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D 18 Input Bore

Discussion in 'Early CJ5 and CJ6 Tech' started by 3b a runnin, Jan 12, 2020.

  1. Jan 12, 2020
    3b a runnin

    3b a runnin Member 2019 Sponsor

    virginia
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    I'm planning to change the t90 in my cj6 to a t15. Knowing the D18 had a small input bore, and the T15 needs the bigger bore, the question is, has anyone modified the bore of an 18 to equal the 4in bore of the D20? I have a spare D20 on hand, but I hate to use the only one I have if the d18 can be bored. Pros/cons anyone??
     
  2. Jan 12, 2020
    duffer

    duffer Rodent Power 2019 Sponsor

    Bozeman, MT
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    You can bore the small index D18 case to the larger index. IMO, that will cost you a lot more than just finding an appropriate large index case.
     
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  3. Jan 12, 2020
    3b a runnin

    3b a runnin Member 2019 Sponsor

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    duffer, that may very well be true. I might possibly be able to get this done at my workplace free of charge, possibly. Is the D20 exactly 4in ? I can measure the input on the T15 when I get it modified to fit the oddfire.
     
  4. Jan 12, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb 2020 Sponsor

    Jasper Georgia
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    First of all I am not sure at all. But I am thinking some of the v-6 jeeps used a t- 14 trans with a larger bore d-18. With that said if you had one of those transfer cases it would probably bolt right up to your new t-15. However the main drive gear in the transfer case will probably not be compatible. The transfer case gear the t-14 uses will not fit the T-15 output shaft. also if you are running an overdrive the T-15 it will not fit it either. please ask someone else who has a lot more experience than I for another opinion.
     
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  5. Jan 12, 2020
    colojeepguy

    colojeepguy Colorado Springs 2019 Sponsor

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    I tried exactly what you're doing, many years ago. Long story short, it didn't work. Find a large bore D18 case, or you can even use a D20 case with the 18 gears in it.
     
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  6. Jan 12, 2020
    duffer

    duffer Rodent Power 2019 Sponsor

    Bozeman, MT
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    The large index case is also stronger. And the TeraLow gears fit without grinding the case. All the V6 CJ's used the large index case, T86AA or T14 trans.
     
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  7. Jan 13, 2020
    47v6

    47v6 junk wrecker! 2020 Sponsor

    Washington DC.
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  8. Jan 14, 2020
    3b a runnin

    3b a runnin Member 2019 Sponsor

    virginia
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    OK, thanks guys, I guess i'll just use the D20 case I have on hand and keep an eye out for another for a spare.
     
  9. Jan 15, 2020
    Twin2

    Twin2 not him 2020 Sponsor 2019 Sponsor

    Virginia Beach, VA
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    not having a T15 to look at . does the cluster and reverse shaft stick out like a T14 in rear of transmission
    D18 case has cut out for this
    DSCN0330 - Copy.JPG

    where the D20 does not
    DSCN0329 - Copy.JPG
    just asking
     
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  10. Jan 15, 2020
    TIm E

    TIm E Member 2019 Sponsor

    NW Arkansas
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  11. Jan 16, 2020
    colojeepguy

    colojeepguy Colorado Springs 2019 Sponsor

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    That's the problem I had trying to put a D18 behind a T15. See the thread that TimE linked above....
     
  12. Jan 17, 2020
    nickmil

    nickmil In mothballs.

    Happy Valley, OR
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    Pretty sure the shafts protrude from the back of the T-15. The location of the reliefs in the 20 can depend on transmission it was behind so you’re will have to be test fitted. T-150 used in ‘72-‘79 CJ’s the shafts were flush. Some automatic 20’s had no reliefs at all.
    The 18 can be opened up to the 4” bore size with little strength issues.
     

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