Detroit Diesel 4-71 Engines

They ran great but they did use/leak some oil. About a gallon every 24 hours run time. I had a mechanic make up a homemade Airsep and that helped with the oily mist problem.

all the DDs leak. The "ho"made remedy was to put a milk jug on the end of the crancase breather tube and put a "diaper" in there. I never had a DD with an AIRSEP, so I can't speak to that. I'm sure Ski and Dmarchand can speak to this issue.

That is pretty much what we did. Used a fuel filter without any element in it. The oil would stick to the sides of the filter canister. It had a petcock in the bottom so I would drain it every 20 hours or so.

Sailor of Fortune said:

I would love to have a boat with a 4 or 6-71... They aren't sexy,efficient,quiet or modern, but at the end of the day,feed them clean fuel and they won't fail you. Many engine diapers required..

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I disagree. Nothin sexier than a pair of Detroit’s in a big ass sport fish.

Comodave said:

They ran great but they did use/leak some oil. About a gallon every 24 hours run time. I had a mechanic make up a homemade Airsep and that helped with the oily mist problem.

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I don't understand that. My twin 6-71's were from a WWII minesweeper and the little soda-can drip cups I hung on the breather tubes collected "drips" but no way was it close to a huge amount. I ran at 1350 rpm. Are you running yours at 1800 full throttle, or perhaps they are ready for an overhaul??

Detroits don't have to leak large amounts. I run my current 671 naturals at 1800 (max continuous) and don't have a noticeable mist. I have pans under the engines that don't seem to collect oil. Several POs ran at 2100, but 10 knots is fast enough for me. The airbox will always leak a small amount. Excessive amounts can be from worn parts. The basic 671 block was designed for about 200 hp. If you're pushing 450 hp the rings and sleeves aren't going to last as long and you'll get more leaks.
The engines are heavy duty and the design allows for ways to ensure each cylinder puts out the same power continuously. Blocks are marked with numbers by each bore. The numbers refer to minor machining differences in manufacturing. They correspond to a correct sleeve for that cylinder. Some mechanics just put the same sleeve in all cylinders so while some cylinders are carrying the load, others are loafing.

My twin 4-71's have 60,000 hours on them. First started in 1980. Commercial use mostly at about 1300 to 1400 rpm. Fuel consumption abot 3.5 gph each at 1400 rpm. While rated at 1800 rpm continuous duty 115hp. I am taking about 65 hp out of each engine at 1400rpm. 34 inch 4 blade propellers. 3:1 gears. In nearly 40 years use these engines have never had their heads off. Still running great. The Detroit diesel 71 series non tubo engines are one of the best least problematic engines ever made. They just don't break.