ring is inside. I suppose that yes, elsewhere, my cylinder will have some marks, but cylinder is clean.

Yes, the 105 has a lean run due to pressure unplugged.
The others I do not know because it is second hand engines.

But, I run the 105 like this over more than 20 flights without any problem.
The only problem was a little bit of compression lost, but with 30% Nitro it was very powerful.
I also noticed that I needed to put the idle screw very rich, because my idle seemed very lean,
maybe due to this, I do not know.

Just to be relaxed, I have decided to change the piston and the ring. On my two engines.

[Sirolf]

Did you buy the 105 new?
You could try getting a new piston and ring under warranty.

I luckily did the last time.

Check the bottom side of the conrod. Any scratch markings?

[mat081278]

No, OS 105 was also a second hand.
But I changed bearing, and there was the pin.
No scratch anywehre, pistons are ok and cylinder too.

On a French forum, a guy told me that possible reason, was a start with to much fuel inside the motor, with a powerful starter. Because the fuel in not compressible, ring is push inside the piston.

And yes I did some kinds of start.
What about you ?

So I will change piston and ring, and try to do soft start next times.

[Sirolf]

Could be... to much fuel could make it's way through the holes on the top of the piston towards the ring.

We'll see how it holds up now. Contacted OS about this once again but they don't
really seem to wanna respond..

[mat081278]

After less than 10 flights, my new 91 Piston pin has disappear.
I wrote to OS US and told me to send them the piston. I will do.
Order one time more a new one.
Same problem on OS GT15 and OS has solved it with a new piston, with pin coming from top to bottom.

[Sirolf]

I also wrote OS about this...several times amd still no reply.
Easier to just buy new stuff and hope for the best..

[Sirolf]

Well.... happened again last saturday.

[marc8090]

Something is wrong to keep causing this.

[Sirolf]

I hope the fix of a new stainless steel pin will fix the problem.
This has been by an experienced guy who told me he drill the hole out a bit further and
inserted a longer pin which i think now can only be removed by destroying the
piston itself..

[mat081278]

I am very interested by this fix. Where do you find the longer pin ?
How do you pressed it in ?

[Sirolf]

Don't know the man, but it's a friend of a flying buddy of mine who apperantly who
is very handy and experienced and a real problem solver!

As soon as the weather clears up out to do some serious testing..

If it holds up great!! if not i have 2 more pistons which can be modded a different way.

[thefrog]

Are you installing the piston facing the right direction? Are you making sure the ring gap is over the pin? Are you installing the sleeve without twisting it as you slide it in?

It seems like you're not doing one of these things and the ring is possibly pushing the pin into the piston and it's falling down the inside.