The number one cause of anything loosing up is vibration. What has more vibration than anything? Your engine. Take a bolt with a split-lock and torque it to a rating. Then, loosen it and watch your off-torque. The initial break-free torque will be 10-20% less than the on torque, then it drops to nothing. Does that mean you should tighten your bolts 10-20% more? Absolutely not. The split lock acts like a spring and pushes the head of the bolt up. Now, do the same test with either a flat-washer or nothing and note the difference in off-torque. Big difference.
As a general rule, on a course thread, only about 30% of it is actually making contact. For a fine thread, there is about 45% contact. The rest of that space is air. In this space, fun things can occur like rust and other chemical reactions. This is not ideal, by any strecth.
What thread locker does is that it fills the air gap preventing any adverse chemical reactions, and it adds a layer of resistance. Install the same bolt using a thread locking agent, torque it to a specified level, then let it sit for 24 hours or so. So come back and try to take it off. The initial off-torque rating will be higher than the on-torque and you will have resistance throughout the entire bolt/nut loosening. This is ideal.
Hardware doesn't always like to stay tight in aluminum, therefore thread locker is highly recomended for aluminum assemblies. For example, every time I changed the oil in one of my cars w/ an aluminum cased auto tranny, every bolt on the the bottom pan was good for a turn and a half at the minimun, even with lockwashers. I removed and thread locked each bolt, then torqued them down and never had another problem for the next 40K miles before getting rid of the car.
BTW, I had a 3 hour seminar w/ a Loctite rep at work the other day, so I'm the 'expert of the hour'
Al
