TH350 To TH400 Swap - Impala Tech
Transmission & Driveline Transmissions & Differentials

 
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post #1 of 11 (permalink) Old 12-02-2015, 01:39 AM Thread Starter
 
 
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TH350 To TH400 Swap

Hello to all,

My soon to be tow rig, a '70 Kingswood wagon is getting a new motor and trans. I am removing the tired 350 / TH350 in favor of a mild 454 and TH400.

Question is what should the donor transmission be? I assumed a short tailshaft TH400 but am not really sure? Found an Impala guy here in the Seattle area and he "says" he has a drive shaft from a '69 or '70 impala. Great, but what length should it be to mate up to the TH400? I read here in a post from Big Dave that "B" bodies got a longer tail shaft trans....

Also can i use a TH400 from a truck that has a bolt on yolk, remove it and replace it with a slip yolk easily?

I am also trying to wrap my head around the crossmember part for 1970, what do I need as the one I have seems to have an inverted shape and mount bracket on the back side compared to the TH350.

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Thank you all for your responses,

Dave

Last edited by davidcmay; 12-02-2015 at 01:58 AM. Reason: added Picture
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post #2 of 11 (permalink) Old 12-02-2015, 07:57 AM
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The full size cars got the longer tail shaft trans. You can use the short one but will need a different driveshaft. The stock crossmember fits both TH350 and 400 and the frame is drilled to move it back for the longer 400 trans.

I'm not familiar with a bolted yoke, but it most likely bolts to the out put shaft of the trans. If the shaft has the same spline as the slip yoke model, you may be able to use the slip yoke. If the spline is different, then the out put shaft would need to be changed, which would require complete dis-assembly of the trans. It may also be longer than a car trans...

As an aside, the TH400 uses a larger diameter yoke/driveshaft than all the other auto's and you'll most likely need an adapter u-joint at the diff for the larger diameter driveshaft.

The local driveshaft, if correct for a '69-70 full size with TH400 should be correct for the long tail shaft application.

PS, if you were so inclined to have the TH350 rebuilt and upgraded for towing, it would handle the task just fine with less hp drain than the TH400 and no driveshaft issues, as it should bolt right back in the same location with the 454.

'62 Impala SS 409 TH350
'66 Chevelle SS 496 M20
'70 Chevelle SS 396 M20
'67 Camaro ss/rs 350 PG
'38 Chev coupe street rod
'54 Chev 210 2 door
'69 Chev C10
'89 Chev R3500 roll back
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post #3 of 11 (permalink) Old 12-02-2015, 10:09 AM
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The TH350 isn't as strong as the TH400 because the TH350 uses mostly aluminum for it's internal parts, where the TH400 uses cast iron for the same parts. This is why the TH350 is given a lower torque rating. It is the torque of a 454 that will easily break a TH350 unless you spend two to three times the price of a rebuilt stock TH400 to replace the weak stock parts with billet and hardened forged steel parts to get a faster shifting TH350 that can stand up to a big block.

As a an automatic transmission shifts it stops rotating in one direction and reverses the direction of rotation on the next shift. It is this rotating internal mass that has to stop rotating, and change the direction of rotation of every shift (three times in a TH350 or TH400) that sucks up horsepower but only while it is shifting.

Once both are in high gear both the TH350 and the TH400 have the same parasitic drag (horsepower loss converted into waste heat when compared to a standard transmission). So driving down the road there isn't any more power lost with a TH400 than a TH350 or a 700R4 or any other automatic.

The TH400 has a larger diameter output shaft with 31 splines instead of the smaller 27 splines used with the TH350, early (course input spline) Muncie or the older PowerGlide. Later model Muncie (26 spline input shaft), and the Borg-Warner T-56, Tremec and TKO series of five and six speed manuals use the same 31 spline output shaft size as the higher torque rated TH400. As such you will need to change the front driveshaft yoke that slides into the transmission (there are four different lengths of these yokes so find one that matches the length of your TH350).

Bolt in yokes are generally associated with a four wheel drive application. You have to totally disassemble the TH400 transmission and replace the output shaft from bolt in to slip yoke to change over tot he slip on yoke with a longer tail shaft as the yokes do not directly interchange.

Additionally as the TH400 is used in trucks as well as cars, there are different size universal joints. Your TH350 used a Spicer 1310 size and there exist a yoke in that size in junk yards (used). Light trucks used either a 1310, 1330 or a 1350 universal joint depending upon the GVW of the truck. You can buy mixed size universal joints that will have two 1310 and two 1350 trunion caps on one universal but they cost more than a stock joint and might not be available for a while if your wagon breaks down on the road.

Finally because the TH400 is two and five eighths of an inch longer than a TH350 your existing drive shaft has to be cut down and rebalanced. I might add though the TH400 will bolt up to a TH350 cross-member; the stamped steel fabricated cross member has a lip that will cover the oil pan bolts of the transmission preventing easy access to change the fluid and filter. This is why the TH400 uses a round bent pipe with a tab welded onto it so it will allow easy maintenance.

Big Dave
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post #4 of 11 (permalink) Old 12-02-2015, 12:21 PM Thread Starter
 
 
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So here is what I need to do this swap so far.

1) bolt on yolk , bad for this use.
2) Need to shorten the DS and change the yolk on original, or find the original 400 DS with the correct yolk.


Things I am not yet clear on.

1) Long or short shaft TH400? What length is correct for original?
2) Crossmember fitment? Do I use the original? Move it or flip it?
3) Length of the OEM TH400 drive shaft?
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post #5 of 11 (permalink) Old 12-02-2015, 12:33 PM
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Quote:
Originally Posted by davidcmay View Post
So here is what I need to do this swap so far.

1) bolt on yolk , bad for this use.
2) Need to shorten the DS and change the yolk on original, or find the original 400 DS with the correct yolk.


Things I am not yet clear on.

1) Long or short shaft TH400? What length is correct for original?
2) Crossmember fitment? Do I use the original? Move it or flip it?
3) Length of the OEM TH400 drive shaft?
Bolt on yoke is fine but only if you want to convert your wagon over to a 4x4. Around here people put 4x4 chassis (frame and drive train) under the strangest collection of cars (VW bugs, Novas, even a on an old Rolls Royce).

Yes a TH400 requires a dedicated used drive shaft or shortening your existing shaft.

Your wagon came with a long or nine inch tail shaft (the TH400 uses a 6, 9, or 12 inch tail shaft).

On full size cars some where offered with both mounting locations for either a TH350 or a TH400. If you have only the one mounting location the original needs to be cut off and re welded onto the frame or buy a set of used mounts and weld them onto the frame to accept the TH400 transmission cross-member.

That I do not know. The wagon used a leaf spring rear suspension system, like a pick-up truck, instead of being coil sprung with four control arms securing it the way it is under most other B-body cars.

Big Dave
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post #6 of 11 (permalink) Old 12-02-2015, 12:54 PM Thread Starter
 
 
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Dave

Thank you for your reply,

Yes my wagon has a TH350 crossmember and seems to need the mounts modified.The wagon I have uses real coil springs, I believe they changed over to Leaf springs in '71.
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post #7 of 11 (permalink) Old 12-04-2015, 08:20 PM Thread Starter
 
 
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Found this information today. Transmission crossmembers on all Impala's were the same. On 454 cars 2 sets of mounts were on the frame for the crossmember. I guess I will have to fab up a couple of mounts unless I can find an aftermarket repro?

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post #8 of 11 (permalink) Old 12-04-2015, 10:21 PM
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Quote:
Originally Posted by davidcmay View Post
Found this information today. Transmission crossmembers on all Impala's were the same. On 454 cars 2 sets of mounts were on the frame for the crossmember. I guess I will have to fab up a couple of mounts unless I can find an aftermarket repro?

Attachment 13162
I forgot that the wagon's had an enclosed frame, like a convert. Yep, you'll need to extend the welded bracket for a TH400.

Bring it over and we'll fab up a piece and weld them on.

'62 Impala SS 409 TH350
'66 Chevelle SS 496 M20
'70 Chevelle SS 396 M20
'67 Camaro ss/rs 350 PG
'38 Chev coupe street rod
'54 Chev 210 2 door
'69 Chev C10
'89 Chev R3500 roll back
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post #9 of 11 (permalink) Old 12-06-2015, 11:02 PM
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I recently did a similar swap in my shortbed C10 pickup. Yes I know, different application, but it should still apply here if you have a long tail th350 currently.

I removed my smallblock/th350 and replaced it with a garden variety 454/th400 from a truck. The th400 was a short tail and had a bolt-on yoke. Overall length was shorter than the 350. You can simply unbolt the yoke and use a slip style, but the stock driveshaft was too short and the yoke was too small. So I used a slip yoke from an 80's K10 with an np208 transfer case on my stock driveshaft. It's a longer yoke to make up the length difference and has the needed 32 splines. Also, since it was from a 1/2 ton, it didn't use a larger 1350 joint.

'69 Impala 4-door hardtop
'70 Impala Custom coupe
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post #10 of 11 (permalink) Old 11-12-2016, 02:18 PM Thread Starter
 
 
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Found a Source

I finally found a source for the frame rail mounts I need.

https://www.impalaparts.com/products...-BRACKETS-CONV
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post #11 of 11 (permalink) Old 03-18-2020, 02:30 PM
 
 
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Can anyone tell me what the measurement is to relocate the trans crossmember brackets on my frame. I am swapping from a TH350 to a TH400 abd there are no predrilled holes nor is there a second set of brackets. Need a measurement as if i lay my tape down along the framerail butted up against the existing brackets. Distance between the two will be great. Thanks
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