I found this on Pipedia
. It echoes the advice of @OneStrangeOne
Fixing a loose stem
Even if you're careful to never remove the stem from a hot pipe, you may occasionally be faced with a loose stem. Often this problem will fix itself with time, but if the stem is so loose that it is in danger of falling out, then something must be done. The safest bet is to take the pipe to a tobacconist or send it to a repairperson. These people will have a great deal of practice performing this task, and they will do it for a very modest fee. It is remarkably easy for an amateur to crack a shank while attempting this repair, as many of us can sadly attest.
Nevertheless, if you are determined to do this yourself, you must first determine what sort of stem you have. If the stem is lucite, the easiest fix is to apply a very thin layer of clear nail polish to the tenon, allow this to dry *completely*, and then carefully sand the tenon to fit. A vulcanite stem, on the other hand, is a bit more complicated, as you will need to heat the tenon and expand it in some way. There are a number of variations to this procedure, but the most common one is described below.
First, remove the stem from the pipe and insert a pipe cleaner into the stem so that it just reaches the end of the tenon (this is to ensure that you don't collapse the air hole). Next, carefully heat the tenon over a match for about five seconds (the intent is to soften the vulcanite, not melt it). Then gently press the end of the tenon against a flat surface, keeping the tenon as perpendicular to the surface as possible, taking care not bend the tenon to one side or the other. After the stem has cooled, test fit it. If the stem is still too loose, repeat this procedure. If it is now too tight, then see "What should I do with a stem that's too tight?" below. NOTE: It is *very* easy to ruin a perfectly good pipe with this technique, and I feel I should reiterate my earlier statement that this job is best undertaken by a "professional."
A variation on the above that has less chance of bending or ruining the tenon is the following: Insert a tapered mandrel into the tenon. Apply heat to the mandrel (an alcohol flame is recommended). As the heat from the mandrel transfers to the tenon and softens it, move the mandrel further into the tenon. Repeat as necessary to get the desired expansion. Remove the mandrel and place tenon in cold water to set. Note that PIMOmakes a 'Stem Tightening Kit' that uses this principle.
A less radical (and *much* safer) procedure that has been recommended to me by several people is to simply rub the stem's tenon against a block of beeswax until the tenon is well coated. Once this is complete, reinsert the stem. I am told that the joint will tighten after a smoke or two.
Another less radical approach to try if the beeswax method doesn't work, is to simply heat the tenon and then allow it to cool. Very often the tenon will have expanded just enough to make a decent fit. Rather than an open flame, I suggest carefully using a heat gun, or a handheld hairdryer on high heat aimed at the tenon.