aviator2002

Hi
I have a GP Christen eagle with a US 41cc gas engine fitted but my problem is that I cannot keep the muffler/silencer to stay fixed to the engine, I have a J`Tec snuffler made for the engine but even when I use all four fixing bolts plus red thread lock it still comes loose and now the mounting hole which is cast into the cylinder fins has broken off so I now only have threee mounting points to use. A fellow club memeber has suggested I fix a support from the silencer to the bulkhead but I am not sure since if the engine moves and the silencer is fixed solid then something else is going to give and I may be worse off that at present.

Has anyone else had problems with this engine muffler/silencer set up and if so did they manage to fix it and if so how?

Frustrated aviator[]

Tired Old Man

My Feedback: (1)

Several things shake a muffler loose.

One is the bolt size used. Many engines use a 5mm screw at the exhaust flange. That's easily mistaken for a 10-32 and if the wrong one is used loose screws are pretty much assured. The threads will have been damaged if that was done.

Incorrect length screws loosen easily. The distance the threads should mate with the cylinder should equal a minimum of double the screw diameter. That distance is after passing through the flange on the muffler. Unfortunately there are many engines where only 8 to 9mm of a 5mm diameterscrew can mate with the cylinder threads.

Use of a gasket can cause loose screws as the gasket material expands and contracts during heating of the engine when running and cooling after shut down. Eliminate the gasket and use Permatex Ultracopper compound in it's place.

Long mufflers act as levers on the screw locations as engine vibration acts upon the muffler. Adding an additional support for long mufflers greatly assists muffler retention.

Dirty screw threads will not permit thread lock to correctly bind to metal. Use a "Q-Tip" type product and acetone to clean the screw threads in the cylinder. If they are really dirty first chase the threads with the correctly sized tap.

aviator2002

Thanks Pat Roy for all this information, most of which I did carry out prior to fitting the silencer but when I take it out of the plane again I shall check my bolt lengths as that may be the problem, these are 6mm dia as the original tapped holes were shot when I got the engine secondhand so I re-drilled and tapped to 6mm. The J`Tec Snuffler mufflers are quite long with the snuffler cans fixed in place so this could be the other cause as you suggest the lever effect could be letting them wobble a bit. Gasket wise I used a high temp silicone liquid gasket but will try your recommendation as an alternative.

This engine muffler combination runs really well and is also very quiet so if I can sort out this problem I will have the perfect flying model.
Thanks again for your suggestions and I shall now dissapear back into the workshop to begin the strip down and hopefully the cure, I am hoping that the remaining threaded holes still have the threads intact since there is not enough meat left to tap out any bigger so it would mean getting the holes welded up and starting again.

John

aviator2002

Hi again
I have now removed the engine /silencer from the airframe and found that 2 of the three tapped holes had suffered from the vibration and so would not tighten down again so I have had all three holes helicoiled to 6mm dia. and have now re-fitted the silencer but because the helicoils could be removed if I used threadlock on the screws next time I take them out I decided to go the wire way and so drilled the heads of all three capscrews to accept a wire link which I have done and also made support brackets to clamp the cans of the J`Tec Snuffler Muffler to the bulkhead which should stop any unwanteed movement from those and am now ready to test my system again tomorrow.
Hopefull all will stay put this time and I can get on and enjoy this model to the full.[)]

Tired Old Man

My Feedback: (1)

An interesting note. Certain mufflers come with bolts that can be safety wired through the screw cap. I recently used a peir of that type mufflers on one of my twins. Both had the bolts well tightened, red Locktite, Permatex Ultracopper for the gasket material, and safety wired. Roughly 10 flights later one muffler was laying on the covering inside the can tunnel and the other had loose screws. So the bolts fell out of one head and loosened inside the other in spite of all the precautions.

Similar happened on a single with a different muffler, which destroyed the bolt threads inside the head at the muffler flange. This happened several times and I was forced to replace the head to continue using the engine. My solution was to support the muffler at another location other than the muffler flange. Nothing has loosened or fallen off since.

The lesson I received was that if I intended to use a muffler long enough to leverage the two little screws way up front was that if I wanted to keep everything in one place and not spend more money I needed to better support the muffler. That's what I got out of it but others may find the effort too much work or effort and be perfectly content to continually deal with loose mufflers and their effects on other essential components.

aviator2002

We will always have a task to perform or perfect with this wonderful hobby of ours, certainly keeps the interest up and I always enjoy trying to overcome the problems that crop up from time to time in all aspects of the hobby.

Thanks again for your input and hopefully it will be helpful to someone else with the same sort of problem.