Using Delrin Tenons

A Delrin tenon is a joint formed by preparing the projection that seats the mouthpiece into the shank from a separate acetal rod and gluing it into the mouthpiece, rather than turning it from the mouthpiece material itself. Delrin is DuPont's trade name for its polyoxymethylene (POM)-based acetal resin; the resin is on record as having entered mass production in 1960. The material's arrival at the pipe bench is much more recent, and its rationale is not aesthetic but entirely practical.

The Logic of a Separate Tenon

With a tenon turned from the mouthpiece itself, the diameter is left to the maker's hand and the lathe's setting each time; with a ready-made rod, the diameter is known from the start. Because acetal is manufactured to tight tolerances, every rod of a given size is, in practice, the same diameter, so the fit becomes repeatable from pipe to pipe. The second gain is on the repair side: a broken acetal tenon can be removed and replaced with a new one without sacrificing the mouthpiece itself.

There's no binding standard for diameter selection; workshop habit is what decides. A 5/16-inch (roughly 8 mm) rod is most often preferred; 1/4- and 3/8-inch sizes are also used. The decision is made according to the final thickness of the mouthpiece and shank.

Two Faces of Acetal

Two behaviors make the material suitable for this job. The first is dimensional stability: because the piece holds its size under heat and pressure, it doesn't drift while being drilled. The second is its low friction and the accompanying self-lubricating behavior; the mouthpiece slides in and out of the mortise smoothly, without squeaking.

The same slipperiness exacts its price at assembly. POM is among the materials most resistant to adhesive; the bond forms not chemically but mechanically. In practice, this means cutting a groove around the area to be glued, then deepening these grooves further with a file at three points around the circumference. Once the epoxy cures, these irregularities form a lock that keeps the rod from spinning within the mortise.

A Fixed Rod, a Variable Hole

The hard part of the job isn't the rod but the hole meant to receive it; while the rod arrives at a fixed size, the hole is affected by three factors. The first is the drill bit: bits stamped with the same size can give different actual diameters from brand to brand, so a maker has to test the bit they're going to use in advance and set aside the ones that have worked. The second is the material. Testing must always be done on briar pieces, because other woods don't react the same way when drilled and can give a misleading result; in vulcanite and acrylic, meanwhile, the bit that gives a correct fit in briar often runs too tight, so separate bits may be needed for the body and the mouthpiece. These differences among mouthpiece materials are covered separately in the article on mouthpiece materials. The third is drilling technique: because heat and pressure from chips accumulating in the mortise can change the final diameter, the bit is fully withdrawn and cleared at short intervals.

From Gluing to Final Fitting

The bottom of the mortise drilled into the mouthpiece is left flat, and the end of the rod is prepared to match this bottom; the aim is to minimize any gaps where epoxy could pool. The resin is applied to both surfaces in a thin, even layer, the pieces are pressed together and rotated to release air bubbles, and any excess epoxy is wiped away before it cures. Drilling or taking measurements is not attempted before the cure is complete; fast-curing resins shorten the working time but reach full strength later.

Two approaches are followed for opening the smoke channel. Some makers drill the channel while the rod is still separate; others glue the rod to the mouthpiece first and then drill both together, without removing the piece from the lathe. The second route is often recommended because it reduces the risk of misalignment; the bench-side details are described in the article on drilling on the lathe. In the final stage, the tenon is shortened to match the depth of the mortise in the body, and its end is lightly chamfered so it doesn't score the mortise. How a fit that has loosened or tightened over time is adjusted is covered in the article on pipe repair and mouthpiece maintenance.