Rtel

I'm having trouble keeping the muffler on my new ZDZ-80 tight. I have lock washers under the screws but I think the heat takes the "set" out of them and they end up not doing the job.

I wonder how some of you guys are securing your mufflers so they don't come loose?

Also the threads on one of the screws appears to be pretty buggered up. Not sure how that could have happened. The screw wasn't cross threaded. Looks like the screw took the worst of it and not the case. I believe this is a 4mm screw. I plan on buying a 4mm tap and trying to clean up the threads in the case.

Thanks,

Rick

Mike Dooley/RCS

Muffler bolt size on all ZDZ's is 5mm. If you are using a muffler with the bolt heads recessed, do not use any washers or lockwashers. Clean the bolt threads in the case and the bolts themselves, apply a bit of Ultra Copper Sealant and reassemble. Wait overnight to run. Also be sure you are not using a soft mount and your propeller is balanced properly. If you are using a muffler with exposed bolt heads, drill them and use safety wire.
Thanks,

Rtel

Thanks Mike. After closer examination it appears that the bolt had aluminum material packed into some of the threads. It looks like it "galled". In any event I have a 5mm plug tap coming and will chase the threads before I install new 5mm bolts. I'll load up the threads with the "copper" colored sealant and try it again.

I'm pretty sure there is enough thread left to work but in case there isn't I guess the alternative would be to drill and tap to the next size up, even an english size if it's between 5mm and 6mm. Hopefully there is enough meat around the original hole to do that if neccessary.

Rick

Big_Bird

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Rtel

Ken, thanks for your input. I should have said that I was running the Bisson Pitts style muffler so I didn't have the crush problem. I have experienced that on a muffler on a Brison 4.2 though. By the time I found it the muffler was trashed after it had loosened due the crush effect you described.

I have now chased the threads with a 5mm tap and used longer 5mm bolts. There was some loss of threads on one side put the longer bolt will hopefully pick up enough additional threads to fix that. I made sure that they aren't bottoming out and have coated them with the copper color high temp sealant to help keep them tight. The engine is all back together and I'll give it a try tomorrow. It looks like glorious weather here in Florida for an all day flying session. Maybe I can get some more gas through this great engine and continue the break in process.

I'm not sure that a baffle is needed in this instance. The head just sticks out the bottom of the cowl and there are other inlets to the cowl itself and a generous exit. Part of the muffler is directly exposed to the air outside the bottom of the cowl and I haven't noticed any sign of heat at all after each flight. If everything was completely enclosed then I would certainly go the baffling route.

I hope the fix to date will work out. I should know soon. Thanks again for your insight.

Rick