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D 18 Input Bore

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

  1. 3b a runnin

    3b a runnin Active Member 2023 Sponsor 2022 Sponsor

    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??
     
    3b a runnin, Jan 12, 2020
    #1
  2. duffer

    duffer Rodent Power

    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.
     
    duffer, Jan 12, 2020
    #2
    3b a runnin likes this.
  3. 3b a runnin

    3b a runnin Active Member 2023 Sponsor 2022 Sponsor

    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.
     
    3b a runnin, Jan 12, 2020
    #3
  4. Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

    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.
     
    Ol Fogie, Jan 12, 2020
    #4
    Wllys and 3b a runnin like this.
  5. colojeepguy

    colojeepguy Colorado Springs

    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.
     
    colojeepguy, Jan 12, 2020
    #5
    3b a runnin and Rich M. like this.
  6. duffer

    duffer Rodent Power

    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.
     
    duffer, Jan 12, 2020
    #6
    Wllys and 3b a runnin like this.
  7. 47v6

    47v6 junk wrecker! 2023 Sponsor 2022 Sponsor

    47v6, Jan 13, 2020
    #7
    3b a runnin and Twin2 like this.
  8. 3b a runnin

    3b a runnin Active Member 2023 Sponsor 2022 Sponsor

    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.
     
    3b a runnin, Jan 14, 2020
    #8
  9. Twin2

    Twin2 not him 2024 Sponsor 2023 Sponsor 2022 Sponsor

    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
     
    Twin2, Jan 15, 2020
    #9
    colojeepguy and ITLKSEZ like this.
  10. TIm E

    TIm E Aggressively average

    TIm E, Jan 15, 2020
    #10
    ITLKSEZ likes this.
  11. colojeepguy

    colojeepguy Colorado Springs

    That's the problem I had trying to put a D18 behind a T15. See the thread that TimE linked above....
     
    colojeepguy, Jan 16, 2020
    #11
  12. nickmil

    nickmil In mothballs.

    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.
     
    nickmil, Jan 17, 2020
    #12