1. Registration trouble? Please use the "Contact Us" link at the bottom right corner of the page and your issue will be resolved.
    Dismiss Notice

Gm Sm420 Vs Condiesel (gama Goat) Sm420

Discussion in 'Builds and Fabricators Forum' started by T. M., Nov 18, 2024.

  1. Nov 18, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    I'm in the process of looking for a SM420 to swap into my Jeep, and I have found a NOS ConDiesel M561 Gamma Goat transmission, which from what I have read is just an evolved, post '67 SM420 made for the military. I have read on Novak's website that they are basically the same as an SM420, with beefier internals and the different top cover. I see that it also has no PTO cover, as the PTO port is not milled out. I am trying to see if I can use this in my 4-speed swap.

    My questions are:
    Would this trans work with standard SM420 adapters, and
    Will the top cover directly interchange with an SM420?

    I'm contacting Novak with these questions, but I figure I may as well ask here and see what the learned collective know. Any help is appreciated.
     
    Ol Fogie and Glenn like this.
  2. Nov 18, 2024
    Jw60

    Jw60 New Member 2025 Sponsor 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Trails end,...
    Joined:
    Apr 8, 2008
    Messages:
    5,918
  3. Nov 18, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    Ol Fogie and Glenn like this.
  4. Nov 18, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
    Joined:
    Aug 10, 2003
    Messages:
    24,151
    This seems like quite an obscure question. It's also very much on-topic.

    If you have access to the transmission, why don't you measure it? You may get more info along those lines here (ie comparison to civvy transmission measurements) than specifics about a Gamma Goat transmission.

    Is there a forum for M561 at the G? www.g503.com
     
    Ol Fogie and Glenn like this.
  5. Nov 18, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    Yeah, my questions are quite general at the moment because I'm not finding much info and don't know exactly what to look for or what to ask.

    I don't have physical access, but I could ask for some measurements from the seller.
    I applied for membership on G503 a moment ago. Will report my findings.
     
    Ol Fogie and Glenn like this.
  6. Nov 18, 2024
    garage gnome

    garage gnome ECJ5 welder

    Western MA
    Joined:
    Jun 19, 2009
    Messages:
    3,181
    Call Novak, they are quite knowledgable about transmissions.
     
  7. Nov 18, 2024
    3b a runnin

    3b a runnin Active Member 2024 Sponsor 2023 Sponsor 2022 Sponsor

    virginia
    Joined:
    Jan 11, 2015
    Messages:
    1,467
    This
     
    Glenn likes this.
  8. Nov 18, 2024
    73 cj5

    73 cj5 Not ready for the junkyard yet

    Clinton, Mississippi
    Joined:
    Jan 10, 2014
    Messages:
    2,159
    Glenn, T. M., Fireball and 1 other person like this.
  9. Nov 19, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    Got an email back from Novak today. Unfortunately I was told that "We've only seen a couple of [ConDiesel SM420s] throughout the near 11 years I've been here." Even Novak doesn't have information on these transmissions.

    I was able to get in contact with the transmission seller. He told me that he was able to measure his ConDiesel SM420 up against a normal GM SM420, and all of the important dimensions, bolt patterns, etc. were the same between the two. He told me that he also tried the normal GM SM420 shifter top on the ConDiesel and it shifted through gears correctly.


    Since there is so little info available on these transmissions and I might soon have a NOS trans, I'll probably spend some time and get detailed pictures, notes, and measurements of it. I'm also looking at picking up an early SM420 for dirt cheap to use the top cover, and that would give me a good transmission to get comparisons off of. I would like to compile as much useful information as I can so that either Novak or I can provide something of a write-up on these transmissions.
     
    Glenn and Danefraz like this.
  10. Nov 19, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
    Joined:
    Aug 10, 2003
    Messages:
    24,151
    Suggestion - ask that this thread be moved to the early CJ-5 forum from off-topic. Threads in off-topic are deleted at some point (3 mo? 6 mo?); threads in the technical forums are retained as long as the forum exists. Kinda think documenting this in detail for posterity will be a thankless endeavor. You could give some future owner/builder access to your experience through this thread - kinda think it will be enough if they do an effective search. Not if this remains in off-topic.
     
    Glenn, Tralehead, colojeepguy and 2 others like this.
  11. Nov 19, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    Noted. I realize that it won't be the most useful thing since there obviously aren't lots of people building these transmissions, but I figure it's worth the time to learn what I can and share what I can learn. Sure would help me if I could look it up and find some answers.

    Do I need to contact the mods directly, or is asking on this thread enough? Given the options, I figure it should go on Builds & Fabrication(?). Can the mods move it there?
     
    Glenn likes this.
  12. Nov 19, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    **For clarification, I'm referring to civilian, pre '67 transmissions as "GM SM420s" or "Normal SM420s," and transmissions made by CONDEC (ConDiesel to be specific) "ConDiesel SM420s"**

    I've been working on trying to figure out exactly when the ConDiesel transmissions were made, but I'm not finding much on it. The best I can find is that August 1966 was when the first production contract was put out for complete M561s, and it was when the GM 3-53 Diesel engine was finally chosen over the competition. CONDEC won the bid in February of 1967, and the contract was awarded in June of 1968. Production started on June 11th, 1968 at CONDEC, and one can assume it was at or around that time that CONDEC started production of the ConDiesel SM420s. Of course, assumption is the lowest form of knowledge, and I have no real way to know this.
    Before that, the only reference I can find to the transmission used in the Gama Goats is that a Corvair 4-speed trans was adapted for use in a Gama Goat prototype in January of 1965.

    Perhaps CONDEC bought the tooling from GM and used it in their production. This would make sense, as ConDiesel SM420s look almost exactly like GM SM420s and seem to have the same important measurements, but are less finished (PTO port isn't milled out, input mating surface isn't as milled down, output mating surface isn't as milled down, etc.) This of course is only speculation on my part though.

    Long story short, it seems plausible and I daresay likely that the CONDEC ConDiesel SM420s didn't start production until after GM stopped producing SM420s, which is a little bit helpful. Now I can think of ConDiesel SM420s as a sort of 4th generation of SM420, and I "know" (AKA think) that there is little to no overlap in production years.


    Here's where I am getting most of this information from:
    https://truck-encyclopedia.com/coldwar/us/M561-Gama-Goat.php
    https://www.vought.org/special/html/sgamaa.html
    The second website seems to be a webpage dedicated to Chance Vought's work on aircraft and the Gama Goat. So far it is the most complete information I have been able to find in 2024. Seems There used to be a "GamaGoat.com" that is referred to as the website for Gama Goat information, but it has apparently shut down.
     
    Glenn likes this.
  13. Nov 19, 2024
    Jw60

    Jw60 New Member 2025 Sponsor 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Trails end,...
    Joined:
    Apr 8, 2008
    Messages:
    5,918
    Glenn likes this.
  14. Nov 19, 2024
    colojeepguy

    colojeepguy Colorado Springs

    At the foot of...
    Joined:
    Apr 4, 2010
    Messages:
    7,569
  15. Nov 19, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    Interesting. I guess that negates the whole "GM didn't produce Gama Goat transmissions" that I believed. Looked up that contract number, but I couldn't find anything on it. That trans you linked looks alot more "finished" than the NOS one I'm looking at. But the NOS trans I am looking at has a contract number that dates to January 1983.
     
    Glenn likes this.
  16. Nov 19, 2024
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
    Joined:
    Aug 10, 2003
    Messages:
    24,151
    Dunno specifically. They might notice it. Or you could send a PM. They can move it anywhere they want.
     
    Glenn likes this.
  17. Nov 19, 2024
    termin8ed

    termin8ed I didn't do it Staff Member

    Mason, MI
    Joined:
    Dec 22, 2002
    Messages:
    4,527
    I can be bribed with jeep parts:susp:
     
    Glenn, dozerjim and Danefraz like this.
  18. Nov 19, 2024
    timsresort

    timsresort Well-Known Member 2025 Sponsor 2023 Sponsor 2022 Sponsor

    South Lake Tahoe CA
    Joined:
    Jan 22, 2014
    Messages:
    1,658
    Glenn, dozerjim and Fireball like this.
  19. Nov 19, 2024
    T. M.

    T. M. Rubicon or bust!

    Boise, ID
    Joined:
    Jan 11, 2024
    Messages:
    592
    Bah, the one thing I lack! I can offer up a T86 that converts gears into metal flake. Perfect for...uh...stuff. Perhaps you want some bent up 9" brake drums that I forgot to throw away?

    Dad has told me a good 5 times now "Don't turn my house into a junkyard" so I'm keeping parts acquisition at the absolute minimum. I've seen too many deals float by on Facebook Marketplace already. I can't imagine how much junk and/or gold I'll have once I get my hands on a little land...
     
  20. Nov 19, 2024
    timsresort

    timsresort Well-Known Member 2025 Sponsor 2023 Sponsor 2022 Sponsor

    South Lake Tahoe CA
    Joined:
    Jan 22, 2014
    Messages:
    1,658
    If I had a nickel for every time I heard that...
     
    Glenn, dozerjim, 3b a runnin and 2 others like this.
New Posts