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Old 13-03-2015, 13:12   #1
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switching out A4

Tossing old Atomic 4. Would like to get a rebuilt diesel. Need advice since I don't know jack about engines. What hp for a 1971 Bristol 30'? Does it have to be a marine engine? Best brand? approx price (so I don't get totally screwed). I am in Texas, boat is in Ct. have not seen engine, this is on advice of surveyor.
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Old 13-03-2015, 13:36   #2
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Re: switching out A4

Yanmar mount spacing does not lend them to drop in replacement for an A4. The aft spacing is too wide if the hull is narrow. Diesels are also taller than the A4 so be sure there is the headroom. Gas engines don't need a fuel return to the tank. You'll have to add one which could mean a new tank.

As far as HP, depends on how you use the boat. 10HP will get you 5K in flat water/no wind conditions. Beating into winds and seas will probably take more HP. 20HP would be a good all around engine size for cost, weight, and power. If you plan on pulling skiers, might want to go larger. The larger the engine, the more fuel you'll burn which could be an issue if your tankage is small.

FWIW, have 27HP in my 13,000#, 35' boat. It works great for all conditions that I've encountrred. Did fine with 24Hp in a 20,000# boat in more than 10,000 mile cruising.
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Old 13-03-2015, 14:07   #3
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Re: switching out A4

Nothing wrong with an Atomic 4
If it's totally hosed you can get a replacement from
Moyer Marine for 5K
They also have all the parts you would ever need
Could save you a bundle
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Old 13-03-2015, 14:07   #4
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Re: switching out A4

beta marine makes a drop in replacement for the a-4, think they're like 6-8 grand brand new. go to betamarine.net.
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Old 13-03-2015, 14:09   #5
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Re: switching out A4

If you're looking for a direct drop in look at the Beta Marine engines. They make a really nice marine version of a Kubota engine that are very reliable and smooth. The plus is they've set up the engine mounts to fit an A4 almost exactly!

Are you planning on doing some long range cruising? If so definitely go the diesel route. However if you'll just be doing some coastal cruising or weekend use for the boat the Atomic Four is a great engine. Many people are afraid of the gasoline aspect, and it does deserve serious respect for safety. However I believe it is still the most widely produced marine engine in existence. It was made for right about 40-50years. There are many of them still in regular use. If you were to look at a safety report that accounted for the per capita or percentage of the engines in use I'd be the A4 would rate quiet well. I just sold my old boat with an A4, the engine was from 1959, the boat a 1961. The engine had new bearings/ rebuild in 2001, and was running like a top when sold. I've heard in general you'll spend about $9-14k re-powering with a diesel. That will of course depend how much you do yourself and how many parts like the fuel tank you can reuse. Plan on replacing the prop, prop shaft, coupling, engine panel, controls, muffler, and cutlass bearing as well, because they will likely be the wrong size. You can still get a very nice rebuilt A4 from Moyer Marine and get out of the re-power for about $3-5k.

In short our A4 was great it: required very little maintenance, gave .75gph burn at 6.4knts, smooth, and reliable. It did make me very conscious if I left the fuel valve open or not, and of running the bilge blower and checking the engine compartment before starting.

Good luck either way.
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Old 13-03-2015, 15:14   #6
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Re: switching out A4

Great info and thanks to all.
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