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Old 02-05-2021, 03:05 PM   #1
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Diamond series cummins review/info

I'm looking at a 1997 Jefferson that is powered by twin Diamond series cummins engines that are rated at 420hp. I'm having a hard time tracking down any info on this particular model of cummins. My current boat has an older single cummins rated at 210. I love the engine, fuel efficient, simple, and easy to work on. The higher horsepower versions of other motors always seems to decrease reliabilty. Any info would be much appreciated!
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Old 02-05-2021, 04:07 PM   #2
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Higher HP versions usually do decrease reliability...If they're run hard and overloaded.
I love my 8.3L 450 Diamonds.
Check out SeaBoard Marine. They speak Cummins

https://www.sbmar.com/

Good luck on the hunt.
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Old 02-05-2021, 04:27 PM   #3
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In addition to the power (more below) the main reliability/maintenance issue is the sea water cooled aftercooler. If maintained regularly (disassembly, cleaning, new O-rings, reassembly and testing- see Sbmar.com) they can be operated reliably. If you do that maintenance and run them like your 210s they should last just as long.

But 420 hp is a lot for a 6 liter engine and if the PO ran them hard they might be rather worn out. If you want to run them hard, you run the risk of a seemingly small failure trashing the engine in addition to the wear. I would certainly add a Borel raw water alarm to give a bit more time to respond to a R/W cooling system failure at high loads.

Join the Sbmar Cummins forum and read all of the posts about this engine and its somewhat less stressed sister, the 370 to get a feel for the issues.

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Old 02-05-2021, 04:48 PM   #4
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Are they 5.9l or 8.3l ?
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Old 02-05-2021, 05:35 PM   #5
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If they’re 420 horse from 1997. they’re 8.3 liter “420C’s.
Right after or around ‘97, the 450 diamonds (also 8.3) were released at 430 hp. Almost identical motors.
Like his 210’s, solid motors.
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Old 02-05-2021, 05:55 PM   #6
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Here's a great place to start
https://www.sbmar.com/articles/the-c...-engine-story/
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Old 02-05-2021, 06:12 PM   #7
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The 420 is going to be an 8.3liter. Very good engine. Two versions built around '97, one the tag says "420hp", later version says "450hp". Later version has a Bosch P7100 inj pump, which has nicer manners than the earlier one. But both reliable.

I did engine surveys for 25yrs and when it came time to spec an engine for my new build, I went with a used 450. Been nearly flawless for 4000hrs.
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Old 02-06-2021, 09:20 PM   #8
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Thanks for all the replies. The engines have low hours, around 980, I'm hoping they haven't been run too hard. Regardless, I'll get an engine survey if I decide to move forward.
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