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Dana 18 To Dana 20 Conversion Advice

Discussion in 'Early CJ5 and CJ6 Tech' started by Robert Barrix, May 17, 2020.

  1. May 17, 2020
    Robert Barrix

    Robert Barrix New Member

    Wisconsin
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    I have a 1971 CJ5 Dauntless I am restoring. The Dana 18 transfer case has been broken and welded. I was going to replace it though it is difficult to say the least in finding a replacement D18 large hole. I have found several D20 T-cases and I thought I read somewhere on here that they are the same as the D18 large hole (Super Dana 18's). Anyone know if they are a direct replacement or if modifications are needed?
     
  2. May 17, 2020
    bigbendhiker

    bigbendhiker Member

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  3. May 17, 2020
    colojeepguy

    colojeepguy Colorado Springs

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    Make sure you get a case with the filler hole in the side ,not the back.
     
  4. May 17, 2020
    Twin2

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    here's all I know
    D 18 - rear mounted parking brake = offset rear end
    D 20 = no parking brake . output up high = centered rear end
     
  5. May 17, 2020
    bigbendhiker

    bigbendhiker Member

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    Yep, here's some more info regarding the D20 vs. D18 from Novak's website:

    Copied and pasted from The Novak Guide to the Dana Spicer Model 18 Transfer Case

    The Dana 18 can be replaced by a Dana 20, but only with some work. Some individuals may wish to do so for the quieter, direct-drive, or to use a centered axle. We don't believe that this change is desirable, overall. The Model 20 has a higher and longer output section, and even if switching to a centered axle, this creates unnecessary u-joint strain in early CJ Jeeps. Also, one loses the very useful ability to overcome some trail obstacles by biasing the Jeep's underside somewhat right, as clearance is better with both axles offset towards the passenger side. Also, the Dana 18's native 2.46:1 low ratio is better than that of the Model 20, at 2.03:1.
     
  6. May 17, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Not what I understood the OP is asking.

    This is about building a Dana 18 using a Dana 20 case.

    My understanding is that you can take the guts from the big-hole Dana 18 and put them into a Dana 20 case. This is what Doug is refering to above, I think. You need a case with the fill plug on the side of the case, because the brake assembly (?) will interfere with the plug on the rear? I expect you could drill and tap a fill on the side of the Dana 20 case if needed.
     
  7. May 17, 2020
    Oldriginal86

    Oldriginal86 Member

    Pasadena, Md.
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    I read your question with an understanding that you would use the 20 case and install the inner workings from the 18 inside. This is best of both, low gears of the 18 and strength of the 20 case.
     
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  8. May 17, 2020
    Oldriginal86

    Oldriginal86 Member

    Pasadena, Md.
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    we had the same idea
     
  9. May 17, 2020
    Twin2

    Twin2 not him 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    swapping the cases have been covered over and over
    I only can come up with one question . other than the fill plug
    D 18 front of case . large hole
    DSCN0330 - Copy.JPG

    D 20 front face
    DSCN0329 - Copy.JPG
    no body has said what to do about the cluster/ reverse shafts , with the case swap
     
  10. May 17, 2020
    homersdog

    homersdog Tulsa, Ok 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    I understand the '71 dana 18's were built from the factory with a 20 case casting. That is why Novak calls them "super 18's". Dana probably modified them for the oil path Ron is showing above.
     
  11. May 17, 2020
    oldtime

    oldtime oldtime

    St. Charles,...
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    Really need to get away from referring to the case castings as D18 and D20 because they are not ... and the big hole 1966-1971 D225 Jeep D18 proves my point. Here is some info I posted elsewhere.....

    Small Hole Castings
    My oldest pre-civilian Jeep parts are all long gone but....
    Here is a pic of the oldest transfer case that I currently have.
    This casting was from well after the early Brown/Lipe era.
    It's definitely from a 1960 CJ-3B.
    What you see here is the Dana Spicer embossed casting number.
    The C-18-15-10 designation was common to all DS small bore cases.
    The -3 suggests it's the 3rd variation of this particular casting.
    This particular machined casting weighs in at 25-3/4 pounds.
    [​IMG]


    Big Hole Castings

    The earliest big hole casting I have is from a from an early 1966 Dauntless CJ-5 with big hole D18 (built in December of 1965)
    [​IMG]

    This particular casting weighed in at 23-3/4 pounds.
    As you will see all big hole castings were embossed with C 18-15-24
    The "2" indicates that Dana Spicer was already casting their second version of the big hole case by 1966.

    Here we see a nearly identical casting from a 1967 IH Scout Dana 20.
    [​IMG]

    This particular casting weighed in at:24-1/2 pounds.
    I can only detect one simple difference between the 1966 Jeep D18 casting and the 1967 IH D20 casting.
    The Jeep D18 was tapped for clutch control while the IH D20 casting was not.
    So these two cases obviously have the exact same Dana Spicer casting number.
    Yet the Jeep part number and the IH part number are completely independent.

    Here we see a -5 version of the big hole case.
    This particular casting weighed in at 24-1/2 pounds.
    [​IMG]

    Here you see the only design change that occurred sometime between the -2 version and the -5 version.

    Here again is the -2:
    [​IMG]

    And this is the -5:
    [​IMG]

    Here we see a -6 version of the big hole case.
    This particular casting weighed in at: 25-3/4 pounds.
    [​IMG]

    The -6 version retains the same Texas pattern re-design as the -5 casting.
    But beginning with the -6 the left side fill plug was ommitted and replaced with a rear fill plug.

    Remember....Due to rear bearing cap location the rear fill only works with D20 assemblies and not the D18 assemblies.
     
  12. May 17, 2020
    oldtime

    oldtime oldtime

    St. Charles,...
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    If you must refer to them as either dual shift rail castings or single shift rail castings.
    But better yet refer to them by the embossed numbers located at rear of casting plus the date code as required
     
  13. May 17, 2020
    nickmil

    nickmil In mothballs.

    Happy Valley, OR
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    To add more, if you are still using the stock bellcrank style clutch linkage make sure the boss casting is on the driver side. Some did not have the boss cast in, some had the boss but unmachined, and some had the boss machined and drilled and tapped for the pivot ball. It’s easy enough to drill and tap the boss for the livid ball.
     
  14. May 17, 2020
    Robert Barrix

    Robert Barrix New Member

    Wisconsin
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    Great information. What I gather is that if I can find a Dana 20 casting that matches my C18-15-24-5 Dana 18, then I could swap out gears move rear output to passenger side and install the OD unit. I suppose finding a Dana 20 with that casting number will be about as much fun as finding the Dana 18......

    Thanks everyone!
     
  15. May 17, 2020
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
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    Reading the above my take is that any D20 case can be built up with D18 guts.

    Really these cases, or the small holes for that matter, are not hard to come by, place a wanted ad in the classifieds & you'll probably source on in a few days.
     
  16. May 17, 2020
    Twin2

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    OK will you address my question
    what do you do . with the T 14 reverse / cluster shaft sticking out of rear of transmission
    that's what the two recesses are in both photo's
    when you use a D 20 case , which has none
     
  17. May 17, 2020
    colojeepguy

    colojeepguy Colorado Springs

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    If you look at old time's pics you can see they still have the recess for the cluster and reverse shafts, which yours doesn't have. The 20 came stock behind the T14 so it would have to have that recess. Do you know what your tcase came out of?
     
  18. May 17, 2020
    Robert Barrix

    Robert Barrix New Member

    Wisconsin
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    OldTime - Excellent information!! I was able to locate a D20 with the same casting numbers (version - 5) online. Saving this post for the information! Thanks Again!
     
  19. May 17, 2020
    Robert Barrix

    Robert Barrix New Member

    Wisconsin
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    Yes, That is what I meant to say. Keep the Dana 18 current gears and set up and just move over to a Dana 20 case. I was able to locate the same cast number and version as mine (Version 5) online. This was a very helpful thread for me. Thanks for all the replies!
     
  20. May 17, 2020
    3b a runnin

    3b a runnin Active Member 2023 Sponsor 2022 Sponsor

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    I swapped 18 gears to a D20 case, and used the rear fill hole behind the park brake. Tapped the hole a little deeper and it worked fine.
    On the last conversion, I had the small hole d18 bored out to 4in., same as the d20 to fit a T15 tranny. Still working on the clutch on that one, but that's another story.
     
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