BOMB SQUAD: Atomic-4 Gasoline Engine

Universal Atomic-4 engine

It seems hard to imagine now, but for about 50 years most auxiliary sailboat engines were gasoline-powered. By the mid-20th century the majority of these (in North America, at least) came from just one source, the Universal Motor Company. From 1947 to 1985 Universal’s famous Atomic-4 engine (aka the Atomic Bomb) was installed in over 40,000 sailboats ranging in size from 25 to just over 40 feet. At its peak in the 1960s and 1970s, Universal controlled about 85 percent of the market for sailboat auxiliaries.

Some 20,000 Atomic Bombs are still in service today. For cruisers who favor older, less expensive sailboats, it is still quite common to find these antique power-plants lurking inside engine compartments. Learning to live with a Bomb is thus, for some, still an important part of owning and maintaining a boat.

One reason for the Bomb’s great success was that it was not merely a marinized car or truck engine. It was instead designed and conceived from its mounting bolts up as a marine engine, thus was considerably more reliable than its competitors. The old Bomb did, however, have certain weaknesses.

Most Bombs, for example, had raw-water cooling systems (sometimes referred to as “open” systems) where “raw” seawater is circulated directly through the engine block to keep it cool. The build-up of scale and corrosion inside the block over time ultimately leads to blockages that can cause serious overheating problems. Also, raw-water-cooled engines tend to run at lower temperatures. This leads to condensation in the engine and less efficient fuel combustion, which causes carbon to build up on valves and piston rings.

By contrast, almost all marine engines today instead have freshwater closed systems. Here fresh cooling water circulates in a closed loop between the engine and a heat exchanger, through which raw water from outside the boat also circulates, drawing away heat from the fresh water. This allows for more efficient fuel combustion and creates fewer corrosion problems.

As a gasoline engine, the Atomic-4 also has an electric ignition system with spark plugs. These often suffer in the damp and corrosive marine environment. Finally, of course, gasoline is a very volatile substance; it is also considerably less efficient than diesel fuel, which effectively doubles a boat’s range under power without any increase in fuel-carrying capacity.

For all these reasons, the current conventional wisdom is that only diesel engines should be installed on sailboats. Owners of older boats with Atomic Bombs aboard are often urged to repower with diesel engines, and for the past 20 years replacing old Atomic-4s with new diesels has been a cottage industry of sorts. At least two manufacturers (Westerbeke, which bought Universal, and Beta Marine) currently offer drop-in replacements that fit easily into engine spaces originally configured for the Atomic-4.

Repowering a cruising sailboat
A Beta Marine diesel drops aboard to replace an old Atomic Bomb

But consider this: most surviving Atomic-4s are found on older, smaller boats with market values ranging generally from $8,000 to $30,000. Installing a new diesel engine costs roughly $7,000 to $10,000. A new engine does increase a boat’s resale value (in fact this is one the very few systems upgrades of which this is true), but even if you sell your boat when the new engine is young, you’re unlikely to recoup more than half your investment.

For casual coastal cruisers who don’t need extra range when powering, learning to live with a Bomb may be what makes the most sense. There is a strong, mutually supportive subculture of Bomb owners, parts are still available, and the old engines can be tweaked and improved. For example, a freshwater cooling system can be installed, a more reliable electronic ignition system can be added, oil filters and water pumps can be upgraded, and so on.

As for the old bugaboo about storing gasoline aboard, most cruisers carry more than enough gas for their dinghy’s outboard to blow up their boats in any event. As with so many things, safe operating procedures are the key. An Atomic-4’s engine compartment should always be well ventilated with a blower fan prior to starting the engine so as to evacuate lingering fumes. Follow this simple precaution and there’s little chance a Bomb will detonate. Indeed, as far as I am aware, there are no known instances of a Bomb ever exploding.

If you’re a bluewater cruiser, however, you should almost certainly jettison your Bomb in favor of a diesel. Diesels are much easier to maintain over the long run and on long voyages the extra range they provide is invaluable. The only bluewater sailors with gas engines I’ve ever encountered were extreme minimalists on smaller boats (or simple multihulls) that carried only outboard engines for auxiliary power.

If you do have a Bomb on your boat and would like to maintain rather than replace it, you should definitely get in touch with the folks at Moyer Marine. They are extremely helpful and are a veritable fount of Atomic wisdom (and  parts). Indigo Electronics is also a good source for after-market upgrade kits.


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7 Responses
  1. vicy

    About the wonderful, I am very pleased to see this article, learn some things, and view the text is recognized. Thank you for sharing. At the same time ,i love buy snow thrower , diesel engine supplier , Modern bathroom cabinets and Solenoid switch .I think I would be more helpful, thank you!

  2. M. Ferris

    A load of crap…. what is it with sailors and diesel. They smell, they’re noisy, and all those around me with diesels have had far more problems with them than we have had with our old A4. It’s quiet, easy to service and runs well.

    What about your dinghy engine.. is it a diesel? What about all the power boats out there are they all diesels??

    This article is the biggest load of crap I’ve ever read.

  3. Bill Morris

    This article gives sound advice regarding diesel engine safety and economy. Ferris disagrees because he is probably defensive about not being able to afford a proper diesel. I dumped by Atomic Four 17 years ago for a Beta Marine and circled the globe on a 1966 Cal 30. I couldn’t imagine motoring directly into 35-knot winds in the northern Red Sea with some funky gas engine. The money for the diesel was well spent. Bill Morris, S/V Saltaire, Los Angeles Harbor

  4. Gak

    Hello all atomic 4 lovers. She’s smoking like crazy , seems to be holding oil and starts right up. Any ideas just got her last week.

  5. Navy Snipe

    BULL! Twice the distance on the same fuel allocation is BULL as well. These engines are now going for nearly as much as a diesel. LOOK at the pricing at Moyer Marine. They are LESS problems once you get an electronic ignition module and electric fuel pump installed than what could ever be expected from most diesels. Gasoline doesn’t grow the black sea snot which kicks up in the tank in ALWAYS heavy weather and clogs the diesel’s fuel system. Rebuilding a carburetor on an A-4 is a one hour job TOPS by a careful newbie at best. THOUSANDS can be spent replacing diesel fuel system parts. Bill Moyer of Moyer Marine helped one person I know who was a newbie mechanic rebuild his own with expert advice and guidance. He took the engine home with him for the off season. 800 dollars in parts later he had an engine good for another 30 years without even having to over bore the block. Most can be rebuilt the 1st time around without over-boring. A fresh water cooling kit is a VERY good investment when it comes to these engines. The single LARGEST issue with them is that MOST of the props that were mated to them were too much and prevented them from attaining their true performance potential. Most were 12″ dia. 2 blade models with 12″ of pitch. These props lugged the engine and kept the RPMs WAY below where they should be with these engines. Indigo offers a 10″, 3 blade, 7.4″ pitch prop which they specifically designed for this engine. With it the engine will drive an 11,200 lb Tartan 34c at hull speed (6.7kts) with 2,400 RPM, still 600 RPM below the red line. I heard MANY stories of how these engines were not worth running over 1800 RPM because all that was there was smoke and noise and nothing else. This is typical of the mating of a bad prop to these engines. These engines will in fact outlive many of the diesels which replaced them if kept well maintained. People are now waking up to what these engines in fact have to offer and many are now, more than ever, hanging onto them. And that sailor who rebuilt his own for 800 dollars? The yard wanted 8,000 dollars to do it. Once on the bench these engine short blocks can be easily honed and rebuilt in a day. And unlike those who got screwed by buying the cast iron Farymann diesels of the same vintage, you can still get parts for an A-4 UNLIKE the those Farymann engines which were offered up by many boat builders during that same period.

  6. Navy Snipe

    Gak Sunday, 23 July 2017
    “Hello all atomic 4 lovers. She’s smoking like crazy , seems to be holding oil and starts right up. Any ideas just got her last week.”

    Strip the manifold, transmission, alternator and starter off her. They get real light and can be taken out that way by hand easily. Take her home and look into for parts. You can probably get away with just honing the cylinders and sticking new rings, valve springs, and bearings into the short block and running her for another 30 years. If she was raw water cooled you may want to take her in and get her hot tanked. If there is corrosion where the raw water runs through the head gasket you may want to spend some additional money and get the head milled and a copper head gasket shim from Moyer to restore the low 6.4:1 compression ratio. These engines have NO center crank shaft bearing and raising the compression ratio or ANY performance mods is a very bad idea. The crankshafts last so long on these things that you can’t even give one away. If you put an electronic ignition module on make sure you get a BALLASTED ignition coil to go with that. Getting rid of that diaphragm fuel pump and going electric is the best thing there though you will most certainly want to change out to at least a new OEM unit as very bad things can happen when those diaphragms start leaking fuel into the crank case. These engines take very few specialty tools to work on. Those you can get from Moyer too at a reasonable price. The transmissions are simple too. You can get a rebuilt reversing pak for around 450 dollars or rebuild it yourself too for far less. If the transmission still feels good I would not even mess with it. These gear boxes used ATF, NOT gear oil. You will roast the bearings in them if you put in regular gear box oil. I would at least change out the ATF in the gear box though if I were doing this and replace the front and rear transmission seals while I had the thing out.

  7. denis

    i purchase a oday27 with a atomic4 1973 .original engine burn oil on 2!!! i sail in lake champlain i use21 liter this year .i start to removing it for a rebuilt or maybe put a other engine.i am a mechanic so !! ok gas,diesel,or electric…depend what you do with your sailboat….the danger of gas vapor a big bla bla bla.90%of cruser have gasoline engine 99%of the car are gasoline and no fan wow!!!!and 95% of sailboat have gasoline and propane …..if you have a diesel many boat dont have a fan to evacuate the vapor so if you have a leak of propane or gasoline for your dinghy a big boom!!! my subjestion is to put a fan on all boat

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