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SM420 to bell housing question

Discussion in 'Early CJ5 and CJ6 Tech' started by p38L5, Jul 5, 2012.

  1. Jul 5, 2012
    p38L5

    p38L5 Member

    Napa CA
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    Feb 5, 2012
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    94
    I am building up a Buick 231 to swap into my M38A1, I have an Advance Adapters bell housing to mount an SM 420 and the adapter to mount my dana 18 to the 420. There are 4 mounting holes on the 420 where it mates to the bell housing. The top two are drilled to allow the bolts to thread into the bell housing, the bottom two holes are threaded - so it appears those bolts are threaded in from the bell housing. All four corresponding holes on the bell housing are threaded to allow the bolts to be threaded in from the tranny side. My question: What is the preferred method for the bottom two holes - drill out the tranny side, drill out the bell housing side, does it matter? Would it be beneficial to use studs in the bell housing in place of bolts?

    Thanks,

    g
     
  2. Jul 5, 2012
    '72CJ5

    '72CJ5 Member

    Modesto,...
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    If I remember correctly when I was researching my np435, I was thinking about running the 420 and ran across this issue discussed a few times through google and people were saying that they drilled out the lower two tranny holes to bolt through the tranny and into the bell housing. The only thing i see benefiting you this was is that you can remove the tranny without pulling the bell off also. I would search google for answers as well as here and go from there but if it was me, i would drill out the tranny threads and bolt straight through the tranny and into the bell housing. happy searching.
     
  3. Jul 5, 2012
    Smokeater

    Smokeater Sponsor

    Browns Summit, NC
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    Best my memory serves me, drill out the bottom two bellhousing holes and assemble tranny and bell before matting to the engine. Do a search as this has been discussed in detail many times.

    Sent from my MB855 using Tapatalk 2
     
  4. Jul 5, 2012
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

    cordele, Ga.
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    Do what is logical. If you drill out the tranny holes then you have the option of removing tranny only or both tranny with bell. If you drill the bell then you have only one option. For me it is a no brainer.
     
  5. Jul 5, 2012
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    So would either option leave the nuts on the outside?

    My first inclination is to drill out the aluminum side, since threads in cast iron will be stronger than threads in aluminum.
     
  6. Jul 5, 2012
    Warloch

    Warloch Did you say Flattie??? Staff Member

    Falcon, CO
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    Mar 4, 2003
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    As Tim states - the threads in the cast will be stronger. As others point out - think it through for what works for you as pulling the whole thing can be a PITA for some. I had to pull the full drive train to fix a clutch issue a couple weeks ago. I could have just dropped the tranny/tcase, but since the bottom P bolt had to have a nut inside the bell (like you would have the bolt head if you drill the bell) the full drive train had to come out.

    Now I have built my rigs with that in mind, though I could have saved time and hassel by not having to pull the engine too. I also have an overhead hoist in my shop with a good tranny jack, so I can do it all myself.
     
  7. Jul 5, 2012
    '72CJ5

    '72CJ5 Member

    Modesto,...
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    My tranny is bolted TO the bell as is most every other tranny I have seen or dropped. But as stated, whatever works better for you will be the way to go.
     
  8. Jul 5, 2012
    nickmil

    nickmil In mothballs.

    Happy Valley, OR
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    I have seen factory SM-420's bolted both ways. May be application or year, not sure on that. One thing to check before drilling out the holes in the transmission is if you will have clearance for the bolt head or nut if using a stud AND a tool, whether socket or wrench to turn them....
     
  9. Jul 6, 2012
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

    cordele, Ga.
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    Novak gives both examples for the SM420 install depending on which bell is used. One is to drill the lower tranny holes and the other is drill the bell.
     
  10. Jul 6, 2012
    p38L5

    p38L5 Member

    Napa CA
    Joined:
    Feb 5, 2012
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    Thanks guys,

    Upon closer inspection, (actually get down and look at the tranny with glasses on) one lower hole on the tranny is blind, and the other ear has ridges which will interfere with a bolt head or nut. So, on this tranny it's top bolts into the bell housing, bottom bolts into the tranny.
     
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