Stuart ST Oscillator - Part 1

I thought i'd have a little break from Tich for a bit and have a go at another Oscillating engine. The Stuart ST (also known as the S.T. Oscillator) is a classic beginner’s model steam engine from Stuart Models. It’s a compact vertical single-cylinder double-acting oscillating engine with a 7/16 in bore and stroke, standing only about 2½–3 in tall, with a 1 in diameter flywheel. The design is elegantly simple: the cylinder itself rocks on a sprung pivot, alternately aligning ports with those in the fixed standard to admit and exhaust steam.

This first part of the build focuses on the core rotating and reciprocating components: the crankshaft/crank, flywheel, cylinder, top cylinder cover, knurled nut (spring tensioner), and piston. The kit supplies bar stock and an extrusion for most of these (the standard is the main casting), along with drawings, fixings, an O-ring, spring, and gaskets. A small lathe and basic milling capability are ideal; some operations can be done carefully with hand tools if needed.

Crankshaft / Crank

If you follow the drawings, the crankshaft starts as a piece of mild steel bar (typically 5/8 in diameter × 3 in long) & is turned as one piece from solid. I decided to make the crank shaft and crank web out of 2 seperate parts, then joined. Lastly i added the crank pin.

Flywheel

The flywheel is supplied as a short length of brass bar (1 in diameter × ¾ in). Some builders swap it for steel for better momentum and a different appearance. I decided to stick with the brass.

Chuck the bar, face one end, drill and ream the centre hole to a firm push-fit on the crankshaft. Turn the outer diameter true and add any decorative grooves or chamfers shown on the drawing. Part off to thickness, reverse, and face the second side. Drill and tap for a 5BA (or equivalent) grub screw in the boss. The finished flywheel should spin smoothly and truly with minimal run-out. A well-balanced, true-running flywheel is one of the most satisfying parts of any small engine.

Cylinder

The cylinder is a brass extrusion. Clean up the outer surfaces, face both ends to length, and square the port face. Drill and ream the bore (7/16 in) carefully so it is truly cylindrical and perpendicular to the ends—this is critical for free piston movement and sealing.

On the port face, drill the two steam ports at the correct spacing and angle so they intersect the bore properly (the drawings show the geometry; typically angled holes that meet the main ports). Drill and tap the pivot hole on the opposite side for the stud that allows the cylinder to rock. Take care not to break through into the bore. Last operation was to add the lagging holes can be drilled and tapped ready to fix the aluminium cladding in place.

The finished cylinder should rock freely on its pivot once assembled, with the port face remaining flat and smooth for good sealing against the standard.

Top Cylinder Cover

The cylinder covers are made from phosphor bronze bar (7/8 in diameter stock). The top cover is the simpler of the two.

Turn the outer diameter and face both sides. Machine a central boss or decorative feature if shown. Drill the clearance holes for the fixing studs or screws that secure it to the cylinder. A light gasket will seal it. The cover simply closes the top end of the cylinder; the bottom cover usually incorporates the gland for the piston rod. I will cover that in part 2.

Bronze machines nicely but can work-harden, so use sharp tools and steady feeds.

Knurled Nut (Spring Tensioner / Pivot Collar)

The cylinder is held against the standard’s port face by a spring-loaded pivot. The knurled nut (or pivot stud collar) is the adjustable part that sets the spring tension.

Start with brass bar (around 5/16 in diameter). Knurl the outer surface for grip—fine or medium diamond knurl works well. Turn the body to shape, drill and tap the centre for the pivot stud (commonly 5BA), and part off. The finished nut threads onto the stud, compressing the spring so the cylinder is held firmly enough for a good steam seal without excessive friction. Adjustment is by finger pressure while the engine is running or during initial setting-up.

Piston

The piston is turned from brass bar (½ in diameter stock). Face the end, turn the outer diameter slightly oversize, then reduce it carefully to a close sliding fit in the cylinder bore. A groove is turned for the supplied O-ring, which provides the seal.

Drill and tap the centre for the stainless-steel piston rod. Thread both ends of the rod 7BA - one for the piston, the other for the big-end/crosshead connection to the crankpin. Part the piston rod to length and clean up. The finished piston + rod assembly should slide freely in the bore with the O-ring fitted, yet seal well under pressure. Many builders leave the final diameter until the rod is fitted so everything stays concentric.

These six components give you the basic rotating and reciprocating heart of the ST. Once they are finished and checked for fit and freedom of movement, the next stages (standard, bottom cover/gland, big end, assembly and port timing) bring the engine to life.

Take your time with the bore, the crank throw, and the port geometry - these determine how sweetly the little oscillator will run. The ST is forgiving and rewarding; even a first-time builder can produce a lively, reliable engine that will run for years on compressed air or low-pressure steam.