I have taken an engine apart for cleaning and inspection. Everything looks good inside, just a lot of dirt to clean out after a crash. should I use thread lock on the head and back cover when i reassemble?

Thread locker is not required for engine reassembly. I suppose you could use it, but it simply isn't needed.

It is not a good idea to use thread locker on your engine reassembly.

3 reasons (not listed in any particular order of importance)

1 - It is not necessary
2 - The engine gets hot so only the high temp product will have any real effect
3 - You are screwing a steel bolt into aluminium. The thread locker will cause the threads to strip out of the aluminium when you go to undo the bolt later. (It can and does happen).

Make sure that the threads on the bolts and in the engine are clean before re-installing the bolts.
Tighten the bolts to the correct torque and evenly - do not overtighten them. Things like alen keys are a certain length to assist with this. They allow you to obtain the correct torque for steel to steel with normal hand force.
Use new spring washers wherever they existed originally.
Use new gaskets.

.....it is normally only used on the muffler bolts.

FBD.

D'oh - I forgot about that old method of retaining mufflers

I discovered a product over here that the guys use for muffler retention. I don't know the english name for it - it is some sort of a two part "acrylic/urethane/epoxy" that sets in about 2.1 seconds from the time the tubes are opened. (My name for it is rocket fuel). I apply it to the muffler flange instead of the gasket and it works as a great muffler gasket material. I've never had a muffler come loose since I was told to try it. It's an interesting thing, when it is time to remove the muffler, the bolts come undone easily(no thread locker used) the muffler comes off as easily as if the normal gasket was used and the "glue" comes off as easily as an old gasket.

Only problem with it is it stinks and it sets real fast.

What about the saitos with the plastic back plates? I think they have the stuff on that. Cheers the pope

Loctite makes a Purple color (Loctite 222) threadlocker that is Low Strength for smaller fasteners. I've used it with no problems.

I've never had a problem with engine screws coming loose.

Terry in LP

Back in the racing days we used the blue locktite on threads. It is particularly useful for keeping a TD 049 in one piece. The venturi, backplate, cylinder and glowhead can all unscrew. Jut use a tiny bit and make sure the surfaces are clean. I actually haven't used it in years.

A drop of oil on the screws is all I use, except muffler screws, which I use purple or blue loc-tite on.

Yes, Saito does use threadlock on the backplate screws.

I use epoxy for those and muffler bolts.

I agree with aussiesteve, I have used loc-tite on muffler bolts and found it to cause stripping of the threads in the aluminum muffler. I read in another thread that Teflon pipe thread tape works great and experience had proven that to be true. I have never used any thread locker on any other engine screws and have found no reason to do so.
bhady

Ok Guys, so I am learning my lesson the hard way. I used blue loc tite on my muffler screws and am now unable to remove a Bisson Pitts muffler from my OS 55 AX. I've pretty much stripped the bolt heads round. Any suggestions would be greatly appreciated. I'd like to replace the muffler. Thanks

Use a propane torch to heat the muffler right at the lugs for the screws. When the muffler starts smoking it's hot enough. Use needle nose vise grips to crack them loose.

Buy new screws and a new wrench. I use blue on my muffler screws and haven't had any problems with screws stripping.

Just dont use red Loctite. It takes heat( lots) to release, but if you let cool it grabs again. They also have one that tightens up when you try to loosen it!

Remember, a properly cross threaded fastener requires no Loctite.

You don't have to 'drown' the threads with the blue for it to work. Just enough to fill (not cover) three or four of the very fine threads is usually plenty.

For muffler screws, i use loctite pipe thread sealant - kind of like liquid teflon thread tape. I know it's not meant for screw threads, but it works great. It won't set hard - it always remains a bit pliable, so getting the screws out again is no problem. It also has a reasonably high temperture rating. And a little bit goes a long way. I'm still using a tube that i bought about 11-12 years ago. For the other threads, just make sure the holes are clean, and put a drop of oil on the threads and snug them up until they 'feel right'. IT's very important for heads and backplates is to make sure that each of the screws are tightened gradually and evenly.

What does the oil on the threads do?

It keeps the threads from galling, allows you to tighten the screws without damaging the metal. Oh, and for 4-stroke mufflers, I use teflon tape.

I've got an OS91 four-stroke, bought from a guy who used threadlock to re-assemble. If I could find the guy again, I would get him to dis-assemble it, because I've got the head of a ball-driver broken off in the socket of one of the headscrews. The engines don't come with threadlock. I've flown engines by the dozen, most of them disassembled and reassembled one or more times, never one with threadlock. Just tighten them up with an L-shaped allen wrench, tighten to the first little squeak, and they won't come out unless you stick a wrench to them. Teflon tape is the way to go on muffler threads.

Saito uses it.

I have never had problems using Loc-*** on model engines. I have used both colors and have never had problems with stripping. But I only use one drop and if it is very small I use less than a drop.