Making a Webley Junior piston

In the previous post “Modifying a Webley Junior piston”  I mentioned that I would be making a replacement piston for a Webley Junior pistol.  I had picked the gun up very cheaply because for some unknown reason it lacked a piston. Apart from that, it was in excellent condition.  I have now finished making the piston, and I thought the following pictorial account might be of interest to fellow tinkerers.

I started by getting the correct dimensions from an authentic piston:

And then cutting a short length of ¾” mild steel bar:

The trickier, more precise, stage was carried out first, namely cutting the sear groove.  This was carried out in the lathe with a threading tool.  It is always a good idea to do the trickiest steps first if you can, as if you make a mistake you can easily start over again without wasting a lot of time and material. The rest of the bar was turned down to give the correct piston profile:

Next the piston was drilled out to ½”,  flattening the base of the hole with a ½” end mill.

The piston head was drilled with a 4.7mm hole in readiness for the seal screw thread, and the head trimmed to the correct thickness:

The hole was then threaded for 2BA:

This is the final result, shown alongside an original piston:

Having checked that the piston functioned correctly in the pistol, it remained to make a leather seal for the head.   For this a simple die was knocked up by drilling a shallow ¾” diameter hole in a short length of scrap steel round bar, and fitting it with a smaller diameter plunger rod. The base of the die was drilled with a small central hole, which would prove useful later for drilling a perfectly central  hole in the leather seal

A small piece of 1.7mm thickness leather was soaked in water/washing up liquid for a few hours to soften it, and then pushed into to the die with the plunger.  Finally the plunger was forced home with maximum pressure in the jaws of a vise, and the leather was allowed to dry fully in the vise overnight.     

Then, with the die still in the vise, the excess leather was trimmed off with a Stanley knife:

Still in situ, a hole was drilled into the base of the leather seal through the die :

The  seal was then removed from the die:

Next the central seal holding disc was machined from PTFE rod, including a countersink hole for the screw head.  The leather seal, PTFE disk and 2BA retaining screw were assembled to make the complete seal unit:

The seal was then fitted to the piston:

The piston with seal weighs 75.2 grams.   Guy said his original piston with seal fitted also weighed 75 grams, so there is good agreement between the two.   

The seal was soaked in neatsfoot oil for an hour, and the piston fitted into the Junior:

After a few shots to let the dieseling settle down, the Junior was chrono’d and was giving consistent muzzle velocities around 290 fps with waisted pellets weighing 0.6 grams  (e.g. one string of six consecutive shots:  297, 303,288,289,290,288 fps).  Interestingly,  Lesley Wesley in his book contemporary with the Junior cites a factory muzzle velocity of 290 fps, but does not quote the pellet weight.  

So the piston is working well and all that remains now is to case harden the piston head and the piston flange to ensure this pistol has a long active working life.