TH350 to TH400

Old June 3rd, 2019, 09:06 AM
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Question TH350 to TH400

Hello,

I currently have a TH350 in my 1972 Cutlass, and Mark R just built me a stout SBO 350, and I'm installing a TH400 to go with it. So far, I've only purchased the new trans mount and new parking brake cables, but is there anything else I'd need to make the installation work? Any other oem-style cabling, brackets, etc.?

Much appreciated!
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Old June 3rd, 2019, 09:18 AM
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The transmission yoke is different, the TH400 uses an electric kick down solenoid. I think all the 68-72 A-body cars have the wiring already under the dash, you need the switch, mounting bracket, and add the wire to the existing engine harness under hood. The dipstick and trans fill tubes are different, the vacuum modulator is in a different location. Thatís all the differences I can think of.
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Old June 3rd, 2019, 10:00 AM
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Its been a while since I did this conversion myself. If I remember correctly, the "pad" on the cross member for the trans mount needs to be cut off the front side and welded to the back side if going from a TH350 to TH400. This will not be an issue if you are replacing your existing cross member with one from a TH400 car.
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Old June 3rd, 2019, 10:21 AM
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A drive shaft and a new yoke. The xmember is the same it just needs to be moved back a couple of holes.
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Old June 3rd, 2019, 10:56 AM
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Originally Posted by matt69olds View Post
The transmission yoke is different, the TH400 uses an electric kick down solenoid. I think all the 68-72 A-body cars have the wiring already under the dash, you need the switch, mounting bracket, and add the wire to the existing engine harness under hood. The dipstick and trans fill tubes are different, the vacuum modulator is in a different location. Thatís all the differences I can think of.
Originally Posted by 69 Hurst View Post
Its been a while since I did this conversion myself. If I remember correctly, the "pad" on the cross member for the trans mount needs to be cut off the front side and welded to the back side if going from a TH350 to TH400. This will not be an issue if you are replacing your existing cross member with one from a TH400 car.
Originally Posted by oldcutlass View Post
A drive shaft and a new yoke. The xmember is the same it just needs to be moved back a couple of holes.
Thanks, everyone! Yes, I have a new driveshalf being made once the engine and trans is installed.

So far I have:

-kick down solenoid
-switch
-mounting bracket
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Old June 3rd, 2019, 05:06 PM
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Originally Posted by oldcutlass View Post
A drive shaft and a new yoke. The xmember is the same it just needs to be moved back a couple of holes.

Ah, I did the conversion on a '78 Trans Am. We did not have the option of using other holes as there weren't any.
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Old June 7th, 2019, 10:10 AM
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Hello all,

below is a screenshot of the itemized parts list - I think I have them all. Let me know if I'm forgetting anything!

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Old June 7th, 2019, 11:14 AM
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Originally Posted by 72GoldOlds350 View Post
Hello all,

below is a screenshot of the itemized parts list - I think I have them all. Let me know if I'm forgetting anything!

You don't need to do anything with your crossmember except to move it back. They are the same for all transmissions in the 68-72 A bodies.
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Old June 7th, 2019, 12:22 PM
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On my 70 I had to cut the tab off the gas pedal that the Turbo 350 kickdown cable connected to. It interfered with the switch. On my car the wiring was already part of the harness. I just hooked it up.

http://www.buickperformanceclub.com/conversi.htm
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Old June 7th, 2019, 06:26 PM
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Originally Posted by svnt442 View Post
You don't need to do anything with your crossmember except to move it back. They are the same for all transmissions in the 68-72 A bodies.
I've heard this before, but then others (see below) say they had to alter the crossmember. I guess I could always find out and order the part if I indeed need it.

Also, even if the stock crossmember does work, considering the new amounts of power the engine is going to produce, is that aftermarket crossmember any stronger?

Originally Posted by droptopron View Post
On my 70 I had to cut the tab off the gas pedal that the Turbo 350 kickdown cable connected to. It interfered with the switch. On my car the wiring was already part of the harness. I just hooked it up.

http://www.buickperformanceclub.com/conversi.htm
Thanks for the link! I wonder if that is the same for Olds.
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Old June 7th, 2019, 07:06 PM
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You can modify your old modulator line to work on the T400 and as stated above you just need to move your current cross member back a bit.
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Old June 7th, 2019, 11:22 PM
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Is the aftermarket crossmember any stronger that stock?
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Old June 8th, 2019, 03:40 AM
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Iím sure the boxed crossmember is stronger than stock, they are also much heavier. The G-Force crossmember I used in my 87 442 years ago was a beast.
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Old June 8th, 2019, 09:45 AM
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Originally Posted by 72GoldOlds350 View Post
Is the aftermarket crossmember any stronger that stock?
You're not putting down enough power to worry about it. You'll crack the transmission case before you hurt the crossmember.
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Old June 8th, 2019, 05:40 PM
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I donít think the trans crossmember is ever really under serious stress. Iím guessing chassis flex would be a issue long before the trans crossmember causes problems
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Old June 13th, 2019, 09:13 AM
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Any other parts I may need that are not listen on the parts list I supplied? Just hope I'm not missing anything.
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Old June 22nd, 2019, 09:12 PM
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I'll take that as a negative. I do appreciate all the help, everyone! Love the community on this site. I'll keep everyone posted if I run into anything specific or if there's any updates.
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Old June 23rd, 2019, 03:30 AM
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There is no difference in factory crossmembers from TH350 to TH400. Use your old one. And yes, the accelerator pedal for a TH350 needs to have the tab cut off.
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