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Old 08-12-2019, 12:59 PM   #61
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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.
Understood, that's why I didn't go to the lengths I would go to on most engines. I figured I'd at least match up the components to get the weights as close as I could and they ended up coming out really close.
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Old 08-16-2019, 02:37 AM   #62
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When you put the air box covers on, torque them to spec (I think 35 ft pounds). Over tightening the holding bolt will pull the cylinder out of align when the engine is hot and cause premature wear of the liners, etc.
I've seen a couple engines damaged when unknowing mechanics were chasing air box leaks and just started over tightening cover bolts until the sound went away instead of using a new gasket or a sealer on the old gasket.



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Old 08-21-2019, 12:31 AM   #63
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Next up I dry fit the liners. This allows me to check the height of the liners below the deck prior to final installation and order up any shims that are required to get them to sit below the surface of the cylinder block deck by the specified value. I used a depth gauge to measure them.



With that done, it's back to the piston/rings/rod/liner assemblies.
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Old 08-21-2019, 12:37 AM   #64
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With all the rings installed, I slide the wrist pins in. There is a rubber seal that goes between the piston crown and the skirt. This makes for a tight fit. I use engine assembly lube on any moving parts as I put them together.

The wrist pins just slide into place easily once things are lined up.



With the two stroke design, it is important to keep oil out of the cylinders because it will burn just as well as diesel fuel. Oil can come out of the wrist pin interface and be put onto the cylinder walls. The upper compression rings are not designed to remove oil from the cyliner walls, the lower rings do that. So we have to seal off the wrist pin. A metal plug is installed in the piston skirt.



The plug is then formed into a dished shape that locks it into place to seal things up using special die and a hammer.

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Old 08-21-2019, 12:38 AM   #65
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After dishing the covers, they are tested with a special suction cup attached to a vacuum pump with a built in gauge. If they hold vacuum they are sealed.

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Old 08-21-2019, 12:41 AM   #66
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Now the pistons are ready to have the rods bolted on.



They get torqued down with a torque wrench and the rods are installed.



All six assemblies completed.

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Old 08-21-2019, 12:50 AM   #67
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Now I can put the cylinder assemblies into the liners. I use a ring compressor that is the proper size for the 71 series detroit, 4.250". I bought a $50 ARP brand one, the 'proper' tool is over $400 for the name brand. It's nicer, but this got the job done.



The tool is tapered so it's larger at the open end and exactly 4.250 at the installation end. The taper compresses the rings to fit the cylinder walls with some spring tension. I line up the ring gaps to match the specifications and then place the piston into the tool and get it ready to install into the liner.



I then line the tool and piston assembly up on the BOTTOM of the cylinder liner and give a firm quick push to install it into the liner. You have to use a smooth, firm motion or the rings can pop out between the tool and liner and refuse to go in and then you have to start over. With the design of the rings, you need to insert the pistons into the bottom of the liner.



With that completed 6 times, along with pushing the rod bearings into place that completes the cylinder assemblies and they are ready to go into the cylinder block.

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Old 01-25-2020, 10:09 PM   #68
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Haven't posted in a bit, but I've been busy working on the engine with some progress.



Getting the cooling system together and will be ready to fire it.
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Old 01-26-2020, 01:48 AM   #69
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Old 01-26-2020, 06:43 AM   #70
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sbman, I noticed that your original post was from last April. Are you just working part time with no hurry on your boat rebuild, or spending more time on the hull bottom, flybridge, etc., instead of the engine?
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Old 01-26-2020, 10:21 AM   #71
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I thought my F.L. pistons were big, those are HUGE. Are they new or were you able to reuse any of the original, bet they were costly

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Old 01-26-2020, 01:49 PM   #72
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DD pistons are 4.25" dia, about the same bore (??) as the Ford. But the DD ones are much taller as they have to cover the intake ports fully when the pistons are all the way up.

They are also cast iron which is different. Most small diesel pistons are Al.

And not expensive at all. A full set of liners/pistons/rings/bearings/seals/gaskets/etc for a 671 is something around $2k, but it has been a while since I looked one up. Much less than other engines.
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Old 01-26-2020, 02:53 PM   #73
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Great post, thank you for sharing your work so far.
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Old 01-26-2020, 08:24 PM   #74
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Great photos and your comments are very educational.
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Old 01-26-2020, 08:32 PM   #75
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This will be aprox. my 10th full size engine rebuild. Over the years, I've rebuilt multiple Toyota engines (including their big 1FZ-FE inline 6), a couple of ford 390 V8, a couple of honda engines, a Lycoming aircraft engine, a datsun, and a couple of others that slip my mind...
I, too, find this very interesting and appreciate your extra effort in posting this. My last boat had 892TAs (J&T, 790hp), that had to be rebuilt twice. I didn't think I was up to the task although I had successfully rebuilt several small block chevys and a Jaguar in-line 6 (and its transmission). How much more skill is necessary to a Detroit rebuild?
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Old 01-26-2020, 08:50 PM   #76
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Mako: the boat project is a hobby for me, I work on it in my spare time so with all the projects that need done, working by myself after work and after family commitments, etc, it takes a long time to get things done. I probably have 150 hours into the rebuild so far. I'm also reconditioning most of the parts myself. Cleaning hardware, making hydraulic hoses, even just the gasket scraping requires more time than you would think. Dealing with rust has been the hardest part.



Pete, Ski has it right. The rebuild kit included pistons, rings, liners, bearings, wrist pins, and a full gasket kit, it was about $1500. It cost $1800 to have the head rebuilt at a machine shop. I also had the blower rebuilt for $800. In all I expect to have about $7000 in the overhaul total.

MyTraveler: This engine Is actually simpler than most of the other ones I've done. The challenge is just in the weight of the parts. Having a lift to move the parts is necessary. I also had to get some larger tools just because things are big. If you have done V8 engines, a Detroit is no problem, just get the shop manual and study it.
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Old 01-26-2020, 10:07 PM   #77
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The challenge is just in the weight of the parts. Having a lift to move the parts is necessary.

Yep I remember when I in-framed my 6-71’s. I couldn’t budge the heads and there wasn’t really enough headroom to build a pulley system. So I hired two big gorillas to lift and then set the new heads.

There’s something to be said for an engine with individual heads!
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Old 01-26-2020, 10:29 PM   #78
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We had J&T 6-71N’s in our 1977 42’ Uniflite SF , loved them, my all time favorite Detroit’s. Rebuilt at 4000 hours in Costa Mesa. Those engines took us through thick and thin, never let us down or missed a beat.

Not sure if I missed it, but are you marking each torqued bolt head with a white pen to keep track of your torque sequence?

I did a survey on a rebuilt 6-71N out of a 43 Viking Double Cabin out of Marina Del Ray in the 90’s that had a low hour failure after one of the rod cap bolts were missed in the torque sequence.
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Old 01-27-2020, 08:51 AM   #79
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I can't believe I didn't see this thread until now. It's really interesting to see how differently these engines are put together compared to a gas engine which are the only things I've rebuilt.


What is a typical rebuild interval for a 2-stroke DD? One person said his had been rebuilt twice, and just now someone said at 4000 hrs. That seems quite frequent compared to modern engines where I think 10k hrs is a min expected interval, and many run 20-30k hrs. Now granted, marine (harsh environment) and pleasure (intermittent use) is probably the worst use case, but the rebuild interval still seems low. I realize that in this particular case the rebuild is all because of abuse so isn't a good example by itself.
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Old 01-27-2020, 09:33 AM   #80
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I too think this is a marvelous thread. Not only is Ski's great knowledge and experience exemplary, but on boatdiesel Randy Sparks is a wonderful DD source.

Years ago I managed a place that had many different diesels in service, one being the 6-71s. Due to shift schedules the rebuilds didn't always go well with different hands getting involved. The best results were to assign one and only one mechanic to each engine. In following your thread I'm again reminded as to the old adage "too many cooks spoil the broth."

BTW, the Uniflite model you have is pretty well laid out. You'll have many happy cruising years in front of you. Keep up the posts and especially regarding the sea trials.
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