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Old 02-13-2009, 08:30 PM   #1
Walts_restoration
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1986: np208. 700r4---->th350 swap, need advice

For my 86 k5, i wanna swap the 700r4 in favor of a th350. I dont really care about the overdrive or lower gear ratio, i plan to tow and play with this truck, so i want it to take some moderate abuse.

I know the 82-84 or some years in there used a th350 np208. What all is different between these tcase/adapters and the 700r4s? i thought i read somewhere the th350 and 700r4 can swap without any modificaions.

edit: just found out they are different.

Quote:
Originally Posted by 1-tonmudder View Post
In about 1981 GM starting usin the 208's behind the Th350's.It was only a 1-2 year deal until the 700R4's came out.The 700 and 350's both different adapters.

The Th350/208 adapter,trans side.


By 1983 they were using the 700's and it required another adapter.

Trans side.
So how easily do you guys think ill be able to find a trans and adapter? im gonna call a local 4x4 place see if they have the right trans and adapter, if not, what different transmissions will work (correct length) anhd do you know a place where i can get the right adapter bracket? Will i need to swap the cross member too? im new to this 4x4 stuff and trying to learn before i walk the wrong path. Oh, and i dont wanna worry about a lockup converter, its unneeded in my books.

(the main reason i want to use the th350 opposed to the 700r4 is the engine i have came from a 75 blazer with a th350 np203, so everything motor wise is setup for a th350 already, and plus they are more simple in my oppinion)
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Old 02-14-2009, 09:45 AM   #2
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Re: 1986: np208. 700r4---->th350 swap, need advice

Man ive tried searching for this. Does anyone know of an aftermarket adapter opposed to an orignal gm?

Everyone else is going th350 to 700r4, and im going backwords. I guess i should be finding people going with the grain and get there stuff off them lol.


Any help or info you guys can give me will be appreciated. I just wanna make sure i do this once, and do this right.
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Old 02-14-2009, 12:47 PM   #3
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Re: 1986: np208. 700r4---->th350 swap, need advice

hey heads up, i mis red the PM you sent me..
sorry...
but here's what i got, the PO never stated anything about an adapter etc.. so i honestly have no clue, but here's pictures.. maby they will help


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Old 02-14-2009, 02:32 PM   #4
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Re: 1986: np208. 700r4---->th350 swap, need advice







Best pics i could get of her. Shes in the weeds right now, front right tires flat. Its been sitting so long, bought it from my brother, he pulled the motor 5 years ago. Ive had it in my name for near two years now, and the stars havent aligned enough for me to be able to dorp the motor back in. But my birthday is march 10th, and i really want this done by my 19th.
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Old 02-14-2009, 06:03 PM   #5
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Re: 1986: np208. 700r4---->th350 swap, need advice

I seriously doubt that there is any difference in the mating surfaces of the two adapters.... I would gues you could make it work with your original adapter.

Also, the same spacer/adapter that people are using when they trade in a 700r4 for a th350 could be used in your conversion to make up the space lost by the shorter th350 but I think that requires a longer output shaft for the tranny. I think. But, if the tranny is already hooked to the engine I would think you're just gonna move the engine back.
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Old 02-14-2009, 11:09 PM   #6
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Re: 1986: np208. 700r4---->th350 swap, need advice

you can use the 7oor4 adaptor. One of the two has a slitly longer output shaft that needs to be trimed, but I think it is the 700r4. If you are going to be towing, why not use a 400? just wandering because the 350 is not any stronger than the 700r4.
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Old 02-15-2009, 12:06 AM   #7
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Re: 1986: np208. 700r4---->th350 swap, need advice

th350s are pretty strong, ive never had a problem unelss they were ran hard. There is a guy who had a th400/np208 combo, but to use a th400 id need to change the kickdown. the reason i picked a th350 is becuase the 350 im dropping in is already setup for a th350. And more than likely it will end up be an occasional tow, nothing heavy like a bobcat, at most my trailer and camaro.
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Old 02-15-2009, 01:10 AM   #8
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Re: 1986: np208. 700r4---->th350 swap, need advice

If you don't want to move the crossmember forward & modify driveshafts, you need this:

http://advanceadapters.com/product/1784/50-7100.html

The transmission has to be torn down to put the shaft in though. As I think was said earlier, I think there is a stock length output shaft from a long tail 2wd that comes close, but then you have to do the crossmember & shafts rework. Or, I think there was an oddball "extra long" tailshaft in some vans or maybe subs that would work with the aftermarket case spacer.
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Old 02-15-2009, 01:19 AM   #9
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Re: 1986: np208. 700r4---->th350 swap, need advice

i figured the 350s w/np208 wouldnt have been any different forward or backward than the 700r4s with a np208. Looks like im looking for a parts blazer.. lol.
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Old 02-15-2009, 01:50 AM   #10
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Re: 1986: np208. 700r4---->th350 swap, need advice

Yeah, the 700r4 case is longer. somewhere buried in my trailer I think I have the "couple year deal" TH350 off a 208. A friend has the same in his truck. It's been his daily driver forever. He's getting the tranny rebuilt right now- only because someone else borrowed it and burned it up trying to pull a 4dr pickup & gooseneck out of a hole.
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Old 02-15-2009, 01:55 AM   #11
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Re: 1986: np208. 700r4---->th350 swap, need advice

i just dont wanna go through the hassle of dealing with the factory computer carb junk that was on it. Was a real rats nest, i never saw it before my brother started tearing it apart. But from what i can tell, had an edelbrock performer intake, and the stock computer qjet, so it must of been a real canoworms. just trying to simplify things like my 77 c10, heavy half 350/th350, bullet proof combo imho. Ive never even had a lick of trouble from the trans.
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Old 02-15-2009, 11:57 AM   #12
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Re: 1986: np208. 700r4---->th350 swap, need advice

I would really like to find someone with an 82-83 to take some measrements from to see if the transfercase is in the same place as my 86 or not, becuase that would determine if different driveshafts are required.

There is a th400/np208 on cl but i dont think a th400 would bolt right in a light duty k5 lol.
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Old 02-15-2009, 02:07 PM   #13
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Re: 1986: np208. 700r4---->th350 swap, need advice

well what and where do you want measurements?
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Old 02-15-2009, 10:55 PM   #14
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Re: 1986: np208. 700r4---->th350 swap, need advice

Quote:
Originally Posted by dammitmitchell View Post
well what and where do you want measurements?
Youve got a 4 speed right? I need somebuddy with a th350 automatic, not a 350 small block, different ball games.

Id like to see a front driveshaft and rear driveshaft length, from center of ujoint to center of ujoint. Should pretty much say if the transfercase sits in the same spot.
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Old 02-18-2009, 11:46 PM   #15
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Re: 1986: np208. 700r4---->th350 swap, need advice

Still wondering if anyone out there has a factory turbo 350 np208, especially in a blazer that they could take measurements off of. thanks guys.

Gonna drop advance adapters an email, see what they say about it.
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Old 02-19-2009, 11:36 AM   #16
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Re: 1986: np208. 700r4---->th350 swap, need advice

Hey Walts_restoration,
i have an 82 k10 longbed that i'm pretty confident has the th350/208 combo in it. It probably would not help much for the rear shaft measurement but I assume the front would give you the information you are looking for. I live in Independence, MO but the truck is at my brother's in Tonganoxie (just down the road from Lawrence). I'll drop you a PM and see if we can get together to get the measurements you are looking for. If you want we can meet and you can look at the truck to get a complete picture of what it would take to convert yours.

Ron
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Old 02-19-2009, 01:29 PM   #17
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Re: 1986: np208. 700r4---->th350 swap, need advice

pm'd ya back, thanks for the help.

Quote:
Spencer

The 80-early 82 ton trucks and blazer had lock up converter th350/208 combos. The overall length is not quite the same. Our 50-7100 kit is new output shaft to be installed in any th350 2 or 4 wd regardless of year or lock up converter or not. We recommend taking the trans to a trans shop and having them install the shaft for you (it comes in from the front) You can find a 2wd 6” tailhousing Th350 and we will sell you just the spacer and hardware and you will need to cut the 2wd output shaft.

Hope this helps.



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