Looking for general mechanics education.

Does anyone know of a rule of thumb on when you should use thread-lock on a bolt/nut and when you shouldn't?  Is there a reason you wouldn't use it on ALL structural and engine fasteners?

What about sealant?  Needed ay place else in the engine fasteners except the intake bolts?  Does silicone work in all cases?  I hear RTV mentioned a lot.  Where are all of the spots it should be used?

I use sealant on things that are in liquids like oil. (exp: intake maniflold bolts)

I use thread locking on bolts that need to stay in place under high vibration (exp: fly wheel bolts)

But thats just me

I agree with Brian, but let's add a bit.

You need to be carefull of any sealer or treatment on bolts that need to be torqued.  You need to use the correct, if any, compond to achieve the correct torque.  This will affect the clamping force.  If the  bolt turns easier, it will clamp more.  If it turns harder, it will clamp less.  And clamping force is what we are really trying to achieve.  Torque is just a method of doing so.

Some head bolts require silicone RTV sealer, others must be oiled, others must be installed dry.

Be careful of which locktite you use.  There is a mild grip, the most common choice.  It can be removed fairly easily.  There is a stronger version that is very difficult to remove.  And then there is the most aggressive that requires heat application to loosen and remove.

There are few places where you would want to avoid anything.  Many interior pieces for example.  The extra grip could damge components during installation or removal.  Pot metal parts are easily damaged.

Well that all depends.  here are examples of what I used and where on my current engine build.

Pipe plugs that lead to oil or coolant passages - Use Peratex high heat thread sealer.  Preferably high temp with teflon as the teflon acts as a lubricate during threading as well as adds to the durability.

Main Studs - Mine were outer splayed bolts.  These tend to break into the water jackets.  I used "Green" Loc-Tite.  This is what was recommended by the machine shop because it locs and seals at the same time from moisture.  Its the only thing they have found that will do both.

Center main Studs - No loc tite, only torqued to spec.

Head Studs - Permatex Ultra Black Thread Sealant.  No Locktite,  Torque to spec.

Oil Pan Studs - Oil pan bolts should not be over-torqued.  Something like 55-65 in lbs.  There are pan gaskets that have torque retention inserts that prevent over torquing and crushing of the gasket.  Pa Studs and bolts are one area I feel need a thread loc.  Mainly because there is a lot of oil, debris and road grim that over time will cause them to loosen.  I use the Blue Loc-Tite because it will allow them to come back out.  On studs there really isn't an installer for such a small stud so i loc tite both the stud and the nut.  I also prefer to use a one piece pan gasket with the retention inserts.

Manifold - Four Center Bolts - Permatex Ultra black thread sealer - You could also use the Permatex thread sealer. Outer bolts - I prefer just to use a little assembly lube to torque them in.

Manifold pipe Fittings - Permatex Thread Sealer.

Just remember a rule of thumb.  All fastners that require a torque to install require some type of lubricate to install properly.  That lubricate can be assembly lube, oil, loc-tite and in cases where the threads are exposed to oil or water thread sealant can be used.  You should also place a small amount of assembly lube on the under side of the bolt head so your minizizing friction between the fastner and the install surface to get an acurate torqung of the fastener.

Sensors - When it comes to electronic sensors that use the mating install threads or surface as the ground source, do not use teflon tape.  It will insulate the sensor and cause an inaccurate read or no read at all.  use a paste type thread sealer. 

Front timeing covers.  Here is an area that I do not use any sort of sealer, loc-tite or subricate.  the torque value is so low I just install them and hand tighten them.

Hope this information helps.  I'm sure others have used other types or forms of sealers to do the job, but these have always worked for me.