# TH350 to TH700R4



## CAT 245ME (Sep 10, 2006)

Alright guys here is what I have, 85 Chevy K10 with a shot TH350 and I recently bought a TH700R4 for the truck, now the 85 origionaly had the 700R4 from the factory along time ago so what will be needed to make the swap, the 700R4 is out of a 91 Silverado and I do have the adapter for the transfer case and dipstick.

Any known problems that I will encounter in the swap, & will my drive shaft lenght be ok.


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## B&B (Nov 4, 2006)

Depends on what they did when they swapped the 700 to the 350 in the first place..

If the 350 was swapped in with an aftermarket "kit" that many of the trans vender's used to sell back when the 700's were problematic, then the T-case and drive shafts will still be in their stock locations. These "kits" used a spacer between the T-case adapter and the trans to make up the difference in length between the 350 and 700. If you look closely you should be able to see the extra (usually aluminum) spacer if one of these kits were used. 

Now on the other hand if they sourced an original 350/208 combo (very hard to find) or they put the combo together with bits and pieces of factory components then it's going to take more work. The 700 is 1 3/4" longer than the 350 so your driveshafts would not work without mods. You'd need to shorten the rear one and lengthen the front one. Some front shafts have enough slip in the splines so you may get away with it on the front but the rear shaft will almost be guaranteed to need shortened.

Need to slide the cross member back to the original 700 location but the original holes should be in the frame rails.

Everything else is straight forward and will work with the 700..cooler lines, linkage and other related items. You will need to readjust the T-case shifter linkage but should be enough threads on the connecting rod to do so.

Once you know which way the original swap was done and if it has the spacer or not it will be clear on which way you'd be heading as far as what parts will and wont work.


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## CAT 245ME (Sep 10, 2006)

B&B after looking at the TH350 there is a 1 3/4 spacer between the tranny and t case adapter so this will save some work.

Another concern I have is were the 700R came from a truck with throttle body do I need different linkage on the carb to work with the 700R or will the linkage already work, the reason I ask is a guy told me that the carb linkage was different for the TH350 compared to the 700R or do you think I would need a different downshift cable.


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## B&B (Nov 4, 2006)

Hard to say on the cable. If they reused the original cable bracket on the intake when the swapped the 700 in you wont have to change it. But if they changed the bracket along with the trans to make it work you'll need to get the correct bracket.

The current cable on the 700 should work on your carbed set up though. But if something doesn't look right you can simply change the cable..just buy one for your carbed '85 and swap it. The cables are easy to change.


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## BOSS550 (Jan 19, 2007)

B&B may have some insight on this but when I swapped my 700 into mine I also had to eliminate the wiring for the electronic converter lockup. For a bulletproof swap I put a NP 205 behind my 700 with a different tailshaft from Advanced Adapters.


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## derekbroerse (Mar 6, 2004)

TC lockup should never be eliminated. Keeps the tranny cool and improves fuel mileage, and its just so damn easy to hook up!


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## doh (Sep 23, 2003)

Why?

The TH350 is twice the tranny of a 700R4?

Rember your have the axle ratio's for a non overdrive tranny.

As said 700R4s don't like to heat up. Add a cooler.


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## B&B (Nov 4, 2006)

derekbroerse;526025 said:


> TC lockup should never be eliminated. Keeps the tranny cool and improves fuel mileage, and its just so damn easy to hook up!


Agree... Hopefully when they originally removed the 700 they lleft the 4 pin lock up connector. hanging in the harness somewhere allowing it to simply be reconnected. Same with the vacuum control switch and related wiring on the firewall. If not, it'll take a bit of parts sourcing to get it all back up and working.


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## B&B (Nov 4, 2006)

doh;526362 said:


> Rember your have the axle ratio's for a non overdrive tranny.


What makes you suspect it has the incorrect gear ratios in the rears, if the truck originally had a 700 to begin with?


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## derekbroerse (Mar 6, 2004)

B&B;526368 said:


> Agree... Hopefully when they originally removed the 700 they lleft the 4 pin lock up connector. hanging in the harness somewhere allowing it to simply be reconnected. Same with the vacuum control switch and related wiring on the firewall. If not, it'll take a bit of parts sourcing to get it all back up and working.


Even better if the TH350 had the lockup too, then it would just be plug and play.

I don't think the TH350 is any stronger than a TH700R4, just the earlier OD's had some reliability problems... were pretty much all taken care of by '87 or so.


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## BREAULT69 (Jan 15, 2008)

The reason someone went through all the trouble to put a 350 in was probably due to the fact the 700r4 had the highest failure rate of any GM trans. in history. When they fail they are extremely expensive to rebuild due to large amount of updates required in an attempt to repair the numerous failures. Misadjusting the TV cable by 1/8" can cause failure in less than 5000 miles ! Not having the converter lock up system working properly will cook it on the highway. Best GM trans. for an (older) GM plow tuck is TH400 - bulletproof, but getting hard to find. Make sure and adj. the TV cable to spec with a pressure gauge, do not trust auto adjuster. Good luck.


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## derekbroerse (Mar 6, 2004)

BREAULT69;526965 said:


> The reason someone went through all the trouble to put a 350 in was probably due to the fact the 700r4 had the highest failure rate of any GM trans. in history. When they fail they are extremely expensive to rebuild due to large amount of updates required in an attempt to repair the numerous failures. Misadjusting the TV cable by 1/8" can cause failure in less than 5000 miles ! Not having the converter lock up system working properly will cook it on the highway. Best GM trans. for an (older) GM plow tuck is TH400 - bulletproof, but getting hard to find. Make sure and adj. the TV cable to spec with a pressure gauge, do not trust auto adjuster. Good luck.


As I said, EARLY 700's had their share of problems and there was no aftermarket solution. As for being the worst, I dunno if thats the case, the same vintage TH440T4's were pretty crappy too. Same problem, everything was made real light for fuel efficiency and not enough strength. Once they had their ballgame right, the 700 stuck around for a long time as the 4L60 and eventually the 4L60e and they are plenty strong if not abused. TV adjustment is critical, but you can be a click or two out and it won't just up and die. It allows for a little tuning but usually the 'right' spot is the 'correct' spot.

TH400's aren't perfect either, but they are strong. They have a crappy first gear (2.48 vs. 3.06 of the 700R4, over half a turn of the engine...) for getting up and moving with a load. They have no overdrive, so if its a dual-purpose truck you have to live with that. And yes, they can be blown up too with abuse.

Really the only reason the older trannys are considered better is because they have been developed for so much longer (and of course the 400 is just beefier) that they had the kinks out that much earlier. Nothing wrong with a properly modern-built 700R4. Mines been in my car since '93 or so, and while it doesn't get extreme amounts of mileage it does take full throttle launches on big tires (even sometimes slicks) and just keeps on going, yet still gets me 18mpg on the highway. Not bad for a '68, huh? 

But yes, that was a common solution in the 80's until they figured out how to build a decent O/D tranny. And the cost isn't that much more than a 350/400 any more, certainly a lot less than the late model stuff!!!


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## CAT 245ME (Sep 10, 2006)

Well here is what I know for sure, LMC truck lists the same downshift cable for the 82-86 carbed trucks as the 87-91 TBI blazers (305&350) so I removed the cable from my 89 K5.

I finally got a hold of the GM tranny man at the transmission shop and he said I would need the cable bracket from a 700R4 & carb setup, he also said the most important part was the road test and setting up the overdrive, like some of you posted that if the cable is not lined up right it will burn up. So once the 700R is in the truck, I am going to bring it in and have him road test it and set up the OD to be on the safe side. 

Any one think I should have the tourque converter checked over before the tranny goes in, I know the TC wasnt looked over when the 700R was rebuilt and first installed n the wrecked truck it came out of.

The 700R I have is a newer version from a 90 chevy, the axle ratio's are still the same in my 85, it has 3.08's. but the rear 10 bolt has longed been replaced with a 12 bolt (another popular 80's swap).


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## BREAULT69 (Jan 15, 2008)

It wouldn't hurt to have the converter flushed, if you have a shop to do it. They drill a hole in it & install a plug when done. The guy that used to do it for me here only charged $25 to do it, don't know what it might cost in your area. Did you price a new one ? Any signs of heat on the converter ? Bluish tint ? if so replace. Also feel for groove where seal runs. The cable is actually the "throttle valve cable", adjustment drastically changes pressure & shifts of transmission. Good Idea to have a shop adjust it. We always used a pressure gauge after burning up a reman by using auto adjust but it only happened once out of probably 20 700r4 installs.


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## CAT 245ME (Sep 10, 2006)

UPDATE:

Well this week the 700R was finally installed in the truck, everything went smooth with only one problem, whoever did the original TH350 swap cut out the 4 pin connector from the harness and then taped up the bare wires. I cant understand why because the 4 pin plug can be disconected from the harness, but I did find what I needed off a parts truck.

I also noticed the older 4 pin connector had 2 wires (from 86 Sierra parts truck) and my 89 K5 Blazer's 700R pin connector has 3 wires to the 4 pin connector, any one know what the extra wire is for? or should I just forget about it.


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## B&B (Nov 4, 2006)

Does the converter lock and unlock correctly like it should CAT? 

There were two different wiring configurations on the early 700's depending on whether the pressure switch in the valve body was normally open, or normally closed.

Some used the third wire as a means to ****** the timing at lockup.. if it's working correctly your good to go.


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## CAT 245ME (Sep 10, 2006)

Dont know how it's all workin yet cause the truck hasnt been road tested yet.

After the truck went into the garage, the mechanic went to shut the engine off but the ignition switch wouldnt turn off (stuck).

Not sure what the problem was cause my cousin stopped in and asked if I had a steering column to replace the one in the truck, apparently the problem was more than just the switch, but havent been talking with the mechanic yet.


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