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  1. #1

    Default Mystery part on 6-71 Detroit: Please help me identify it.

    I have a mystery part sitting on the camshaft cover at the rear of each of my two Detroit 6-71's on my boat -- a 42 ft (1976) Uniflite Sportsfisher which was bought in rather poor condition.
    I've attached a photo of one of these mystery parts.
    Can anyone tell me more about what this part is?
    What does it do, and do I even need it at all?
    If, perhaps, it plays an important function, then what do I need to do to service it and bring it back to it's useful life?
    I suspect that it may have something to do with a tachometer which is missing on the flybridge.
    Thanks for any useful input.
    JimClick image for larger version. 

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  2. #2
    Join Date
    Sep 2010
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    Jersey Shore
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    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    Looks like a tach signal generator to me.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  3. #3
    Join Date
    Nov 2009
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    temecula, ca., usa
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    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    I concur with Chris. It is a tachometer ratio adapter AND signal generator.

    The zerk fitting is for lubing the internal ratio drive gears. If you unscrew the cap that is "katycorner" to the generator assembly, underneath it you should find a a receptacle for the end of a cable used with a mechanically driven tachometer. This "feature" would be used by guys like me when working on the engines to monitor RPMs in the engine compartment using a hand held tach. Very fancy and expensive little devices.

    As a matter of fact, I suspect that the only reason you have them mounted on your engines is specifically for this feature. Since it's coming off the cam and it's a two stroke, there is no ratio issue for the drive since it is 1/1. They were absolutely necessary when you needed to take the drive off of the blower shaft because that ratio can be any number of fractions above 1/1. For example: 2/1 for naturally aspirated genset with small bearing blower or 2.05/1 for transport geared applications. And, in that same series,1.95/1 if it were turbocharged.
    I know.....too much information.....but I can't stop.

    They were problematic in that they never got lubed or got over lubed with high pressure shop grease gun systems and the housings would deform. The little bronze shaft bearings eventually fail and they come apart anyway.

    I stopped using my mechanical tach when reliable, compact photo tachs came about in the 80's but, I STILL HAVE IT in my tool collection!

    You could still run a tach with the signal if it still makes a signal. If you don't want to, you can simply remove it and install a cap in it's place to act as a dust cover. otherwise just leave it in place and keep it lubed.

  4. #4

    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    wow, that's some great info. Thanks a lot.
    Come to think of it, I used to have one of those hand held tachs too. It was very useful, but I forgot what for.
    Anyway, I'm in the middle of re-doing all of the "core" 12 starting/battery/alternator/isolater system and I am finding lots of little wires that go no where.
    So, my "core" electrical upgrade project has some project creep, in that, I'm ripping out all that no longer seems useful, and for those things that are important, I'll run new wire, sending units (if I can find them) and gauges. I'm sure you'll see more posts from me in the near future as I come across some of the more mysterious items.
    Thanks again for your considered responses.
    Until the next time...
    Jim

  5. #5
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    Nov 2009
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    temecula, ca., usa
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    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    Looking forward to hearing how you make it happen.
    Good luck with the old girl.
    jimmyd

  6. #6

    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    Thanks jgmo,
    I appreciate your response.
    I finally got this boat up and running. Today I take it out of the slip for the first time since I bought it. Going to the fuel docks to spend some more money!
    The signal generators seem to work, however one of the tach guages is a bit wimpy and doesn't seem to work well. At least I suspect the issue is with the guage And not the generator.
    I'm thinking I'll replace both tach since they read from 0 to 8000 and I only need 0 to 3000. Any suggestions as to replacement guages?
    It would be nice to know more about the signal generators in case I need to fix or replace a part. Any idea about where I can get a schematic and/or parts list?
    thanks again,
    jim

  7. #7
    Join Date
    Nov 2009
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    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    For replacements you might try a place called Diesel Pro in Miami, Fla. I put a link in below.

    http://dieselpro.com/detroit-diesel-...ne-gauges.aspx

    Most big cities will have one or two good speedo and tach shops that can help you get what you already have back up and going right. Although, in the digital age, they are getting harder and harder to find. Mechanical tachs are problematic and I'm not sure their repair is much sought after anymore. I've been out of the game long enough that I have lost my old contacts that I used or I would provide you with someone on the west coast if I still could.

    Hope you get it sorted out and that your maiden sea trial is a success.

  8. #8

    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    Hey, I posted on the thread for 71 series models numbers, needing some help

  9. #9
    Join Date
    Nov 2009
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    temecula, ca., usa
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    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    Hi bmarsh5151

    I sent you a response on the other thread.

    Good luck.

  10. #10

    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    I have decided to quit with the mechanical signal generators, and their guages. Instead, I'll use the AC tap on the alternators to send signals to new tach guages with a range of 0 to 3000.
    The plan is to remove the signal generators and plug the hole.
    Is there anything special I need to know about removing the signal generators from the camshaft and then plugging the holes?
    After I remove them, I'll put them on ebay.com since they are still working fine. It's just they are old and since one of the guages is a bit flakey, and since they range from 0 to 8000,it's time for them to go.
    How much do you think they are worth?
    Thanks for all the discussion and feedback. You guys are great!
    Jim

  11. #11
    Join Date
    Nov 2009
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    temecula, ca., usa
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    Default Re: Mystery part on 6-71 Detroit: Please help me identify it.

    They should simply unscrew from the cam cover. You can't see it in the photo you posted but there should be a "coupler" that looks similar to the one that is on the adapter that is holding the generator. It should allow you to take the whole shebang off and, then, you will only need to cover the opening. Make sure that you retrieve the little drive key if it hangs in the end of the cam. You may be able to use the cap that you can see screwed on to the right side (in your photo) of the unit to cover the hole. I believe the threads will match up nicely.

    As far as what they're worth?? I don't have a clue.

    I hope you have the comparative ratio figured out for your alternators for RPM syncing. If they are the gear driven DELCO-REMY, I believe there are several different ratios that were used. I'll check that and see if I can find what they are. If they are belt driven, then I'm not sure what you may be looking at. It may just come down to pulley diameters. That would be simple.

    Good luck.

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