Saving An Historic Otto

By Staff
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Period advertisement for the 4 HP Otto.
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The Otto being rescued from a 40 year slumber in New York.
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John Rex heats up the Otto's cylinder in preparation for removal of the water jacket and cylinder.
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Close up view of the stepped gas inlet valve for speed adjustment. The pick unlatches completely if the engine stops.
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Nate Lillibridge (left) and John check the Otto in its present home in the Large Engine Shed at Rough and Tumble, Kinzers, Pa.
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This story starts on the outskirts of Boston in the 1950s. Major
construction of the infamous Route 128 (now I-95) was underway, and
in its path stood the historic laboratory and workshop of Francis
Blake, an inventor and physicist who, among other things, patented
the carbon microphone used by Bell Telephone Co. The workshop,
dating from the 1870s, had been closed up in the early 1900s when
Blake passed away. It had remained untouched all that time, as if
waiting for him to return and fire up the Otto gas engine that sat
in the corner, once again setting the belts in motion and the line
shafts rumbling.

With bulldozers drawing ever closer it was time for action. A
New York engine collector, with plans to restore the Otto, gathered
up the engine and all the contents of the workshop, removing them
to a storage building in New York state. Remarkably, before
anything was disturbed, the workshop was photographed just as it
had appeared in Blake’s time.

The Otto Engine

Blake’s Otto engine was manufactured around 1890, soon after
Nicolaus August Otto in Germany made a major breakthrough in gas
engine design with his invention of the compression ignition,
four-cycle engine. Up to that time, gas engines were
non-compressing, examples of which were the Lenoir (which resembled
a double-acting steam engine with an efficiency in the 5 percent
range), and the Otto and Langen vertical free-piston engine. The
Otto and Langen used an explosive charge to move a piston to create
a vacuum; the power stroke was developed by Facing page:
Francis Blake’s machine shop as it looked in the 1950s when
Blake’s 4 HP Otto was removed

the atmospheric pressure then returning the piston. The piston
connected to a rack and pinion, itself connected to an overrunning
clutch and finally an output shaft. The clutch ran free on the
rising stroke and engaged during the descending stroke, allowing
power output. The four-cycle engine, however, made its power by
first compressing its fuel/air charge, followed by the subsequent
explosion and expansion of the charge in the cylinder. Initially,
Otto took some ridicule over his engine, as there was a belief that
each stroke should provide power, whereas he was ‘wasting’
three cycles and only deriving power on the fourth. Otto’s
belief in the four-cycle operation proved out, of course, going on
to lay the foundation for the gas-powered, internal combustion

Second Salvation

Forty years went by, and the well-meant act of saving
Blake’s historic equipment seemed to have been forgotten as the
storage building it was housed in slowly deteriorated. The roof
caved in, collapsing on top of Blake’s equipment and exposing
the contents of the shed to the elements, and vandalism.

In the early 1990s Nate Lillibridge and Fred Hendrickson, two
New England engine enthusiasts who had heard about Blake’s
equipment, wanted to know what had happened to the Otto engine from
Blake’s workshop. They tracked it down, but what they found was
a sorry sight. Most of the equipment had deteriorated and rusted
amongst the rubble, but the Otto had survived sufficiently intact
for Nate to consider saving it. This was, after all, an
historically significant engine, and Nate, who has a legacy of
saving unique engines and putting them on permanent display for
everyone to enjoy, was confident he could breathe new life into the
Otto. In 1992 it was recovered from the rubble and transported to
Connecticut, where Nate solicited the help of John Rex to
orchestrate the Otto’s restoration.

Blake’s Otto is a 4 HP horizontal with a single flywheel,
sideshaft, slide valves and flame ignition. While most of the
engine appeared to be intact, there were some missing pieces, so
Nate, Fred and John went back to New York to search and sift
through the rubble of the Otto’s last home. While the effort
turned up some of the parts, a spur and bevel gear with their
common stub shaft (used to drive the sideshaft) and the connecting
rod brasses were a no show.

A closer inspection of the pile of iron showed the restoration
would have to contend with some mystery elements, as well. For one,
there appeared to be some ‘Blake modifications’ made to one
of the spur gears and timing cams. Blake was, after all, an
inventor and a machinist, and he appeared to have been a tinker-er,
as well. The restorers also had to face the challenge of
understanding the engine’s slide valve, flame ignition and
timing operation – this was not your regular hit and miss engine.
John obtained a copy of Otto’s 1877 German patent for the
engine, researched other literature and picked the brains of engine
enthusiasts who had experience with slide-valve flame ignition.

The only other known Otto of this vintage in North America, a
slightly larger, 6 HP unit with dual flywheels, was in Canada at
the Ontario Science Center. Correspondence with the Canadian
Science Center led to the Center’s generous permission to
disassemble their working engine so that precise measurements,
drawings and photographs could be taken.

As the Otto was studied further, some of its mysteries started
to unravel. It was deduced that Blake converted his workshop to
electrical power when electricity became available some time at the
turn of the century. Traditional Yankee that he was, Blake had made
the transition with minimum investment by belting an electric motor
up to the Otto’s flywheel. He removed the gears driving the
side shaft, disconnected the connecting rod and pushed the piston
up into the cylinder (this was fortuitous, as we will see later).
This allowed him to use the Otto’s crankshaft – which was
already belted up to the line shafts – as a jack-shaft, and by
correct selection of pulley size he effected the necessary speed
reduction between the electric motor and the line shafts. The
electric motor, incidentally, was found in the rubble along with
the Otto.

The Ontario Science Center staff had indicated that the gears on
their engine were very noisy, and inspection showed one of the spur
gears on Blake’s engine has been modified using a then-accepted
method of quieting meshing gears. This consisted of fabricating a
gear in the form of a sandwich, made up of discs of rawhide fixed
between two outer steel discs. The gear teeth were then cut across
the sandwiched gear. There was not much left of the rawhide on
Blake’s engine, which had evidently been a delicacy for the
local rodents. Blake also modified the timing cams on the
sideshaft. Their radial location and profile were suspect, but in
the early stages of the restoration not enough was understood about
the engine to make any changes.

Coming Apart

Disassembly started with removal of the flywheel and crankshaft.
The key holding the flywheel was tight and flush with the hub, so a
threaded bolt (whose end was first machined to the same size as the
key) was welded to the key. With a support frame set against the
flywheel, the bolt worked as a puller to extract the key. The
piston, which was stuck, was removed next. Luckily, it had been
pushed up to the top of the cylinder beyond its normal travel, and
the oil and oil residue that had accumulated there kept it from
seizing. It turned out that the piston, piston rings and wrist pin
were all in remarkable and useable condition. The cylinder on the
Otto was an interesting design, consisting of two concentric cast
cylinders; an inner cylinder in which the piston operated and an
outer sleeve, with the space between the two forming the water
jacket. This benefits of this design were referred to in Otto sales

‘… the water jacket is also easily replaceable in case of
accident by frost. It is a separate piece and not connected with
the cylinder, the latter will always remain uninjured.’ After
persuasive application of heat, the two parts of the cylinder were
removed for inspection. The inner bore of the working cylinder was
in reasonable shape at the high-pressure end where the piston had
been lodged, but at the low-pressure end it was rusted and pitted
from accumulated moisture. By judicious sanding and filing it was
brought back to some smoothness, and the entire cylinder was
lightly honed.

The slide valves and side camshaft were then disassembled, and
the missing gears were replaced. For the missing 1:1 bevel gear,
measurements taken off the remaining bevel gear were compared with
stock gears from Boston Gear, and one was found that could, with
modification, be used. A new stub shaft was turned and keyways were
machined in to take the gears. However, it was necessary to
manufacture a new spur gear to provide the 2:1 step-down for
fourcycle timing. Measurements taken from the Canadian Otto were
scaled down and a new gear was cast and teeth cut.

Blake had clearly experienced trouble with the gear meshes
(hence his modification), possibly due to loose tolerances on the
original engine. Measurements on the meshing tolerances and the
bevel gear 90 degree axis alignment showed the spur gears needed to
be closer together by 0.032-inch and the bevel gear needed to be
raised up 0.051-inch to get the bevel gear axis to intersect. John
calculated that by replacing the bronze bearing (in which the stub
shaft ran, located between the bevel gear and spur gear) with a new
one, but with the center hole for the shaft drilled off center by
0.062-inch and then rotated 32 degrees off the vertical, he could
take up both these tolerances. This scheme worked out nicely,
providing clean, quiet-running gears. Working off measurements
taken from the Canadian Otto, John made scaled-down connecting rod
bearing brasses from bronze for the 4 HP Otto.


The next big challenge was figuring out the flame ignition,
slide valve and inlet and exhaust valve timing. The slide is held
in place by adjustable, spring-loaded retaining bolts and is driven
off an eccentric on the end of the sideshaft. The slide valve
oscillates back and forth across the top of the cylinder, and cut
inside it are cavities and passageways. As it starts its slide over
the head it first lines up the gas and air inlet passageways so the
mixture can be drawn into the cylinder on the piston’s backward
stroke. As the piston moves toward its inward compression stroke
the slide continues across the head and seals off the inlet
passageways. Ignition is effected by exposing a flame to the
compressed mixture inside the cylinder, which is accomplished by
using a carrier flame contained in its own cavity within the slide.
This cavity has its own gas supply, which is ignited as it passes
by an external pilot flame. The outside of the cavity is then
closed, the other side of the cavity is exposed to a touch-hole
through the head wall, and the carrier flame ignites the cylinder
gases – just like firing off an old cannon. The carrier flame is
blown out in the process. The piston moves down the cylinder on its
power stroke, and on its return an exhaust poppet valve opens to
purge the cylinder of spent gases For the slide valve to work it
has to form a gas-tight fit, accomplished by machining an
absolutely flat surface on both the slide and its mating surface.
At the time the Otto was built, this was done by choosing a roughly
correct dimensioned block of metal for the slide block and then
using a technique of ‘metal scraping’ to obtain the
required surface flatness. While more modern methods of milling and
surface grinding could have been used to restore the slide back to
its working condition, John decided to use this age-old process in
the Otto’s restoration. He identified high spots on the slide
through a patient process of laying the slide valve onto a master
surface plate inked with blue. Using a hardened metal hand scraper,
John worked the high spots down, went back to the bluing surface
plate to check for flatness, then followed with more scraping,
repeating the process until the whole surface showed blue and the
required flatness was reached.


The timing proved a bit puzzling. The cam lobes on the sideshaft
didn’t seem right, so John sought out a copy of a 1908 German
engineering handbook, ‘Die Gasmotoren,’ to help him
understand the Otto’s timing sequence. With a German dictionary
in one hand and tools in the other, John finally deduced he was
dealing with another one of Blake’s modifications, and it had
to be corrected if he was going to restore the engine back to its
original state and give it any chance of working. The modified cams
were removed, a new cam index position was worked out (together
with the cam lobe rise) and new cams were made and attached to the

Otto Gas Engine Data Sheet

Manufactured under license by Otto Gas

Works, Schleicher, Schumm & Co. Philadelphia,


4 HP

Patent Date




Serial Number





180 revs/min





Flywheel Diam.


Flywheel Width





Crossley (UK) Pendulum type

The final mechanism to be restored was the governor, a Crossley
patented pendulum type. Crossley was Otto’s licensee in the
United Kingdom, and built a nearly identical engine.

The Otto’s slide valve operates off an eccentric driven by
the sideshaft. The large, spring-loaded knurled nuts allow
adjustment of the slide’s sealing pressure once the engine is
running. They are set tight at start up, then loosened once the
engine is running to reduce friction. Barely visible running off
the sideshaft is the belt for the automatic oilers.

The pendulum attaches to an oscillating slide mechanism that
operates in a hit-and-miss mode when the engine is unloaded, the
pick blade either hitting and opening the gas inlet valve or
missing it. An interesting feature of this mechanism is a fine
adjustment that comes into operation when the engine is under load.
This consists of a ‘staircase’ series of steps cut in the
end of the gas valve actuator that the pick hits, the steps
providing modulation of the gas supply. When the engine runs slow
the pick hits higher up the staircase, pushing the gas valve
further open. When it runs fast the pick hits lower on the
staircase, pushing the gas valve less. If the engine stops, the
governor pick rises clear off the gas valve to ensure it shuts

Final Tribulations and First Start

As light finally started to appear at the end of the tunnel a
few tasks remained, including cleaning up all the metal surfaces
and shafts (done by extensive use of draw filing) and cutting the
two-part gaskets for the dual cylinders and water jacket.

Once all the engine’s components were finally acquired they
were sandblasted, cleaned, and then primed and painted in ‘Otto
Black.’ Final assembly commenced, final adjustments were made,
the Otto’s oilers were attached and the water jacket checked
for leaks. A hydrogen gas supply (hydrogen gas is the best
substitute for the illuminating gas the Otto was designed to use,
which was made up of approximately 55 percent hydrogen) was plumbed
in and the pilot flame chimney and burner were set up. The pendulum
governor was assembled and adjusted, the slide mechanism was
adjusted and tested, and everything was checked, and checked

The raised words ‘FEET OFF,’ cast into the end of the
Otto’s frame, give a chilling warning to not start the Otto by
putting one foot up on the frame while pulling on the flywheel. The
crankshaft bearing provides a perfect guillotine as it rotates past
the end of the frame casting. With everything finally set they
turned the flywheel, lit the flame for the ignition, and with a
disconcerting banging and clattering (accompanied by lots of smoke
and concern the cylinder was letting loose), the Otto came to

The noise and smoke at start-up, it has since been learned, is
not unusual to this type of engine, caused by the slide valve
lifting up off the head and allowing some of the explosive gases to
blast out. With some adjustments and refinements to the starting
technique, engine starts are now a lot less dramatic, and the
engine runs smoothly and consistently at its rated speed of 180

The restoration took two years to complete, and in 1995 the Otto
made its debut at the Eastern Connecticut Engine Show in Norwich,
Conn. Nate now has the engine on permanent display in the
‘Large Engine Shed’ at Rough and Tumble Engineers in
Kinzers, Pa., and it can be seen running during their show

Contact engine enthusiast Ivor Hughes at: 212 Rotax Road,
North Ferrisburg, VT 05473-9757. Contact Rough and Tumble Engineers
at: Box 9, Kinzers, PA 17535 or

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