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Atomic 4 & Stevedore
The Universal Atomic 4 is a four-cylinder, 64.46 cubic inch, 30 horsepower (22 kW) gasoline engine produced by the Universal Motor Company between 1947 and 1984 for use as auxiliary power on sailboats. Over 40,000 of the engines were produced during that time, with an estimated 20,000 still in use today. This “workhorse” engine dominated the expanding sailboat market starting in the 1960’s, and between the years of 1965 and 1975 they were installed on up to 80% of new sailboats in the 25-40 foot (7.6-12 meter) range. Their longevity and reliability is proven by the fact that thousands of sailors still depend on the engine. The Stevedore is a lower horsepower version of the Atomic 4 engine.
Universal Atomic 4
History and Lineage
The Atomic 4 is descended from an earlier Universal Motor Company design called the Utility Four, which was used extensively in World War II by the United States Navy and allies to power the lifeboats for the ships, barges, and tankers of many navies and merchant marine fleets. The Utility Four model was replaced by the Atomic 4 in 1947.
The Difference Between Early and Late Models
Most parts are interchangeable between early and late model engines. However, a few parts (particularly some of those used in fuel, ignition, and cooling systems) are unique to early or late models.
If you have an original engine from the mid-seventies or later, with an oil fill tube over the flywheel housing in front of the engine, you can be reasonably sure that your engine will require all late model parts.
If your engine does not have an oil fill tube over the flywheel housing, it may require a mix of late and early model parts. You will therefore need the following information before ordering:
1. To order ignition parts (including the “Ignitor”), you will need to know if your distributor was manufactured by Delco (used on late model engines), or by Prestolite (used on early model engines). Prestolite distributors are distinctively smaller in size, and their condensers are mounted on the outside of the distributor housing.
2. To order cooling system parts, you will need to know if your engine has a thermostat housing (a small dome-shaped casting) located on the front corner of the head, nearest the starter. This thermostat housing would indicate that your engine has a late model cooling system.
3. To order carburetor parts, you will need to know if your carburetor is made of aluminum or cast iron. Aluminum carburetors were used on late model engines. All carburetors were manufactured by Zenith or Bendix, so the manufacturer does not help in determining your carburetor vintage.
4. To order gaskets, you will need to know (1) and (2) above, plus the shape of your transmission cover. Late model engines used a square cover, and early model engines used a rectangular cover with a curved top (“tombstone-shaped”).
5. Late model (Delco) starter ring gears have slightly smaller teeth than early model Prestolite ring gears. For this reason, Prestolite and Delco starters cannot be exchanged between early and late model engines unless the ring gear (or entire flywheel) is changed with them. In addition, late model ring gears have a slightly larger diameter than early models, so when moving a late model flywheel to an early model engine, either a late model housing will also have to be moved along with the flywheel or the six (6) small boss areas the inside of an early model flywheel housing will have to be ground down. These boss areas are for the ¼ – 20 bolts that hold the flywheel cover on, and they will have to be ground down almost to the bolt holes themselves.
Top End: The Atomic 4 engine is a type of side-valve design, commonly called a “Flat Head” or “L-head” engine with all the intake and exhaust valves located in the cylinder block beside the cylinders rather then in the cylinder head. The valve train consists of the valves, return springs and tappets. Unlike the more complex valve-in-head type design, no rocker arms, push rods or large and heavy “ported” cylinder heads are needed. Unfortunately, a significant drawback to this simple design is the limited compression ratio that is attainable due to the large amount of space required for the side valves to open into the combustion chamber. Surprisingly, the side-valve design is making a comeback in light aircraft engines. Read more about the Belgian D-Motor flat-fours and flat-sixes at Wikipedia.
Like most side-valve type engines which suffer from low compression as described above, the Atomic 4 would undoubtedly benefit performance-wise with “Intake Air Charging”. Supercharging is however NOT recommended, because of the Atomic 4’s “Weak-Bottom-End” described next.
Bottom End: Unlike most 4-cylinder inline engines, the Atomic 4 crankshaft has only two main bearings, one in the front and one in the rear (see crankshaft above). Instead of a main bearing in the middle (between #2 & #3 connecting rod journals) the crankshaft has a counterweight that helps the crankshaft maintain its stability. Because of this engine’s low compression ratio, low power output, and relatively low operating RPM, it does not develop very high loading of the crankshaft bearings and therefore normally never needed the 3rd, 4th, or 5th crankshaft main bearings like other 4-cylinder inline marine engines. However, the lack of support for the middle of the crankshaft does make the Atomic 4 a poor candidate to “soup-up” for competition racing. The Atomic 4 crankshaft can flex and even break, especially during high engine output and/or when under high rotational, gyroscopic and/or inertial loads from the pistons, rods, crankshaft and flywheel. Such loads on the crankshaft can become very high, such as when a small vessel attains high speed in choppy water.
Lubrication: The Atomic 4 engine was equipped with pressurized lubrication to all crankshaft bearings (note the oil feed holes drilled into the crankshaft above). Unfortunately, it lacked an oil filter which would seem to be a worthwhile addition considering that the integral transmission runs in the engine lubricating oil and generates additional wear metals in the oil. A bypass type oil filter can easily be fitted to this engine. Some new engine blocks from “aftermarket” sources like Moyer Marine are machined to be fitted with full-flow type oil filters.
Cooling: Most Atomic 4 engines originally lacked fresh water cooling. The standard raw water cooling system caused cylinder block, cylinder head and exhaust manifold casting breaches especially in salt water from simple “rust-thru” which has resulted in far too many Atomic 4’s being thrown away due to the resulting internal damage. Engines run in salt water would undoubtedly benefit from being fitted with a fresh water cooling system. Freeze damage resulting from a lack of proper winterization has also taken it’s toll of these engines. Fortunately, new engine blocks, cylinder heads and exhaust manifolds have become available from “aftermarket” sources like Moyer Marine.
Exhaust: The Atomic 4 was fitted with a 1 1/4” pipe thread plate at the end of the exhaust manifold for the engine installer to design and fit a proper exhaust system for the particular vessel. The exhaust system could be a “dry” system or a “wet” system depending on the preference. Unfortunately, many exhaust systems were not designed and/or installed according to good practice, and consequently failed, often with disastrous results. Make sure that your engine’s exhaust is properly designed and installed before it fails.
The engine’s exhaust system is one of the vessel’s systems that requires vigilant attention, however it is often the most neglected. Exhaust system components deteriorate rapidly. Make sure that your engine’s exhaust system is in good condition before it fails.
Toxic fumes, flooding bilges, flooded engines, and vessel fires are just a few of the consequences of failed exhaust systems. Make sure that your exhaust system is not going to kill you.
Considering that very few of these engines have suffered failure from any inherent weakness, such as a manufacturer’s design deficiency, manufacturing fault, or defect in materials, its no wonder that this tough little motor has outlasted many of it’s critics and has gained a fiercely loyal following which can be ultimately attributed to it’s rugged simplicity and strength in numbers.
Specifications For Universal Atomic 4 & Stevedore
4-Stroke Cycle Gasoline (Petrol) Marine Engines
⊗ = Data Not Available from Data Source. ? = …? = ¿…? = Data Not Confirmed.
DS = Data Source: …+ = ++ = Data combined from Multiple Sources. BD = BoatDiesel.com = …B.
^ …f = Forum. …w = Webpage. …c = Catalog. …b = Brochure. …s = SpecSheet.
^ …o = Owner’s/Operator’s Manual. …m = Service/Repair/Technical/Workshop/Shop Manual.
^ …1,2,3,A,B,C,etc = Source #, Version, Revision.
CYL = Cylinder Configuration-Number−Liner Type:
^ Cylinder Configuration: u… = Vertical (Upright). I = In-Line.
^ Liner Type: P = Parent/Native Bore. b… = Borable Oversize. …s = Sleevable.
^ ^ Liner type sometimes had to be determined from Shop Manual and/or Parts List.
BORE & STROKE: …mm = Millimeters. …” = …in = Inches.
DISPLACEMENT: …cc = Cubic Centimeters (cm³). …L = Liters (Litres). …ci = Cubic Inches (in³).
⇑ = Base Engine Manufacturer. ⇒ = Engine Marinizer (Mariniser).
MODEL: EC = Engine Code. Cylinder Block Capacity ID Code. TC = Turbocharged.
RATING: See the Engine Duty Ratings at the end of the table.
ASP: Aspiration-Fueling: N = Naturally Aspirated. C = Carbureted.
POWER: kW = Kilowatts. HP = Horsepower. MHP = Metric Horsepower.
@RPM = Power Ratings @ Revolutions Per Minute.
YEARS MFR’d: Beginning-Ending. Trailing “–” (Dash) without an Ending Date = Still in Production.
Click on Data Source Links for Catalogs, Brochures, SpecSheets, Operator’s Manuals, Shop Manuals, etc.
|Universal Atomic 4||Ub||uI-4−P||⊗mm||⊗mm||2-9/15*in||3-1/8in||⊗cc / ⊗L / 65ci|
|Universal Atomic 4||Uh||uI-4−P||⊗mm||⊗mm||⊗in||⊗in||⊗cc / ⊗L / 64.46ci|
|Universal Atomic 4||Wi||⊗-⊗−⊗||⊗mm||⊗mm||2-9/16in||3-1/8in||⊗cc / ⊗L / 64.46ci|
|Universal Stevedore||Ub||uI-4−P||⊗mm||⊗mm||2-9/15*in||3-1/8in||⊗cc / ⊗L / 65ci|
|MODEL − RATING||DS||ASP||kW||HP||MHP||@RPM||YEARS MFR'd|
|Atomic Four − ⊗||Ub||N-C||⊗||7.3||⊗||1000||1947-1984|
|Atomic Four − ⊗||Ub||N-C||⊗||11.9||⊗||1500||1947-1984|
|Atomic Four − ⊗||Ub||N-C||⊗||16.2||⊗||2000||1947-1984|
|Atomic Four − ⊗||Ub||N-C||⊗||20||⊗||2500||1947-1984|
|Atomic Four − ⊗||Ub||N-C||⊗||25||⊗||3000||1947-1984|
|Atomic Four − ⊗||Ub||N-C||⊗||30||⊗||3500||1947-1984|
|Atomic Four − ⊗||Uh||N-C||22||30||⊗||⊗||1947-1984|
|Atomic Four − ⊗||Wi||N-C||⊗||5||⊗||600||1947-1984|
|Atomic Four − ⊗||Wi||N-C||⊗||7.3||⊗||1000||1947-1984|
|Atomic Four − ⊗||Wi||N-C||⊗||11.9||⊗||1500||1947-1984|
|Atomic Four − ⊗||Wi||N-C||⊗||16.2||⊗||2000||1947-1984|
|Atomic Four − ⊗||Wi||N-C||⊗||20||⊗||2500||1947-1984|
|Atomic Four − ⊗||Wi||N-C||⊗||25||⊗||3000||1947-1984|
|Atomic Four − ⊗||Wi||N-C||⊗||30||⊗||3500||1947-1984|
|Stevedore** − ⊗||Ub||N-C||⊗||7.0||⊗||1000||1947-1984|
|Stevedore** − ⊗||Ub||N-C||⊗||11.0||⊗||1500||1947-1984|
|Stevedore** − ⊗||Ub||N-C||⊗||14.5||⊗||2000||1947-1984|
|Stevedore** − ⊗||Ub||N-C||⊗||17||⊗||2500||1947-1984|
|Stevedore** − ⊗||Ub||N-C||13.8||18.5||⊗||3000||1975?-1984?|
|Stevedore** − ⊗||Uh||N-C||13.8||18.5||⊗||3000||1975?-1984?|
NOTES: *2-9/15in is a error in the brochure. It should read 2-9/16in. **Stevedore was Canadian limited horsepower version of Atomic 4. Stevedore: Model UJ – 5101 (direct drive) / UJR – 5102 (2:1 reduction gear) / UJVD – 5103 (V drive, 1:1, 1.29:1, 1.67:1, 2:1 reduction gear options). Available as Automatic. Model UJS – 5111 (direct drive), Model UJSR – 5112 (reduction drive), Model UJVD – 5113 (V drive),
GENERAL INFORMATION – Atomic Four:
L-Head. Compression Ratio: 6.3:1.
Engine Rotation: Counter-clockwise from flywheel end.
Firing Order: 1-2-4-3 (no. 1 on flywheel end).
Reduction Gear Ratio: 2.04:1 or direct 1:1 drive.
Maximum Operating Angle: ~15 degrees.
Fuel: regular leaded gasoline (will function on unleaded fuel with precautions).
Lubrication Oil: SAE 30.
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Universal Motor Company
Engine Duty Ratings
Con = Continuous = Commercial Int = Intermittent = Pleasure Craft Gen = GenSet = Generator (Constant Speed)
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