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Old 09-01-2009, 01:04 PM
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T-350 & T-400 swap questions fullsize c-10

My tranny is going out (only have 1st and 2nd gear) it was even rebuilt two years ago but the warrenty is up.

The truck is a 1984 shortbed c-10 with one piece driveshaft.

My buddy has a th350 and a th 400 I would like to swap in the th400 but correct me if im wrong , I will have to change yokes shorten my driveshaft 1.5inches and move the crossmember aswell as . Is this what I will have to change ?

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Old 09-01-2009, 04:07 PM
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You will need to adress the electric "kickdown" switch that the 400 uses. The 350 uses a cable. The 400 can have a switch on the carb/linkage or some GM cars use a pedal mounted switch.
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Old 09-01-2009, 04:58 PM
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In addition to what I have already said? I talked with some one else and they have had to do no swapping of anything going from a TH350 to a TH 400 in v8 s10 build so im really at a loss right now. He said aslong as its a short tail shaft th350 ill be good to go with my slip in yoke and legenth.
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Old 09-01-2009, 05:14 PM
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I just did this swap on my 82 Chevrolet 3/4 ton Trailering Special. Originally, when new, the truck had a 700R4, that only lasted 16K miles. A TH350 was backdated, and G.M. covered 85% of the cost; a 9" TH350 was used, and other than a "plate" added to the trans mount to correct the location, it was a bolt in. That trans lasted a few years, the truck was passed on to me, and I replaced it with an exchange/rebuilt TH350. When that trans went, I'd had enough of TH350's in a truck that big, so I swapped in a TH400 short shaft. The truck has a 2 piece driveshaft, and I found a LONGER slip yoke that made up for the difference in lenth, the front shaft needed to be longer. I used a stock wiring harness and kickdown switch used in trucks that CAME with the TH400 trans. I WISH I HAD DONE THE TH400 SWAP YEARS AGO! This trans is the best; no lag between shifts, shifts smooth, but when it shifts at WOT, you know it. Your trans cross member can be easily moved, the frame probably even has the correct holes, if not, drill them. Depending on which lenth TH350 you have, you too can probably find a yoke to compensate, but it will probably need to a "conversion" U-joint to mate to your driveshaft. You will be happy with the TH400, I sure am.
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Old 09-02-2009, 03:04 AM
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ppl tend ot have a hard time with the many different trans lengths there were, and with the cetain lengths used in certain vehicle years and models

for the 4x2 version the "short" tail th350 is the 6" tail and existed in every year that the th350 was ever made and available, '68-'86(in '68 only in some corvettes and camaros, nothing else(otherwise they were '69-'86 only))

this is a shorter length than the very common '70s and '80s th400 trans length.,,, by 3"

a full size '73-86 4x2 truck will have the very common 31.5" "long tail" overall th350 trans, from factory.

these have the 9" tailshaft housing length

same length and output splines as the 700r4 trans

the th400 trannies used in the '70s and '80s trucks in 4x2 form were super close to the same length as the 9" tailshaft th350. close enough to work fine,

if you you measure the th400 overall length WITH the output yoke installed on the output shaft you will see that it is the same as the 9" tail th350 length overall,

the truck th400 output yokes are bolted on to the end of the shaft with a threaded bolt into the end of the output shaft.

you should just need a conversion u joint from napa that has both sizes of joint caps if they differ.

and they would differ in size between 1/2 and 3/4 ton of course

just remove the th350 yoke off the driveshaft and install the 400 yoke on it. use whatever u joint needed

the length would be close enough thta it would work, only if going from 9" tail th350 to a common '70s or '80s th400 with truck bolted on style output yoke

but the problem is that if its a single piece driveshaft then you wont have slip

so you will need to find a slip type th400 yoke from a car or other lighter vehicle, etc.

or use and install a common two piece driveline from any '73-87 2wd pickup with th350 or th400

the length form end of the tail of the trans back to the axle pinion yoke on the longbed 2wd pickups with th350 is 74"

most were one piece shaft and this is the longest and most common single piece driveline gm used under the light duty pickups(1 ton and under)

i have several of these shafts in my driveshafts stash still even after having quit stripping and parting and junking old trucks years ago now.,

but now, i see that you have a dshortbed

so that will make a little more difficult

try to find a shortbed th400 driveshaft,

or a non-bolted on style th400 yoke from a car

then you would be ok

so as long as you use a th400 that is the common 70s and 80s truck lenght

i still have one truck with such th400 under it, it is a '79 C-20 2wd, and we use it to haul the car trailer to the auto shredder when i junk a vehicle, also as a wood truck when cutting down and cutting up and splitting up hickory maple or oak trees to use as firewood for the wood burning furnace in the shop.

there is also one under the '80 K-20 plow truck but it is a 4x4, so it doesnt count, in this case.

If you have the 9" tail th350 trans currently under it

also keep in mind that the th400 and its variants use a unique trans mount because the two bolts that hold the mount to the trans case are wider apart than the metric 200 tranny variants and th350 trannies and 700r4 trannies etc

autozone has a th400 trans mount for 5 bucks i bough tone recently because i was goin to put my 87 c-10 305 th400 trans(3L80HD) in a 78 pickup i had. 87-up were longer length than the old th400 trannies, overall length 34.5" 87-92 years.
oddball ones. new design with HD cast on the bellhousing and tage code LJA 87. also has 27 spline output unlike the old th400 in which are all 32 spline. still has same th400 body and pan and parts otherwise.

gm did some funny things in later years with the 400 trannies after killing off the th350 in '86.

hope any of this helps and doesnt confuse anymore.

if you need exact measurements or pics etc then let me know and i should be able to help in some way or another surely.

have fun!

Last edited by fast68; 09-02-2009 at 03:28 AM.
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Old 09-02-2009, 01:24 PM
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I just got back in from looking at the truck and it is a one piece drive shaft and has a slip yoke! not the bolt on yoke I was expecting to see.
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