Adapt 4x4 Th350 in Place of 700r4

[TRAZOII]

I have a question about the swap. I did the kit swap with the longer tail shaft. Everything fits perfectly. Only issue I have is I think the torque converter is too small. What size TC would be used for a 85 k10 suburban in a th350? My truck seems to be missing power after the swap. Almost like its slipping. The th350 was fresh from a rebuild. And the shift points seem to be lower. Any one have any ideas?

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[Jason Deckard]

There is not a standard size that is used. It is based on the power curve of your engine. A stock 400 turbo converter is 12" I think. A stock 350 turbo is 11 inches. The smaller the diameter, the more stall speed you get out of it. The stall speed also has a lot to do with the torque output of your engine. If you have a bigger camshaft, you would want a smaller converter to help with idle-in-gear problems and to allow your engine to Rev a little higher to get that cam working. I have used a 400 converter in a 350 transmission and vice-versa. I don't know where you are lacking power, but you also have to consider that the 700r4 you had was equipped with a locking converter. There is a clutch that is in the convertER that locks up after it got up to a certain speed. Chances are your 350 doesn't have that. They were junk.

Hope it's helps

 

[HotRodPC]

I have a question about the swap. I did the kit swap with the longer tail shaft. Everything fits perfectly. Only issue I have is I think the torque converter is too small. What size TC would be used for a 85 k10 suburban in a th350? My truck seems to be missing power after the swap. Almost like its slipping. The th350 was fresh from a rebuild. And the shift points seem to be lower. Any one have any ideas?

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Have you counted your shifts and certain you're taking off in 1st gear? Do keep in mind, the 700r4 has a 3.06:1 first gear ratio, and the Th350 has a 2.52:1 or so first gear ratio so there will be a hair of difference since 700r4 has a deeper 1st gear ratio.

 

[fast68chevy]

hmm well with our bone stock 82 K10 shortbed it was a 700r4 truck factory and it still is the 208 case and we stuck a th350 tranny in it and didnt change swap anything else out at all. so i dunno what ppl are talking about being different.,. cuz nothing was.,., been driving it 20 years now with the th350 now.. still has the 700r4 OD indicator plate in cluster. 82 to 84-1/2 700r4 were weaker and higher failure rates didnt have the aux valve body and other upgrades yet. so going to th350 was a upgrade actually.

 

[bucket]

hmm well with our bone stock 82 K10 shortbed it was a 700r4 truck factory and it still is the 208 case and we stuck a th350 tranny in it and didnt change swap anything else out at all. so i dunno what ppl are talking about being different.,. cuz nothing was.,., been driving it 20 years now with the th350 now.. still has the 700r4 OD indicator plate in cluster. 82 to 84-1/2 700r4 were weaker and higher failure rates didnt have the aux valve body and other upgrades yet. so going to th350 was a upgrade actually.

That's because it's a direct swap if you install a th350 that already had a 208 behind it. But most people don't have that as an option and are using a 2wd trans along with a spacer. So THAT'S what people are talking about being different.

 

[HotRodPC]

hmm well with our bone stock 82 K10 shortbed it was a 700r4 truck factory and it still is the 208 case and we stuck a th350 tranny in it and didnt change swap anything else out at all. so i dunno what ppl are talking about being different.,. cuz nothing was.,., been driving it 20 years now with the th350 now.. still has the 700r4 OD indicator plate in cluster. 82 to 84-1/2 700r4 were weaker and higher failure rates didnt have the aux valve body and other upgrades yet. so going to th350 was a upgrade actually.

The Th350 with a NP208 is hard to find from the factory. GM did convert many 700r4 trucks back to Th350. Yours obviously wasn't one of them since you mention the gear indicator is still 700r4. When GM did it, they converted the indicator and plate in the column too.

 

[scenic760]

I tried posting up a thread but you guys sound like you may have already worked through what I was asking....

I have a th350 with a short shaft I would like to swap into my 1981 Suburban that has a th350 in it now and an np208. I have read a few different forums that you can take of the tailpiece and lop off 3/4" of the output shaft and everything bolts right up? Does that sound legit?

 

[HotRodPC]

I tried posting up a thread but you guys sound like you may have already worked through what I was asking....

I have a th350 with a short shaft I would like to swap into my 1981 Suburban that has a th350 in it now and an np208. I have read a few different forums that you can take of the tailpiece and lop off 3/4" of the output shaft and everything bolts right up? Does that sound legit?

I can't swear to that since I've not dealt with a NP208 and Th350. I think those are a hair rare since by the time the NP208 was used, the 700r4 was used most of the time. Most of, but not all, most of the Th350 would have had the NP203. It would seem like it could work though. @bucket probably knows more about what years a NP208 and Th350 were OEM. It couldn't be to many.

 

[bucket]

I can't swear to that since I've not dealt with a NP208 and Th350. I think those are a hair rare since by the time the NP208 was used, the 700r4 was used most of the time. Most of, but not all, most of the Th350 would have had the NP203. It would seem like it could work though. @bucket probably knows more about what years a NP208 and Th350 were OEM. It couldn't be to many.

They were mostly all made in '81. The 700r4 became available in '82 and from what I've seen, most trucks got that option.

I'm a bit foggy on the subject, but from what I remember, the th350 used with the 208 was an odd duck. I think it had a longer output and it's own t-case adapter that made the overall length the same as the 700/208 combo. Which would make the short tail th350 not an option to swap in. But don't quote me on that... it's not a combo I'm familiar with, I'm tired and I've been scattered brained lately from being so busy, lol.

 

[HotRodPC]

They were mostly all made in '81. The 700r4 became available in '82 and from what I've seen, most trucks got that option.

I'm a bit foggy on the subject, but from what I remember, the th350 used with the 208 was an odd duck. I think it had a longer output and it's own t-case adapter that made the overall length the same as the 700/208 combo. Which would make the short tail th350 not an option to swap in. But don't quote me on that... it's not a combo I'm familiar with, I'm tired and I've been scattered brained lately from being so busy, lol.

That would seem to reason to me too since all the Tcase adapters after that year seem to make all the dimensions the same as far as driveshaft and linkages. But, If he's just wanting to use his Th350, he should be good to go since the Tcase adapter that's bolted to his NP208 should be the one he needs. I'd day the best thing for him to do is measure the Tcase adapter and compare that to the tailshaft housing of the 2wd unit. I'd almost bet it's going to work as he's been told.

 

[bucket]

That would seem to reason to me too since all the Tcase adapters after that year seem to make all the dimensions the same as far as driveshaft and linkages. But, If he's just wanting to use his Th350, he should be good to go since the Tcase adapter that's bolted to his NP208 should be the one he needs. I'd day the best thing for him to do is measure the Tcase adapter and compare that to the tailshaft housing of the 2wd unit. I'd almost bet it's going to work as he's been told.

That's true, a 700/208 adapter should work fine. You would just have to use the aftermarket spacer in addition to trimming the shaft.

Even a late style "short foot" t-case adapter would work fine if the correct tall one cannot be found. It would just need a "W" or "wavy" style crossmember to match.

 

[Albrigap]

We bought many of these aluminum adapters for the 4x4 350's and cut the tail shaft in the chop saw. Never had a problem.

I had an 82 GMC C15 4x2 6.2 diesel with the 2.42 rear end. I threw away the garbage 700R4 and went to a 400 turbo. Never hurt the mileage a bit. The 700 in overdrive with the 2.42 would have put the rpm's at 55 mph just over idle.

 

[HotRodPC]

We bought many of these aluminum adapters for the 4x4 350's and cut the tail shaft in the chop saw. Never had a problem.

I had an 82 GMC C15 4x2 6.2 diesel with the 2.42 rear end. I threw away the garbage 700R4 and went to a 400 turbo. Never hurt the mileage a bit. The 700 in overdrive with the 2.42 would have put the rpm's at 55 mph just over idle.

Yes, the 2.42, 2.73 and 3.08 with the 700r4 is such a waste and also a trans killer. If you have a 700r4, you need 3.42, 3.73 or 4.10's to not only help out that weak transmission but help your around town mpg and being able to take off from a stop or light or get on the freeway with enough speed to keep from getting ran over.