Once the nuts are done up hand tight on the joint, an additional half to three quarter turn with a pair of spanners (one on the body of the fitting to hold it still, the other on the nut) is usually adequate to make a seal. If the joint leaks, an additional tightening tweak will normally fix it. Note that over-tightening a joint may actually make it leak. Copper pipe works well with compression fittings. Chrome plated copper pipe also works well. Some advocate stripping the chrome from the area of the pipe in the fitting as this allows the olive to get a better grip on the pipe. There is no actual need to do this, but if not doing so, it is advisable to torque the nuts a little higher than with normal copper pipe since its harder for the olive to "bite" the chromed pipe. Place a pipe insert into the end of the pipe to be joined. Pipe inserts, typically made from plastic or steel, reinforce pipes and keep them in a fully circular shape, making connection easier. It should be noted that this only applies to plastic pipes and not copper or steel pipes as they are much stronger

Note that this may not cure a leak that is caused by a scratch on the pipe under the olive. You may be able to wind a couple of turns under the olive - but this is less easy. Copper and brass olives both work fine. Olives can't be removed from a pipe and reused. Even hand tightening a fitting with an olive inside before the pipe is inserted, is enough to make the olive unusable. One of the difficulties with compression fittings is that while the nuts are usually a nominal size, there is not agreed standard tolerance between manufacturers. This can make selecting a single universal spanner difficult. Also not all spanner types can be used in all situations. Hence most plumbing kits will have a number of spanners, including at least one adjustable one.Again, a clean square cut is important, and any burrs removed. Take great care to protect pipe ends when pulling them under floors etc, since its very easy to scratch plastic pipe and this makes it hard to get a good compression (or pushfit for that matter) seal. Compression fittings are very reliable and have been proven over many decades, however sometimes things can go wrong and you end up with a joint that leaks. The common causes are:

Sometimes an olive can be removed by tightening an adjustable spanner closely around the pipe, and then using it as a slide hammer to strike the nut or the olive itself and drive it toward the pipe end.The pipe needs to be cut square and cleanly, with any burrs removed. Pipes cut with a pipe cutter are much easier to use than those cut with a saw. The pipe needs to be clean and free from contaminants like paint or plaster etc. It does not need to be quite as clean and shiny as required for soldered connections. Slide the compression fitting olive (brass ring) over the first length of pipe, and then lay the nut over that The other way is to use a few turns of PTFE tape to the threads. Note that used this way the PTFE will not aid the sealing of the joint, but it will make the nuts easier to torque up.

There is no significant difference. Compression fittings designed for liquid will work fine with gas but if you use optional PTFE tape, make sure it is gas grade. In addition, ensure the fittings are accessible and not hidden behind walls or under floors. Are Compression Fittings Reliable? Non setting lubricants and jointing compounds like that shown above can also be applied under and over the olive. This can be more effective for fixing leaks due to pipe scratches. The vice grip spanner is a combination of an adjustable spanner and a set of vice grip pliers (aka "mole grips"). Very handy for working at arms length in awkward spaces, and also for not marking polished chrome fittings like nuts on taps or service valves. Plastic pipe can be joined with compression fittings, however it is important to use the correct brand of pipe insert to support the end of the pipe, or else the joint may start to leak over time.Compression fittings should be tight enough to create a watertight seal but not so tight that the pipe warps or the thread is damaged as this could encourage leaks and make the fittings much harder to remove later. Proceed with caution. What Should I Do if I Find a Compression Fitting Leaking? Compression joints are designed to be used without any sealant. The nut compresses the olive driving it into the cone shaped profile at the end of the fitting body, causing deformation of both the metal surfaces of the olive and the pipe, zeroing the gap and creating a watertight seal.