Author Topic: TH400 swap opportunity?  (Read 786 times)

Offline krigermoose

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TH400 swap opportunity?
« on: January 08, 2022, 01:59:20 AM »
Hey everyone, long story short I bought a used 350 to drop in my truck after mine blew. This new 350 also happens to be mated to a th400 and the guy wanted to just sell the whole package. It came from his daily driver '75 c20 that he just pulled the engine and trans to drop in a ls 6.0/4l80 setup. Supposedly the th400 has less miles than my truck's th350, and they are most likely in similar condition.

Would it not be a bad idea to just leave the th400 on the engine and drop it all in? Will I need a new driveshaft as well?

Offline dusterdude

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Re: TH400 swap opportunity?
« Reply #1 on: January 08, 2022, 05:29:26 AM »
I would,but you'll at least need a 400 input for your driveshaft or maybe a whole new shaft depending on length differences

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Offline zieg85

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Re: TH400 swap opportunity?
« Reply #2 on: January 08, 2022, 09:12:44 AM »
Be mindful that the TH400 kickdown on squarebody trucks is an electrical switch on the go pedal under the dash.
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline krigermoose

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  • 1986 GMC C1500 5.7L
Re: TH400 swap opportunity?
« Reply #3 on: January 08, 2022, 01:31:24 PM »
I'm considering if it's worth getting a new driveshaft because I'm fairly certain the two transmissions are difference lengths. And regarding the TH400 electrical switch, is it simple to bolt up?

I also do not know for sure if I can mate the old TH350 to the new 350 without a new flex plate? Or can I just bolt it up?

Offline JohnnyPopper

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Re: TH400 swap opportunity?
« Reply #4 on: January 08, 2022, 02:29:54 PM »
the TH400 is certainly step up in many ways, but I would consider the benefits vs the costs.

A TH350 is plenty good for most trucks, so a couple of questions:

Is your use of the truck in need of a stronger trans?
Do you race?
Haul?
Pull out stumps?

If none of the above, I would keep my life simple and sell the TH400.

1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline krigermoose

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Re: TH400 swap opportunity?
« Reply #5 on: January 08, 2022, 03:06:35 PM »
Yeah I don't plan on doing much pulling, racing, etc. I also don't necessarily know the lifespan of my th350 since it sat for at least 10 years.

Offline JohnnyPopper

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Re: TH400 swap opportunity?
« Reply #6 on: January 08, 2022, 06:33:03 PM »
10 years of no wear.  8)

Change the front seal while it's available.

The dipstick seal.

Rear as well.

The trick to these is to keep them clean, they will go forever.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline krigermoose

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Re: TH400 swap opportunity?
« Reply #7 on: January 10, 2022, 10:39:47 AM »
I like your thinking. I'll giver a look.

Offline Shifty

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Re: TH400 swap opportunity?
« Reply #8 on: January 10, 2022, 11:06:42 AM »
While heavier-duty from a stock standpoint, they are also heavier, and require a bit more horse to drive than a TH350.  For a few dollars, you can address the weak point of the TH350, the intermediate sprag....change it out for a hardened 36 element one, and have your cake and eat it too.
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

98 C2500 ext cab longbed

Offline krigermoose

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Re: TH400 swap opportunity?
« Reply #9 on: January 10, 2022, 06:22:55 PM »
Right on, good to know. I'm assuming the engine bolted to the TH400 doesn't need anything to bolt to a TH350? If it needs a flex plate I've got one.

Offline Shifty

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Re: TH400 swap opportunity?
« Reply #10 on: January 11, 2022, 08:08:45 AM »
Most likely, it'll all bolt up, as long as it is the earlier flexplate, you know, 2-piece rear main seal engine, etc...
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

98 C2500 ext cab longbed

Offline JohnnyPopper

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Re: TH400 swap opportunity?
« Reply #11 on: January 11, 2022, 04:28:22 PM »
Same bolt pattern since 1955... 8)
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline krigermoose

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  • 1986 GMC C1500 5.7L
Re: TH400 swap opportunity?
« Reply #12 on: January 12, 2022, 02:47:13 AM »
My truck's old engine/trans setup was '86, this new one is '75. Still seems fine though.