For example, in your nitro engine, the engine has cylinders that pump up and down. There's the cylinder head, which sits on top of the main body of the engine, which houses the piston. Between these two big pieces of metal, is a thin gasket made out of something like graphite. If you just bolted the two bits together, air would leak out in between the gaps since the casting process is not perfect, and there would not be enough compression in the engine, which would cause it to lose power. Similarly, you'll probably find a gaskey at the point where the engine connects to the header pipe or muffler.
Generally, the higher the mAh, the better (and likely more expensive and physically larger) the battery is. In particular, it's easy to get more mAh by simply increasing the battery size; eg a "C" size rechargeable NiCad might be 2400mAh; but getting more mAh into the same size battery is a real challenge.
Also, it's interesting to note that the "mAh" is a unit made to allow people to reason more conveniently. It's not an SI unit for charge, which is of course, the Coulomb. It's not too hard to show that 1 milliAmp hour is 3.6 Coulombs; but if you told someone their battery had a capacity of 2000 Coulombs, they'd give you a strange look, compared saying to 600mAh.
To get total energy storage, just multiply by average voltage. So a 600mAh 1.2V battery stores approximately 720mWh or about 2600 joules of energy.
A safety device used in nitro rc cars. The carburetor in the engines of such such cars has a mechanical throttle on it that controls how much fuel is being burnt and hence how fast the care is going. The throtte is connected to a servo, which pushes the throttle to make it go faster or slower, under the driver's control from his radio control transmitter. But for various reasons (like batteries dying, or a really bad collision which takes out the receiver or something), servos sometimes fail. If it so happens that your servo fails when it is at full throttle (especially if the steering servo also fails), then you have a pretty serious problem on your hands, commonly called a "runaway". A $300+ 2kg piece of equipment travelling at upwards of of 40km/h jammed at full throttle is quite amusing sometimes, but may be (a) dangerous to "civilians" or whatever else it will eventually hit (b) expensive to repair.
Hence you have a throttle return spring. The springs are set up in such a way that if the servo loses power or fails, the throttle will be pulled back by the springs into the idle position and damage will be averted. This means also that the servo has to work against the throttle return spring whenever it wants to push the throttle forward, and will brake faster when it's installed.
___________
|___________|
|////////////|___________|\\\\\\\\\\\\\|
|___________|
^--- screw ^-- hexagonal ^--- screw
thread prism thread
Note that the screw threads are fixed to the hexagonal prism (and also that one of the threads is backwards relative to the other one, so you can still screw it in to two opposite facing screw holes).
The idea is that you put each of the screw threads into holes, and turn the hexagonal bit in the middle with a wrench, then you can adjust the length easily.
For example, consider the setup below:
V-- wheel
--
| | +-- drive shaft
| | V +-----------+
| |=======| |
| | | |<--- chassis
| |--<=>--|
-- ^-- turnbuckle
The above top view shows the chassis of your car. If you twist the turnbuckle one way, it will angle your wheel in, the other way, it will angle the wheel out.
It is typically found in the suspension of your vehicle to adjust things like toe in and camber.