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Old November 27th, 2009, 10:19 PM   #1
jon_xj90
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Default th400 to 700r4 swap

i have an 86 chevy 10k lb gvrw stake bed 2wd dually, bought it for the right price, currently a carburated 350/ th400 / 14 bolt 4.56 gov-lock rear.


i don't haul nearly what the truck is capable of and the fuel mileage is absolutely killing me. my xj on my trailer is only about a 5500lb tow and i need to be able to take this thing across the state at a reasonable cost.

i know i've seen 700r4 transmissions behind big blocks and diesels, how bolt-in is a 700r4 in place of my 400? i know i'll have to switch to a kick down cable in place of my vacuum line that goes to the modulator. my main concern is length and mounting. will i need a new drive shaft and cross member?

and is there a better overdrive trans i should look at?

anything is appreciated!
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Old November 27th, 2009, 10:42 PM   #2
Kodiak450r
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a 700 will do fine if you get one that is beefed up for towing. with some stronger parts and a good trans cooler it could handle what you do just fine. you may need a different crossmember and the driveshaft may require either shortening or lenghtening. i can't say for sure how much as i know 400 and 700's are close in length
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Old November 27th, 2009, 10:51 PM   #3
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thanks just wasn't sure of durability for my application, i've had trucks with both just never tried to swap one. apon reading it looks like the 700r4 is an inch shorter? and has a 27 spline output compared to the 32 on the 400. the cross member should be reusable i'm thinking? maybe just moved a little. anyone who's done it please chime in!
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Old November 27th, 2009, 11:44 PM   #4
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I've done this exact swap in a 1987 3/4 ton suburban 2wd.

The 700r4 has a slip yoke and the th400 has a fixed yoke. you will need an entire new driveshaft from the center bearing forward.

The shifter linkage and cooling lines bolt up. However if you do not switch the steering column you will only have three clicks instead of four, and drive may be where neutral was before if you do not switch the PRNDL indicator.

The flexplate may or may not need to be redrilled. I used the flexplate from the donor vehicle because it did have a different pattern.

Cross member works fine, you need to slide it back a hole on the frame. You will be able to use the stock mount.

Depending on the application you get the 700r4 trans from, you may or may not have a cable drive speedometer housing. The cable drive speedometer housing from an astro van bolts onto the back of the electric speedo trans from a truck, your speedo may or may not be accurate. If you are looking for a trans, just start with an astrovan trans from 86-92.

I have recently parted out the suburban that had the custom driveshaft. I still have the driveshaft, and it might work for you if you use my front with your rear portion. I paid $200 to have it made, I'd take $50.00 for it.
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Old November 27th, 2009, 11:53 PM   #5
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that's more like what i was looking for, thank you. i'm looking to do this sometime after christmas and i get a few dollars back in the bank. looks like i need a solid 700r4 with a cable drive speedo tail, i'll just buy a 700 flexplate, and i'm planning on going with the lokar floor shifter.

i'll pm you come time and see if you still have that shaft there, you're right down in algonac? i graduated form algonac a few years ago and live in east china now.

ever had any trans work done my prator (sp?) he's recommended by mat and ed at brackets auto and i'm looking for someone with some experience with him. he's located right by the trailer park next to the high school i think.
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Old November 28th, 2009, 09:48 AM   #6
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Quote:
Originally Posted by jon_xj90 View Post
ever had any trans work done my prator (sp?) he's recommended by mat and ed at brackets auto and i'm looking for someone with some experience with him. he's located right by the trailer park next to the high school i think.
Yes, I live in Algonac

I've dealt with praytor, but it was a long time ago. He has a good reputation, but he also has a reputation of doing everything at his own pace, and you might get your trans back tomorrow or next year. When he rebuilt a transmission for my 55 chevy years ago, it took him about 4 months to get it back to me. I've put almost 100,000 miles on the trans with no issues.

I'm also not sure if he is licensed, if that matters to you. So if you are in no big hurry, he's probably the guy to go to.

For local trans work I would highly recommend Mt. Clemons Transmission in New Baltimore. Dave has always been very good, very reasonable, and very quick. I've had about 20 transmissions done through him for my shop, and I've never had any issues. Also, every quote he has ever given me has turned out to be exact or lower when I picked it up. He has never jacked up the bill like some other local shops.
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Old December 1st, 2009, 08:39 AM   #7
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When buying a 700r4 try to get a second gen one. gm took care of a few problem that the first gen had. They are same trans but the second gen just has better internals.
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Old December 1st, 2009, 10:23 AM   #8
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You will need to buy or make some sort of set up for the lockup on the converter for it to work properly. that will be the 4 prong conecter at the side of trans. There are a few different aftermarket ones to chose from out there.
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Old December 1st, 2009, 05:19 PM   #9
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i was considering the painless vacuum controlled unit, unless anyone has a better suggestion?
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