th350/np203 to th400/np205 SWAP HELP

kevin lefebvre

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Im wanting to do the swap as mentioned in the title! I have the round bolt pattern 32 spline 205.
What other parts will I need? Will the dirve shafts work? What else do I need to make everything bolt on?

Im not mechanically inclined so all the help I get is appreciated.
 

bucket

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Do you already have the matching TH400 too?
 

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Besides the aforementioned th400, you’ll need the right trans mount perch to match the crossmember.
 

bucket

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No, il try to find one.

If you aren't rebuilding, you need a TH400 with the correct length of output shaft. There's a few charts floating around the internet, I'll see about posting one. You will also need the correct trans-to-case adapter as well as an '85-up style crossmember. Your drive shafts will need replaced or modified.

If the main goal is to ditch the 203 for a 205, there is a much simpler and cheaper option. If you get a 205 and adapter from behind a TH350, you can swap it directly in. You will still need to deal with the shafts, but the particulars will depend on your current 203's outputs and the replacement 205's outputs.
 

kevin lefebvre

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If you aren't rebuilding, you need a TH400 with the correct length of output shaft. There's a few charts floating around the internet, I'll see about posting one. You will also need the correct trans-to-case adapter as well as an '85-up style crossmember. Your drive shafts will need replaced or modified.

If the main goal is to ditch the 203 for a 205, there is a much simpler and cheaper option. If you get a 205 and adapter from behind a TH350, you can swap it directly in. You will still need to deal with the shafts, but the particulars will depend on your current 203's outputs and the replacement 205's outputs.
Im just looking for something strong and reliable for a daily. I swapped out the 350 for a new engine, and my tcase was sounding like the chain was jumping when I stepped on the gas in deep snow on a lake ice fishing. th400 and 205 should be plenty strong.
 

Bextreme04

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You'll either need a short output shaft TH400, or cut the output shaft down to fit the long input on the round pattern 205. The overall length of a TH400/NP205 is significantly different than a TH350/203, so neither the front nor the rear driveshaft will work. You'll need to get driveshafts from a 1980 or later TH400/205 truck. I suggest getting everything else in and then just measure to find you required length. You should be able to source ones the right length from a local junkyard.
 

Craig Nedrow

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a short shaft th-350 and th-400 are very close to the same overall length. Close enough that the same driveshaft can be used. HOWEVER the 350 has a 27 spline output, and the 400 has a 32. I have a 203, at least 100,000 miles offroad, lived it's whole life in the hills, and still does. 203 is sloppy, but if it were me, unless you plan to run tons of HP, just rebuild what you have. Lots of places sell everything you need. I have a 205 behind the BB, and like it, same width as the 203, same mount (both 73 year), If the th400 is out of a truck, or motorhome, will have the short shaft, cars had a long tail shaft, at least all the rigs I have are that way. I have lots of transmissions, and to conclude, do yourself a favor, and study your choices, as a 205 32 spline output will be at least $300+. My 4l80e/205 adapter from advanced adapter was 650, and I have to machine my 205 front bearing to fit. I will die with this truck, not selling, so will be worth it to me.
 

kevin lefebvre

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a short shaft th-350 and th-400 are very close to the same overall length. Close enough that the same driveshaft can be used. HOWEVER the 350 has a 27 spline output, and the 400 has a 32. I have a 203, at least 100,000 miles offroad, lived it's whole life in the hills, and still does. 203 is sloppy, but if it were me, unless you plan to run tons of HP, just rebuild what you have. Lots of places sell everything you need. I have a 205 behind the BB, and like it, same width as the 203, same mount (both 73 year), If the th400 is out of a truck, or motorhome, will have the short shaft, cars had a long tail shaft, at least all the rigs I have are that way. I have lots of transmissions, and to conclude, do yourself a favor, and study your choices, as a 205 32 spline output will be at least $300+. My 4l80e/205 adapter from advanced adapter was 650, and I have to machine my 205 front bearing to fit. I will die with this truck, not selling, so will be worth it to me.
Not selling either so im not worried about prices . The only thing bothering me is the damn drive shafts. Im not in a hurry to swap, but id still like to gather all the parts needed , sadly I cant gather everything since I need to measure the drive shafts at the end once everything is in.
 

bucket

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This first chart shows case length for the TH350 and TH400. You can ignore the rest for a 4x4.

You must be registered for see images attach


The next chart shows the various output shafts used in the TH400. Iirc, a stock, round pattern 205 has the long style input. But someone may have swapped in a short input to mate up with a more common 2wd style TH400.

You must be registered for see images attach
 

Bextreme04

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This first chart shows case length for the TH350 and TH400. You can ignore the rest for a 4x4.

You must be registered for see images attach


The next chart shows the various output shafts used in the TH400. Iirc, a stock, round pattern 205 has the long style input. But someone may have swapped in a short input to mate up with a more common 2wd style TH400.

You must be registered for see images attach
Yes, this is correct. Figure 8 pattern NP205 will use the long TH400 output(mine is like this) and the round pattern 205 has a long input gear that takes the shorter TH400 output. Just as an FYI, all 4wd 4L80e's will have the same size output shaft and are a direct fit on that round pattern 205.
 

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