How to prevent cracked muffler on 912uls

[3kdscf]

200 hours on my engine and my muffler got a huge crack, clearly this is not ordinary wear. Talked to John McBean and his first guess was that the stinger was banging against the engine cowl on startup. I looked and sure enough, saw evidence on the stinger that it had been hitting the cowl. The distance from my stinger to the cowl is 3/8" which would seem like plenty but in a rough cold morning start, clearly that is not enough. Rather than buy another $995 muffler every 200 hours, I think I will increase the distance to 5/8" clearance And while I am at it, going to replace my engine mount vibration isolaters too in case those are shot and causing excess movement.

cracked-muffler.jpgevidence-of-hitting.jpgclearance.jpg

[Shadowrider]

Have you balanced your prop?

[3kdscf]

No, I have not. After reading your comment I headed to the Whirlwind RW-3B installation manual in a panic wondering if I missed something. It appears the prop is statically balanced at the factory because that is not even mentioned in the installation instructions but when it comes to dynamic balancing they say that it is recommended but not required.

I am fairly confident that the stinger hitting the cowl was my issue because the crack showed up after a particularly rough cold morning start. Agree that its definitely worth checking the dynamic prop balance because that could also be part of my problem but its hard to justify a $4000 prop balancer for one test. I just found this awesome thread where a TeamKitfox member is renting theirs out. I will have to contact them.

[Shadowrider]

Yes highly recommend getting the dynavibe and balancing your prop. Static balancing of your prop helps but when its spinning with a spinner is another story.

[avidflyer]

Last summer, I got a 912 muffler that was cracked up pretty good in the same place. Interestingly all the exhaust pipes were stuck in the muffler and wouldn't come out for anything. I ended up making a cut up the length of each nipple and was able to spread/expand the nipples that way and got the exhaust pipes out. Had it all welded back together, and I think it will last a good while in the next project. I ended up with the exhaust pipes being looser in the muffler than they probably were from the factory, but that will likely help things move around more and maybe not as likely to crack. Could be something to take a look at as well. A looser connection may leak a bit of exhaust, but not as much as a cracked muffler I'm thinking. Especially if the entire pipe breaks off. JImChuk

[3kdscf]

Interesting, my four pipes came out of the muffler very easily and also came off the block easily. When I installed my muffler I used loctite LB 8150 anti-seize on the muffler end of each pipe as well as the engine block end of each pipe. It makes sense that if they became stuck that it could lead to cracking just from the thermal contraction and expansion.

[jiott]

My kit was purchased in 2012 and the muffler did not come with provisions for the stinger brace. John McB said the internal construction had been beefed up and the brace was no longer required. Mine has been flying for 1050+ hours now without the brace and no weld problems with the stinger-knock knock.

[Delta Whisky]

Inside muffler.jpgExterior of muffler failure.jpg

Better (promised) pictures and some clarity of the internal "silencing" pipe. After looking more closely at the evidence today, I'm developing the theory that the fairly long stinger, in a vibrating environment, caused the cracking which in turn allowed it to then contact the cowl - not the other way around. I found an airflow deflector I had installed to better direct air to the oil cooler also cracked in two places. Clearly I have a vibration issue that has to be resolved and, yes, I did balance the prop during the first 10 hours of flight. (Now 180 hours.)

[avidflyer]

I wonder how long it would last if you were able to slide in an elbow that inside baffle pipe fit into fairly snug. Then weld the elbow and a patch on the end of the muffler. Might last for a bit till you get another muffler at least.