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As steam engines get hot, there needs to be room for expansion, as an aluminium piston will expand more than a steel piston would through the heat applied to it, which could cause a seizure if it gets too tight in your cylinder and for this reason needs bigger tolerances in the manufacture.
Aluminium is light and reduces the weight moving to and fro giving less engine imbalance, but saving that extra weight in the piston, along with a piston rod and trunk guide plus the connecting rod does not make much difference in the grand scheme of things, so for that reason it is best to stay with steel.
Let us say for example you have a double acting steam engine with a bore of 1 inch diameter and a stroke of 1¼ inches, much like the one in PYRTE, with a cylinder/barrel length of 2 inches.
The first thing you need to do is to decide the length of your piston.
In this case we have to allow for the full length of the bore less the stroke, less the clearance allowed at each end above the piston.
The cylinder covers sit on top of gaskets, but for this example we shall assume they sit flush with the ends of the cylinder.
So for the piston length we have 2 inches (the cylinder length) less 1¼ inches (the stroke) less 2 x ¹/₃₂ (the clearance) = 2 - 1⁵/₁₆ = ¹¹/₁₆ inch.
From this ¹¹/₁₆ we need to incorporate 2 piston rings to help keep the steam pressure at each end of the cylinder where needed and these vary in width according to manufacturer. From this point of view, you need to measure these rings precisely and cut the ring grooves a mere 2 or 3 thousandths of an inch wider than the rings, with a depth equal to the depth of the ring.
This means the ring will sit tight in your bore but will flex to fill in any wider than normal parts if you have not got your bore exactly right. Also, this will allow the rings to open out a little as wear develops.
From the full length of the piston’s ¹¹/₁₆, an allowance has to be made at each end of ⅛ inch before the start of the cut for the groove to allow for oil displacement above the ring on its travels.
Now that we have the requirements covered, take your steel (preferably stainless) piston and face it before turning it down to 1⅛ initially for a length of 1 inch or more.
Centre drill the end and drill it with a ⁷/₃₂ inch to a depth of 1 inch before tapping it at ¼ x 40 teeth per inch and part it off at 1 inch.
Next you need your piston rod which should be ¼ inch ground steel (I have mentioned slide rods being rescued from defunct scanners that are super smooth and accurate, although metric sizes were used here so you may need metric threading to suit) and it needs a thread on it for just shy of 1 inch to match the thread in the piston at ¼ x 40.
The piston has been left oversize so that it can be turned down to size when attached to the piston rod, as I can guarantee your threads will not be cut exactly square, that way everything will be lined up properly.
Thread the piston rod into the piston as far as it will go and part off the piston from the surplus still in your chuck at the 1 inch mark.
Now you need to mount the piston rod in the chuck with the piston sitting proud, preferably by around ⅛ of an inch.
What we are going to do now, with the chuck holding the piston rod tight, is to screw the piston tight onto the rod.
The piston can now be turned to size, to around 10 or 12 thou under the inch diameter to make less friction inside the bore, that way allowing the rings to make constant contact with the bore sides while the piston itself sits within a film of oil.
With the piston to size, lightly cut out the ring slots (not forgetting the ⅛ before the start of the cut), but do smooth the sides of the piston from burrs.
All that remains now is to lightly centre punch the joint in three places between the screw threads on top of your piston to stop the piston working loose from the piston rod, before adding your rings.
In doing it this way, the piston is lined up exactly with the piston rod, regardless of lop sided screw threads.
The piston rod can be left long for the moment, as its length will be fixed according to your trunk guide and trunk piston.
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