Stoking the steam, one engine at a time…
1930 model boat club
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SimplexFully Machined Fun

Type: Vertical, single cylinder, double acting, slide valve.
Bore 5/8 in
Stroke: 5/8in ?
Height: ? in
Availability 1924-1928
Design: Four Comulm Marine

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Cylinder 5/8in bore by 5/8in stroke. All Machining Done.


When we introduced before the war our "Simplex" set of fully-machined castings and parts, we knew quite well that it was a splendid engine, and easily the finest value offered to the public, but even we ourselves did not appreciate how good this set was. Today hundreds of well-satisfied owners, all over the world, testify to the excellence and power of these models.

Many model power boat users have been highly successful in keen inter-club races when using the "simplex" Engine set. This year we have improved on these models somewhat, and to meet the demand of users of high temperature "flash steam" we have introduced another set, with cast iron cylinder, in addition to the ordinary gun-metal set. Both sets are complete and consist of a Fully-machined SET OF CASTINGS and PARTS for a Model Four-Column Launch Engine having a double-acting slide-valve cylinder 5/8 in. bore by 5/8 in. stroke.

The SET comprises -

Machined and bored cylinder, with ports cut, complete with turned covers, and steam chest and cover, faced ready for fitting up. Glands fitted. Four steel columns turned and screwed. Eccentric and strap turned and bored, bed-plate and bearings machined. Flywheel turned and bored for shaft. Crankshaft turned. Machined crosshead, slide bar and connecting rod.
Complete with drawings, instructions and all bolts, studs and nuts, lagging plate, etc.
Price in Gun-Metal, recomended for use with ordinary Boiler .. .. .. .. .. .. .. ..17s. 6d.
with Cylinder in Cast Iron, Steel Piston, Steel Crank Disc solid with shaft and special
flywheel, recommended for use with "Flash" Steam. .. .. .. .. .. .. .. .. .. .. 21s.
Finished Engines .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. £3 3s.

The First Machined Set of Castings for a Model Boat Engine Ever Offered. A marvel of value and a proper piece of engineering work.

Illustrated Booklet, post free, 3d. Write at once and secure one of the first post war batch of these famousEngines.

The New Simplex 1926

The engine has been entirely re-designed and new patterns and drawings prepared. It has a bored crosshead guide cast on to the bottom cover ensuring perfect alignment and obviating the difficult work of fitting the crosshead used with a flat guide. Piston and valve rods have screwed glands and crankshaft and crank pin diameters are slightly increased. The original Simplex has been extraordinarily successful in Power Boat Races, and many records have beenheld by boats engined with it. The new design will be found to develop ,ore power and to be equal to the most exacting demands.


The Simplex engine is a very simple 4 colum marine style single cylinder design. The cylinder is a standard Stuart design with slide valve chest and eccentric running from the crank shaft. The cylinder bottom cover has been extended to provide 4 mounting points for securing to the 4 colums made from steel rod. Fixed to this on one side is the cross head guide that runs down the side of the piston rod for the crosshead to run on. the bottom end is unsurported. It seems that model makes tend to decide which side of the engine to mount the guide as I have seen it mounted on eiterh side. The crank wheel is often machined to form a counterweight shape. The crank is supported by two cast bearings, part of the cast base. A small marine type flywheel rounds of the engine. Availble in two metals, a bronze cylinder and a cast iron cylinder. The bronze cylinder is fine for ordinary steam from a conventional boiler. However for Flash steam which is much hotter cast iron is preferable. I have seen several variations to the crosshead guide design, I am only sure that the original single sided and trunk designs were released by Stuart, however I have seen an engine with twinn guides one on each side similar to if two single stuart guides had been used.

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Cast Iron cylinder version of the Simplex.

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In 1926 Stuart advertised the New Design.

Stuart advert

Video Of Simplex


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