O.S. SIRIUS FR7-420–A Heads-up from Clarence Lee

O.S. SIRIUS FR7-420–A Heads-up from Clarence Lee

rc airplane engine, rc engine, electric rc, rc model, model aircraft

 O.S. SIRIUS FR7-420–A Heads-up from Clarence Lee, author of the Engine Clinic column.

From the November 2010 issue of Model Airplane News


The latest addition to the O.S. Engines line of multi-cylinder, 4-stroke engines is the Sirius FR7-420, 7-cylinder radial. This RC airplane engine is composed of seven 9.95cc (.607 cu. in.) size cylinders which results in an total displacement of 69.65cc (4.25 cu. in.) Each cylinder has a bore of 24.0mm (.945″) and a stroke of 22mm (.866″) making the engine of “over square” design. The power output is not specified.

The Sirius’ recommended propeller size range is from 22 x 20-12 through 26 x 8 with an operating range of 1.500-7.000 rpm. The weight is just a hair shy of seven pounds. This monster should haul that 40% aircraft around with real authority. The engine carries a list price of $6,499.99, but a street price of $3,499.99. Although this could put a pretty heavy dent in your hobby budget, this is actually a reasonable price for a miniature aircraft engine of this size and typical O.S. quality.


O.S. Engines Sirius 7-cylinder 4-stroke radial engine

The brightest star in 4-stroke appearance and performance! The “Golden Era” looks of the FR7-420 Sirius are matched by its superb power output and rich, realistic sound.



•The vintage 7-cylinder design is a scale airplane modeler’s dream

•Individual V-shaped valve covers add to the scale appearance, and make this engine compatible with several airplane styles

•Main internal parts such as the crankshaft, cam holder and reduction gear are all plated to resist corrosion

•Air-fuel mixture is distributed equally to each cylinder to ensure stable, long-lasting idle


·         Stock Number: OSMG1307

·         Cylinder Displacement: 0.6071ci. x 7

·         Total Displacement: 69.65cc (4.25ci.)

·         Bore: 0.944 in (24.0 mm)

·         Stroke: 0.866 in (22.0 mm)

·         Practical RPM: 1,500-7,000

·         Weight: 111.7 oz (3.167 g)

·         Prop Range: 22×10/12 – 26×8

·  &nb
Price: $3499.99

Updated: July 16, 2015 — 10:26 AM

1 Comment

  1. Have there been any reports on how well this engine runs or its actual power output? It’s a beautiful creation, but it also has to be practical if I’m going to put it on a model and fly it. My limited experience with multi-cylinder glow engines has shown a need for an on-board glow system, and the power output is typically not equal to a single cylinder engine with the same displacement. The glow situation is easily remedied with systems available, so that’s not a deal breaker. Would the power output be equal to, say, a Zenoah G-62? The suggested prop sizes indicate that it may well be, but I’d like to see some tests. My last concern is its longevity. There’s a lot more parts spinning around than any other engine, and the likelihood of one of them causing a problem seems rather high. But it certainly is a beauty…..

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