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Old 09-26-2019, 07:08 PM
Join Date: Dec 2006
Location: Fort Worth, TX
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Actually, the fit will never be looser than at room temperature. The aluminum expands more than the steel bolt does, which means a hot engine has more tension on the muffler bolts than a cold engine.

Since this is a confusing topic for a lot of new RC pilots, I'll break down the science (as best I can) on why mufflers come loose.

There are 2 types of elongation involved- elastic deformation and plastic deformation. Elastic is stretching that returns to the original shape, while plastic doesn't. If you've ever stretched the neck out on a T shirt, you have an idea of what plastic deformation is all about.
A new bolt, when put under tension, will immediately stretch elastically. When the engine heats up, it stretches more. The bolt will stretch permanently some, which relieves some of the tension. A couple of cycles of that can stretch it enough for your muffler to become loose, which will let vibration unscrew the bolts. Loctite can stop the vibration from rattling the bolts out, but it can't do anything about the permanent stretching. If you do some searching you can find a few threads from frustrated hobbyists who say they muffler bolts won't stay tight even though they are using loctite on them.
Before long though, the plastic deformation settles down and the bolt returns to its original length after each stretching cycle. You can make the bolt settle down without risking your muffler by tightening it to its elastic limit (evidenced by a sudden increasing in the effort needed to tighten it more) and let it sit for a day or two.
The idea for keeping a muffler attached then is to not let it stretch to its plastic limit when the engine heats up so that it returns to its original length when it cools down. I do that by tightening 1/2-1 turn past snug once the initial stretching has happened. It works for me.
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