Gap filling cement
I have often seen attempts made by fitters to make a square peg fit into a round hole. What do I mean by this? The most common “botch” is to try to fit an imperial and metric pipe / fitting together. Some sizes in the two plastic pipe ranges are only a millimetre or two apart (for example 3? pipe and 90mm which are 89.1mm and 90mm respectively and 1 1/2? and 50mm are 48.3mm and 50mm respectively). When the imperial pipe is inserted into the metric fitting there is a slight “rattling” within the fitting and the temptation is to put a lot of glue on to “pack” the joint. With many types of glue on other materials this would work, as the glues work on contact and adhesion principles. Plastic pipe glue however, works on the solvent principle, and so is called a “solvent cement” rather than “glue”. A solvent cement works not by sticking to the plastic pipe, but by a double process of melting and evaporation. The first step of the process, when the pipe and fitting have been pushed together, is that the outer layers of plastic (of both the pipe and fitting) are melted by the solvents in the glue, and mix together. The solvents then evaporate (hence the strong smell) and as they do so, the two layers of plastic, which have now become one, harden. This forms a physically strong bond, which holds the pipe and fittings together and stops them blowing apart as the pressure rises a seal, and also creates a complete seal.
In the case of pipes and fittings not being compatible (as in the imperial / metric example above) and the gap in the joint being too big, the resulting bond will end up incomplete, as the amount of plastic melted by the solvent cement will not be able to fill the void. This will result in a channelled honeycomb effect in the joint, which will be very weak and leak like a sieve! Putting a bead of silicone of other mastic / glue around the joint will not help if there is any perceivable pressure, as it will, over time blow out and leak.
The moral to this is that if you have incompatible fittings, and it seem like a good idea to save some time and money and try to fit them together…please reconsider as it will cost a lot more in the long run.
Thixotropic Cements are often claimed to have gap filling qualities, and they do, but not in the sense described above. They are designed to momentarily becomes more fluid under pressure so they flow into small channels and gaps within a proper pipe joint, not to fill the large types of gaps formed by two otherwise incompatible fittings.