Overhauling a J&T Detroit 671N

After acid cleaning parts, I leave them submerged in water with baking soda in solution. Acid hides in nooks and crannies and will keep corroding things until engine is in service where it flushes much better. Might leave them submerged for a day or two and agitate occasionally.

Ski in NC said:

After acid cleaning parts, I leave them submerged in water with baking soda in solution. Acid hides in nooks and crannies and will keep corroding things until engine is in service where it flushes much better. Might leave them submerged for a day or two and agitate occasionally.

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That's a good idea and I'll do that.

This info is very valuable to me. Thank you for posting the rebuild.

"This info is very valuable to me. Thank you for posting the rebuild."

If you do not have one purchase the Grey Marine WWII students manual, the best book to hold your hand for every DD6-71 repair or rebuild.

https://www.amazon.com/Instruction-Manual-Diesels-Operation-Maintenance/dp/B000CNMSUA

Dealing with rust

There are a number of parts that are corroded, mainly in the cooling system. Some previous owner put plain water in the cooling system instead of coolant and this allowed it to rust. The barnacle buster was great for the bronze parts, but it eats cast iron up.

I got a bucket of evapo-rust. I'm not sure what the active ingredient is, but it works pretty good. It's a little slow, with heavier rust needing a couple of days to treat, and sometimes multiple treatments.



None of the cooling system parts are detroit, they are custom castings made for/by J&T and are no longer available. Some of them are pretty crusty.

This elbow adapter is a good example:



I scraped and brushed the loose stuff off, dropped it in the bucket of evapo-rust and let it sit for two days. Then a rinse in fresh water and a scrub brush to remove any residue, it looks like this:

Does EvapoRust attack cast iron? I have been thinking about the barnacle buster on my cooling system, in situ, but your comment that it eats cast iron worries me. The system is working fine right now so I dont want to start something that might break it. But not knowing its history I worry that it might need the barnacle buster. Is the EvapoRust only a dip tank or is it usable. in situ as a flush?

jimisbell said:

Does EvapoRust attack cast iron? I have been thinking about the barnacle buster on my cooling system, in situ, but your comment that it eats cast iron worries me. The system is working fine right now so I dont want to start something that might break it. But not knowing its history I worry that it might need the barnacle buster. Is the EvapoRust only a dip tank or is it usable. in situ as a flush?

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Yes, the Barnacle buster attacks cast iron, it's active ingredient is Phosporic acid, I believe. I placed a piece of it in the vat of barnacle buster and it bubbles and fizzes. It doesn't affect the rust much either. It has inhibitors that help prevent it from attacking cooling system parts of brass/bronze and derivatives, but it does slowly attack them as well.

The evapo rust is far more effective on iron oxide/rust and does not attack metal, rubber or plastics. I plan to fill the cooling system on the engine side with it, let it sit for a couple of days and then flush it with water a couple of times, and then put fresh water and coolant mix as appropriate. There are a number of areas inside the engine block i can't reach that I want to remove the rust from.

I should say, the raw water cooling system does not have any cast iron in it's path and I would have no reservation using barnacle buster as directed to clean a raw water cooling system.

jimisbell said:

So Barnacle Buster on the fresh water side, flush and add Volvo Coolant, and Evapo Rust on the salt water side and just flush with sea water. Ok, sounds like one of my projects for this fall.

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No, the opposite. The barnacle buster is designed for raw water systems, and can be put in through your sea strainer with some extra options/tools that is also sold by the manufacturer.

I plan to use evapo rust in the 'fresh water' side of the cooling system which consists of the engine block, heat exchanger tank, various pipes and coolant hoses.

I can't recommend anyone use evaporust in their cooling system, it's not designed for that. I plan to do it for my own engine, while it is out of the boat and can be completely flushed after I'm done.

Just use a pressure washer once you get the block stripped and blast out all the passages. No need for chemicals as it is hard to get it all out of what will be a closed system.

sbman said:

No, the opposite. The barnacle buster is designed for raw water systems, and can be put in through your sea strainer with some extra options/tools that is also sold by the manufacturer.

I plan to use evapo rust in the 'fresh water' side of the cooling system which consists of the engine block, heat exchanger tank, various pipes and coolant hoses.

I can't recommend anyone use evaporust in their cooling system, it's not designed for that. I plan to do it for my own engine, while it is out of the boat and can be completely flushed after I'm done.

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But you are NOT using the evapo rust as a flush???


I guess my confusion comes from the fact that I run in Salt water and call the RAW water side the Salt water side and the cooling system the FRESH water side. Even though I dont even have water in it, just Volvo coolant.

I'm using the Barnacle buster as directed, on the Raw water side which is the salt water flowing system as you describe.

I'm only planning to use the evapo rust on the inside of the engine block, not a full flush of an entire system. I don't think you could do that in-situ with a fully assembled engine and cooling system, it would be very difficult to get it all out. If you have coolant in your engine and have always had coolant in your engine, nothing to worry about there. There are flushes made for cooling systems that could be used on the fresh water side 'as directed'.

The cylinder head is back from the machine shop. 19 valves replaced, all seats replaced, bunch of new guides. Almost done working on the cooling system parts and nearly ready to start assembling things.

So was that a typo or do you have 19 valves? Anyway just thinking that this is the perfect opportunity to give your engine a beautiful new paint job and get rid of the IMO ugly Jimmy-green color. What color is your hull? Can match that. Or perhaps bright fire engine glossy red!

That engine has 24 exhaust valves for six cylinders. 4 exh valves per hole. Usually valves are in good shape, but OP said this engine sat wet and dead for years. So valves might have been a mess.

makobuilders said:

So was that a typo or do you have 19 valves? Anyway just thinking that this is the perfect opportunity to give your engine a beautiful new paint job and get rid of the IMO ugly Jimmy-green color. What color is your hull? Can match that. Or perhaps bright fire engine glossy red!

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Don't change the colour. How else will I be able to identify the make of your engine? I keep a can of genuine colour correct engine paint aboard for emergencies. I can easily identify my own engines that way.

Greetings,
Mr. sb. They are a beast of an engine aren't they? Keep the Detroit green. It goes so well with the oil leaks (just kidding).


Mr. k. "I can easily identify my own engines that way." Um...Can't you just identify them by looking into your engine space?

Yes it has 24 valves. 4 valves per cylinder, and they are all exhaust valves. There was corrosion on the stems of many of them so 19 are now new.

I'll be keeping the Jimmy green, i want it to match it's brother across the aisle in the engine room. As a side note, the valve job was just over $2,000. Thankfully the injector cups pressure tested OK and did not have to be replaced, that would have added quite a bit more to the cost of the work.

Good on keeping it Jimmy green.

My 1983 671N Detroits came white from the factory...

I wonder when they made the switch...?

Ski in NC said:

Good on keeping it Jimmy green.

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It it isn’t green it wouldn’t be a Screaming Green Leaker...

Flybull said:

My 1983 671N Detroits came white from the factory...

I wonder when they made the switch...?

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They switched somewhere between 1977 and 1983!!

I've seen 1980 in white. I think you could order a color too.

I had 1981s that were green.

I think it would be nice to dress an engine up with a bit of the old hot-rod touch. How about black crinkle cut valve covers, with the boat's name embossed... would look great in contrast with the Jimmie-green

Nice A-series valve cover. Mine was blue, but in the end it didn’t have enough clearance for the high lift roller rocker arms.......

Finally some more progress. All the parts are now in and assembly can happen.

First up is to make up the piston assemblies. Am using new pistons, rings and wrist pins. As part of an overhaul kit, all the parts needed are included.

I start out with weighing all the components and matching them up to get the best possible balance. This isn't race-engine style balancing which would be more precise and take COG of rods into account, but this is more than is typically done on a heavy duty engine with very heavy rotating components.

I weigh pistons, rods and wrist pins, make notes and then group the components together to get the weights as close as possible. With this done, each assembly is within 1.5 grams of the others, 1/3 the weight of a sheet of paper. With 7+ pound assemblies that's pretty close.

After the components are grouped, the rings are installed. I use a ring expander tool that allows me to install the rings with minimal chance of breaking them.



There are a lot of rings on the detroit with 5 channels for rings and 7 rings in total. From the top, Fire Ring, Compression Ring #1, Compression Ring #2 and then oil rings ABCD at the bottom. The bottom rings also seal the crankcase air out from the bore.

This detroit uses 2 piece pistons with the piston and skirt as separate pieces. they are held together with the wrist pin and a rubber seal is installed between the parts. The connecting rods bolt onto the wrist pins.

In this picture you can see the two pieces coming together for wrist pin installation. There is a bearing insert into the piston that the wrist pin rides on.



Slid together, ready for wrist pin installation, this piston is upside down in this photo with the crown on the workbench:

Nice. Always liked that "cross-head" piston design.

Note the six cyl two stroke has an inherent imbalance, so getting all the masses exactly the same is not real important. Not the same as a six cyl four stroke. That's the reason for the eccentric weights on each end of the cam and balance shafts. They "mostly" cancel the first order vibe, but engine still has a wiggle.

Nice work.