ZDZ muffler coming undone in flight

[NeoLogic]

Hi,

Recently our ZDZ 160 Champ has had it's muffler come undone in flight for the past 4 flights, despite all the attempts I have made to secure it, short of lockwire. I would lockwire it on, but as some of you may know, it's impossible due to the fact that the fore screws are inserted into the muffler, not just over the flange.

Red lock-tite doesn't work after getting hot, I have tried RTV, slit/spring type lock washers.

Any ideas on this subject?

Thanks,
Drew

[JoeAirPort]

longer bolts, big washers over the holes so the bolt heads stick out (or a plate)...then safety wire it?

[cjcyclesrc]

I've had the best results with Permatex Ultra Grey and no gaskets. I tighten up the bolts then heat the muffler, bolts, and cylinder up with a heat gun and retighten them again. Then leave it at least overnight to dry.

[NeoLogic]

Hey Charles, do you also put the RTV on the threads of the bolts? I have been using an RTV with a high copper content and heating up the engine, and retorquing the bolts, but that doesn't work. I have a heat gun, but I found running the engine up for 10 min, the retorquing seems to get everything hotter than a heat gun.

[Zeeb]

Couple of thoughts for your consideration; are you sure you've got the right bolts? I don't remember the size right off hand but IIRC they are metric and there is an SAE size that is very close but slightly loose. I just get mine from RCS so I've not really chased that issue. The other thing and it's not exactly what you want to hear, is that from seeing a fellow modeler fight this issue with his ZDZ-80RV; once it happens it tends to "wallow out" the threads inside the head. Subsequent losses contribute to more slop in the threads. He finally wound up putting helicoils in the head and that along with some red RTV fixed the problem.

As to your question about putting RTV on the threads, I do that with my ZDZ's and have not had an issue so far with mufflers coming loose. I also do as you noted in getting the engine warm and re-torque the bolts.

[cjcyclesrc]

	Quote: Originally Posted by NeoLogic
	Hey Charles, do you also put the RTV on the threads of the bolts? I have been using an RTV with a high copper content and heating up the engine, and retorquing the bolts, but that doesn't work. I have a heat gun, but I found running the engine up for 10 min, the retorquing seems to get everything hotter than a heat gun.

Yes, I put some on the threads also. I used to use the high temp copper permatex. I have had much better results with the Ultra Grey. It's a high torque instead of high temp, but it is still good up to 500 deg. I don't run the engine until the permatex has had time to dry. If you use the heat gun you can heat it without pressurizing it. Zeeb brought up a good point about the bolts.

[NeoLogic]

Okay great. I feel that since I have had them fall out several times, it's has developed some slop. I am using metric bolts, so they are as tight as they can get considering.

Helicoils are a great idea along with the Ultra Grey. I think with the two of those ideas implemented, it should solve my problems.

Thanks

[Zeeb]

Well if you decide on the helicoils, my friend went to an SAE size as metric helicoils are not easy to find around here and rather than wait on an order he just switched standards.

[Zee]

Try NAPA autoparts store, I have been getting metric helicoil from them for years.
Zee

[dmichael]

Neo- Some questions-
- is this a new issue that didn't exist before with this engine or has this happened from new?
- is the engine hard mounted?
- is there any flexing going on with your motobox that allows the engine to move?
- are these mufflers or headers?
- do you get what you would consider excessive vibration?

Dave

[NeoLogic]

	Quote: Originally Posted by dmichael
	Neo- Some questions- - is this a new issue that didn't exist before with this engine or has this happened from new? - is the engine hard mounted? - is there any flexing going on with your motobox that allows the engine to move? - are these mufflers or headers? - do you get what you would consider excessive vibration? Dave

Hi Dave.

Some great questions that I have thought myself, however not sure how to interpret the answers, or even find solutions to.

1. This is a new issue that just started to happen after several hours of operation. I don't understand why it just started, and once it started, it won't stop, and happens very very frequent.
2/3. Engine is hard mounted to a ply motor mount box. No rubber mounts, or anything that allows the motor to move any way shape or form.
4. I suppose it's a muffler. There is a chamber to quiet the note of the engine.
5. There seems to be some rather low freq vibrations at idle, but the bolts don't seem to come loose at idle, only in flight. Anything above idle seems to be pretty smooth.

I'm headed to the store now to pick up some of that RTV mentioned by cjcycle

[NeoLogic]

I found the proper helicoils at Mcmaster Carr, and our company gets orders from then within 24 hours. Thanks for your help tho.. Great idea on changing the standard with the proper helicoils

[dmichael]

Neo- if there's one thing I have learned many times in RC it's that if the plane is talking to you, listen to it. Something seems to have changed. For example, I once noticed that my cowl bolts were backing out for some reason- never happened before and all of a sudden they were sticking out after I had landed. I didn't think that much about it at the time.

Not that much later I had a deadstick and, upon landing, discovered that one of the jugs had come loose- snapped a couple of the mounting ears on the jug. Two of the four bolts were loose and eventually the others gave out. In retrospect it was easy to see what had happened- the bolts were coming loose (it was one of the first of that particular engine and didn't have lock washers). The engine was vibrating more than before due to this condition and causing the cowl screws to back out even though they never had before.

So- look for things that may have changed. Tighten bolts on the engine and the mount. Check propeller to make sure it's snug. Have you changed props and is it out of balance? Things like that. Given that this wasn't a problem previously, something seems to have changed.

Good luck.

Dave

[NeoLogic]

Dave, thank you very much. We actually have had cowling screws come off in flight recently, which has never happened before.....same as the engine muffler bolts. The problem started the same time as the muffler bolts.

We'll go through the entire plane with a fine toothed comb. Thanks again Dave.

-Drew