This is the frequently abused and mis-used stuff dolloped on to bolts/nuts to stop them coming undone. Having seen and heard of many horror stories concerning this stuff, I thought a few words of caution and common sense wouldn't go amiss.

There is a plethora of types on the market with no easy reference as to which to use for what application except in certain cases. Loctite have a very useful guide, their dealers generally able to help with choosing which is best for any given application. The trouble comes when folk buy stuff unwittingly from their local 'do it all' motorist/car spares place. Good stuff is expensive, and it doesn't go all that far. But at least it does its job when used in the correct applications. Unfortunately many buy a cheaper variety with not quite the same application strength, and then apply twice as much hoping it will do the job. It doesn't. In fact it can compound the very reasons that even bolts thread-locked as such come undone. Without tending to the why's and wherefores as far as the chemical/science side of the stuff goes, a few small guidelines.

Use a recognised make, not some spurious copy. Use the right compound for the required application. I always use Loctite products, as they are ultra-reliable and consistent. When using alternative makes, see if you can get a direct alternative for: Loctite 270 - described as 'Studlock', a green coloured very high strength adhesive where near-permanent fixing or high-yield strength is required (flywheel and damper pulley bolts); Loctite 243 - described as 'Nutlock' where a semi-permanent bond is needed but can be easily undone with standard hand tools (cam nut, diff and transfer gear housing retaining fastenings); or Loctite 222 - described as 'Screwlock' where there is a great deal of thread engagement or where regular easy removal is needed.

Whichever you use, the two most important points to understand and carry out - 1) make sure the threads on fastener and receptacle are clean, dry and free from oil/grease, and 2) you only need a very small amount, not lakes of the damn stuff! If the one small drop a third the way up the engaged area, or two drops spaced one as previously described and another a few threads further up where a longer threaded area is encountered. DO NOT put it on the end of the bolt - it will have been spread too thin over too larger an area to be properly effective. This is suitably illustrated by original equipment bolts fitted to the later transfer gear casing bolts (used instead of studs, nuts and lock tabs - engineering from the ark) where, on removal, you will find the locking compound is situated a way up the thread, not at the end. Too much compound can also get under and in-between things causing either a great deal of swearing when trying to remove these components, or powdering in use, making them loose. The results can be devastating - like when used on rod bolts/nuts. They become loose, lose their clamping-load developed by torquing them down, break, and destroy an engine/gearbox. And genuine Loctite isn't that expensive. Used correctly, it'll do a lot of fixings and they have an indefinite shelf life since they only cure when deprived of air.