Loctite Threadlocker, or thread sealer is one of those items that every do it yourself mechanic should have in their tool chest. Loctite makes three different types of threadlocker depending on the application. Each is a different color which can help you identify the type of threadlocker to be used when replacing nuts and bolts.
|242||Blue||Used for securing general purpose parts, that can be removed again without extra force or impact tools, such as ball joints, starter motor bolts, etc..|
|271||Red||Used for more permanent or extreme conditions, such as engine mounts, caliper holder to spindle mounts, or subframe applications, etc..|
|290||Green||290 is not used often, and was designed for sealing threads after assembly, especially where you need to protect the threads from corrosion, since it wicks down into the threads to seal from the elements.|
The most common Loctite you'll need is 242, or blue. You can still use Loctite, even if the shop manuals do not call it out. For example, Volvo routinely uses threadsealer on all brake caliper bolts; while Subaru does not. You can basically use common sense for determining when to add Loctite if it is not called for in the instructions. If you are worried about a part vibrating loose, then add a little blue Loctite. When using blue Loctite on parts that do not specifically require it, just remember that you are now assembling the parts using what is known as "Wet Torque" values. Wet refers to the fact that the parts now have some sort of lubricant reducing the friction when they are assembled. Typically wet tightening torque values are lower than the dry values. For example, if a 13mm bolt is to be torqued to 66N-m dry, then tighten it to 49N-m wet (this table will show you some other examples: http://mobilebridges.tpub.com/TM-55-1945-205-24-4/css/TM-55-1945-205-24-4_743.htm).
A word of caution here If instructions do not specifically call out for Red, or 271 Loctite, then do not use it!! 271, or Red Loctite will require a bit of extra muscle to remove later on. Most applications of Red Loctite typically require the nut and bolt to be replaced after each use, since they are usually for extreme vibration or shock conditions, and require tightening torque values that can stretch the bolt.