O.S. FS-91 II Surpass 4-Stroke Ringed w/o Pump

Tower Stock# LXSY82  •  Manufacturer Stock# 35920
O.S. FS-91 II Surpass 4-Stroke Ringed w/o Pump (large view)
Notes from our tech department:
This is the FS-91II Surpass 4-Stroke Engine.

This engine does not use a choke valve OR use a return fuel line to the tank.


New rear-mounted, updraft 60P Carb, Intake Manifold, and Coverplate.
Helix gear-driven camshaft supported by ball bearings.
Ringed piston.
No longer utilizes a choke valve
Grease-packed twin-sealed corrosion-resistant rear ball bearing.
Two Year Warranty


One Assembled Engine w/Carburetor.
One type #F Glow Plug.
One Muffler w/Manifold.
One Set of Instructions and Decal.
(Wrenches, Choke Valve Extension, and Mounting Template Are No
Longer Included per O.S.)


Fuel: 5%-15% nitromethane with 18% lubricant. Synthetic, castor, or
a synthetic/castor blend may be used.
NOTE: Certain four-cycle fuels cannot be used due to low oil content.
However, SOME four-cycle fuels such as the Wildcat 4-Cycle 15%
(CATP9109) may be used because it has 18% oil content.
When choosing fuels for this engine, the modeler must be sure to
select a fuel with at least a 18% lubricant content.
Propeller (see SPECS)
Starting and Field Equipment


Bore: 27.7mm (1.09")
Stroke: 24.8mm (.976")
Displacement: 14.95cc (0.912 cubic inch)
Power Output: 1.6 BHP at 11,000 RPM
Practical RPM Range: 2,000 - 12,000 RPM
Crankshaft Thread Size: 5/16-24
Weight-With Muffler: 24.2oz (686g)
Weight-Without Muffler: 22.6oz (641g)
Valve Clearance: Between .04mm and .1mm (or between .0015" and .004",
measured between the valve tip and rocker arm)
Length: 91.5mm (3.60") from backplate to front of drive washer.
Width: 42.6mm (1.68") width of engine neglecting the mounting flanges
52mm (2.05") distance between the mounting hole centers
Height: 123mm (4.88")
Suggested Prop(s): Stunt planes: 11x11-12, 12x10-12, 13x9
Scale models: 13.5x8, 14x7, 15x6, 16x6, (12x8 & 12.5x7-3 blade)
Construction: Aluminum and Aluminum alloys

Needle Settings: High Speed: 1.5 turns out from fully closed
Low Speed: 2.5 turns out from fully closed with the
throttle fully closed


A fuel tank of approximately 300cc capacity is suggested. This allows
up to 12-13 minutes flying time.
Some Differences Between 4-Stroke and 2-Stroke Engines;
4-Strokes are Quieter.
4-Strokes get Better Fuel Economy.
2-Strokes Generate More RPM (11,000-13,000 RPM with Prop
while 4-Strokes rev to around 9,500 RPM)
4-Strokes generate much more Torque and will Swing a larger Prop.
4-Strokes have a realistic Sound; like a Real Aircraft Engine.
OS no longer includes the gauges, tools, and choke valve extension.

The new Surpass engines (Version II and III) have more power than
their predecessors and also have more corrosion prevention
on the inside of the crankcases.
Although it is cost prohibitive, you can convert this to the pump
version using the following items:
- OSMG2533 45983000 CARB #60N FS-91-P
- OSMG6491 45968000 INT PIPE ASSY FS91-P
72508001 PUMP UNIT PD-07
- OSMG4153 45907010 COVER PLATE FS91-P
MAF 5/4/00
updt jxs 7/21/04
updt tjt 080804
updt OA 5/19/08
updt lmm 3/31/09

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