Stoking the steam, one engine at a time…
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SunHot Flash

Type: Horizontal, twin cylinder, banjo driven piston valve or slide valve, high speed marine engine,
Bore 3/4 in
Stroke: 3/4 in
Height: 4 in
Availability: 1928-1990
Design: Enclosed twin cylinder

For ordinary or Flash Steam Boilers
3/4 x 3/4 Twin Cylinder, Single Acting, for 4-ft. 6-in. Boat, or two for 5-ft. 6-in.
Twin Screw Boat

Weight 2-lb. 9-oz.
Height 4-in.
Length(overall) 4 3/4-in.
Width(extreme) 3-in.
Castings and all of Materials required for building the Engine.

£2 0s. 0d.0s.0d.

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The Engine is suitable for large racing craft, for high steam pressures, especially for flash steam.
The Engine is simple to construct, the long bearings have excellent wearing ensures that they, as well as all other qualities and enclosure of the gear wheels
working parts, are flooded with The steam inlet is opposite the lubricator.'.


Cylinders and Valve Chest of close grained cast iron ; very light cast iron Pistons ;

Steel Crankshaft ; stainless steel Piston Valve ; gunmetal Bearings ; we-designed Connecting Rods ; and machine cut gear Wheels.

The cast iron pistons are as light as possible and provided with oil grooves ; piston rings cannot be fitted.

Extra-Lubricator, 16/6
Foreign Orders should include remittance for 7-lb. Postage.


Just like the Star engine, a single main casting contains most of the moving parts. A valve chest casting sits on top and a baseplate seals the bottom. This engine is however designed for flash steam use. Flash steam is generally a lot hotter than ordinary steam and is usually much drier. The Star engine with brass cylinders and pistons would rapidly wear under the scalding heat from a flash boiler. To enable this, the Sun's main casting is from cast iron. Several changes have been made to accomodate the extra heat. The cylinders are now machined into the casting and the cylinders are cast separated allowing air to surround them completely. The vertical valve rod now runs in a tube separated from the cylinders preventing heat transfer. The Crankshaft bearings are disks turned from brass and are held in the main casting by three equidistant bolts. The crankshaft is now a forged steel with a square profile but the same stretched 'Z' layout as the Star. Brass conrods connect to very lightweight cast iron pistons that are machined with oil grooves. The engine is also a wet sump design which splashes oil under the pistons. Vent and oil filling holes are usually found adjavent the valve rod.This is where things get interested as Stuart provided two distinct versions of the engine, one with a piston valve similar to the Star and the other with a Slide valve. This was accomodated using two different valve chest castings either of which was supplied with the same set of remaining castings. The Sslide valve version can be seen below. It also used a banjo to transfer motion from one end of the engine to the other but instead of a piston a slide valve rod. Inside the rectangular valve chest the setup is essentially the same as any other. To allow for the displavement lubricator the valve chest cover is a casting including a pipe running across the top for connecting steam on one end and the lubricator on the other. The slide valve version was not as popular as the piston version as while I have seen over a hundred Piston Suns I have only come across two Slide Valve versions.

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Sun engine with Piston valve. The two screws in the top fill holes required to machine the steam pathways, they are normally blanked and filed flush.

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Sun engine with Slide Valve.

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Sun engine with Slide Valve cylinder head.

Video Of Sun


T. W. Manchester, writes ;-
"I already have many of your Models in boats, and I have just completed a speed boat four feet long fitted with your 'Sun" engine. First trial I obtained a speed of eighteen miles an hour."

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