Saving An Historic Otto

Hidden for over 40 Years/ a Rare Otto is on Regular View at Rough and Tumble in Kinzers, Pennsylvania

Otto gas engine

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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 engine.

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 literature:

'... 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 sideshaft.

Otto Gas Engine Data Sheet

Manufactured under license by Otto Gas Engine

Works, Schleicher, Schumm & Co. Philadelphia, USA


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 off.

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 again.

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 life.

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 rpm.

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 dates.

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