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swap from a 700r4 to a th350

Discussion in 'The Garage' started by rlk5, Feb 20, 2006.

  1. rlk5

    rlk5 Registered Member

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    I'm currently swapping the stock 700r4 out of my 84 k5 to a slightly beefed up th350. I was wondering if someone who has done this could tell me what all i need to change out/modify. pictures of any modifications would really help. by the way i think have a np205 transfer case...i can't really tell due to 22 years of mud and dirt. any input will help.
     
  2. rlk5

    rlk5 Registered Member

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    anyone. help. please!
     
  3. kgblazerfive

    kgblazerfive keymaster Premium Member GMOTM Winner

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    84 would be a 208 t-case
     
  4. kgblazerfive

    kgblazerfive keymaster Premium Member GMOTM Winner

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  5. roadnotca

    roadnotca 3/4 ton status

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    Spec page says '84 = 208; my '85 has 208. I'm gonna do this post in parts, already did it once only to crash. I did my swap ~two years ago, so if there are some new products on the market, I don't know. I surfed, I went to tranny shops, etc. I already had a "built myself" TH350, so I was very familiar with it. The tuff part is the output shaft off the trans has to be shorter to couple with the 208. The folks that sell kits, want to sell you a kit with a replacement shorter shaft, for X more $$$ (yes that is three $$$s). Replacing the shaft requires a complete tranny teardown, some really big $$$$ from a shop, if they rebuild the trans along the way. The shops and the retailers can also get a less expensive kit with just the (Adv) Adapter and a dipstick and a kickdown cable assy. To use the lesser kit requires cutting the shaft off the tranny, intact. Pretty quick and simple, but you also got the (nice shiny worthless) shaft. You have to check spline count so the 700 and 350 is/was the same, either 28 or 32 IIRC but I think the 208 only came in 32, I'm not sure. So if tranny needs rebuild, do it all at once/right. If the trans is good to go, see if you can get a kit designed for cutting, a lot cheaper and quicker.
     
    Last edited: Feb 22, 2006
  6. Citizen Rider

    Citizen Rider 1/2 ton status

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    This was done to my 83 when i bought it. I beleive its a straight swap onto the motor. You will have a couple wire connectors that will now just hang there because a th350 is not computerised. Mine has a np203 on it that originally came on the th350 so i dunno what it would take for a 208.
     
  7. roadnotca

    roadnotca 3/4 ton status

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    According to the spec page, '83 was avail with either 350 or 700 so the swap was probably a 50/50 go/no go.
     
  8. 468k5

    468k5 1/2 ton status

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    been a few years ago,but I believe we cut one inch off the tail shaft of the th350
     
  9. roadnotca

    roadnotca 3/4 ton status

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    Yea I don't remember, except IIRC it wasn't exactly "1, but its forgiving, say plus or minus 1/8?
    So you've been there/done that?
     
  10. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    did the same swap in the last couple months. it's theoretically easy, technically it can be a difficult job - tho not always. definately would recommend a tranny jack if you are able to pick one up and a floor jack, otherwise 3 big floor jacks work, tho it kind of sux that way (one for the trans, tcase, engine). make sure you have a kickdown cable for it - don't use your tv cable, you won't have 3rd gear. it's easier to manuever the transmission and tcase w/out the crossmember attatched. this was done to an 84 k5 - both transmissions hooked up just fine with no length issues, but both had the 208 tcase. good luck.
     
  11. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    also, if you can leave the front driveshaft off until you get the bellhousing bolted up, that is very helpful - helps to keep disribution of weight a little less uneaven. then attatch the driveshaft, and then finally, the crossmember. oh, and you might have a helluva time with the tranny cooler lines - they're a serious pain. worked on those for hours before my buddy and i just gave up and he opted to go get some rubber lines and hook it up that way. i'd recommend you pick up some new hard lines so you don't have to fight your old ones.
     
  12. roadnotca

    roadnotca 3/4 ton status

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    Yea, I guess '83/'84 were transition years, so compatible.
     
  13. 468k5

    468k5 1/2 ton status

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    measure how much of the tail shaft sticks out on the 700 measure the 350 and cut
     
  14. MTBLAZER89

    MTBLAZER89 3/4 ton status Premium Member GMOTM Winner

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    The splined part only goes up so far. The more you cut off the less spline engangement you will have. You are creating a weak point by cutting off to much of the splines.


    The correct way to do this is get a TH350 that had a 208 behind it originaly ~(81-83) could come with that combo, or buy the adaptor and the shaft. I did this swap a couple years ago if you search my name and TH350 you'll prolly find it. there was some good info. Just my .02
     
  15. roadnotca

    roadnotca 3/4 ton status

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    In this case, if the TH350 needs shortening, the only length that will work is the same as factory, neither shorter or longer. Strength wise (Engr.), the spline engagement only needs to be about 2X the diameter of the shaft, 2-3 inches IIRC; any extra length is dead weight. The output shaft from the 350 is very good alloy, it will dull all but the very best blades. Plasma would have been sweet when I cut mine, I spent hours and $$ with my Sawzall. I used a 7" right angle sander/polisher to dress the new "nose" on the splines.
     
  16. alf4444

    alf4444 1/2 ton status

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    okay i'm confused I'm getting ready to do the opposite swap. I'm pulling out the 350/203 and replacing it with a 700r4/205 And I had to buy a spacer to mate the 205 to the 700r4...SO wouldn't that mean the spline on a 700 is longer than one on a 205?
     

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