A hard backfire may have blown this gasket out, letting the muffler loosen up.
Text, photos and video by Tom Hintz
Posted – 11-13-2014
At roughly the same time I started using gasoline engines on my planes I began having mufflers loosen up in flight. I tightened them down again and within several flights, occasionally after one flight they were loose again. Thread locking material was added (bolts and holes cleaned with alcohol before applying the locker material) and once again after several flights the mufflers were loose again.
I mentioned this problem at the flying field and a friend there told me to lose the gaskets and just use gasket sealer in their place. I did as he suggested and my mufflers stopped coming loose. This last weekend the muffler on my DLE 61 came loose again but after roughly 70 to 80 flights, not the three or four it used to take. A contributing factor could be the sharp backfire (the only one on this engine) I had during startup for this flight. I wonder now if that did not blow out some of the gasket sealer to release tension on the bolts. However it started this pointed out the need to check the muffler bolts occasionally but also provides the opportunity to document this procedure so it is always available on my site.
Any time we are dealing with gaskets or gasket surfaces it is very important to start with a clean surface. Since exhaust sealer (Permatex Red RTV High Temp Gasket Maker #81160) is a type of adhesive a clean surface is just as important. I scrape the mating surfaces lightly to remove larger debris and then scrub the surfaces with a brass brush and then with alcohol on a paper towel.
Next I apply a light coat of the Permatex Red RTV High Temp Gasket Maker to both mating surfaces. Don’t get a bunch of the material in the exhaust port itself. I spread the material over the surfaces and then use a toothpick or a cut off from a plastic tie strap to clean excess material from the port and bolt holes. The Permatex Red RTV High Temp Gasket Maker label directions call for applying the material to one surface but in a bead. I think I am better able to prevent getting this material into the ports by spreading a lighter coat to both surfaces.
The directions then call for assembling the parts but only tightening them until the Permatex Red RTV High Temp Gasket Maker begins to ooze from the gap between the parts. Let the assembly set for one hour and then torque down the fasteners good and tight. I can’t find a specific torque for these bolts so we have to be careful to get them as tight as we think we can without damaging the bolts of thread holes in the cylinder. After the final tightening we are to leave the assembly “cure” for 24 hours before starting the engine.
I have used this exact procedure on all of my gas engines and so far the DLE 61cc muffler coming loose is the only problem. My DLE Twin 40cc and DLE Twin 60cc engines fly frequently and have never had gaskets in them. Both perform great, have each flown dozens of times and the mufflers have not come loose. I am going to check the muffler bolts whenever the cowls are off of my planes but that is just common sense. I have used the Permatex Red RTV High Temp Gasket Maker all summer (2014) with the DLE 61cc muffler coming loose just two days before this writing. I think that demonstrates success and I will continue to forgo gaskets in favor of the Permatex Red RTV High Temp Gasket Maker. My engines seem to like it and I enjoy flying more and repairing lanes less.
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