Stem keeps loosening

[Bocephus]

I bought an old Kaywoodie pipe. The stem fit fine til after the first smoke, then I saw it sagging in the rack the next day. The stem was totally loose. I heated the stem and inserted a rod to expand it and all was well until after the next smoke, again, loose. I'm not fiddling with it as I smoke. Is there some permanent way of tightening it up? The old vulcanite seems to keep shrinking from the heat of smoking. I don't know if I can smoke it cool enough not to affect it. I've repeated the tightening process several times.