TH350/NP205 Original set-up question

[kmoore]

I had this over in the Suburban message board and only received one response.

HELP!!

Looking for some insight from you experts with knowledge of what was original in my 72 K2500 GMC Sub. This is out of my league, know enough about an automatic transmission to be.......well in the position I am in now!

Problem started with a leak that was spraying fluid all over the exhaust while I was driving and smoking pretty bad, looked under the truck (after cleaning everything up to find the source) and drove it again with the same problem. The only place that it was obviously coming from was the transfer case, well that did not seem right.....that would be gear oil....right? Tightened everything up that I could on the transmission and transfer case again and still seemed to be coming from the transfer case. I had Transmission issues anyway so time to spend some money on the Burb since it has been a while right? Towed it to an AAMACO dealer who stated that they had experience with 40 year old transmissions and this would be a piece of cake.

Fast forward to now: There should be a seal between the transmission and the transfer case but there is none. I do have fluid in my transfer case but they have never seen that before nor his sources/advisors. "They can put it all together and should be all right"

If there was not a seal it has been that way during the many miles I have driven it with no issues before. He said everyone stated (his transmission sources) that it should have been a TH 400 since it is 4 wheel drive and 3/4 ton. I am not the original owner but my build sheet and all paper work shows TH transmission with an * but no other denotation of what the* means or 350 or 400 listed.

The question is should this have been a TH400 or did they put TH350 in 3/4 four wheel drive units? All mounting holes/brackets and geometry match to what is in there now and no sign of any changes.

What about the seal issue and tranny fluid in the transfer case? May have made a poor decision in AAMOCO but I thought I was doing good with national brand and resources.

If there was an seal/plate available that would change all the spacing of the mounts and would then lend one to think it was not originally set up that way....unless the TH400 was a tad bit shorter. Now I am confused.

First 3 pics are my transfer case/TH 350, and following 2 are from his source of a TH400/TC with a seal or plate.

I do appreciate any help or insight that anyone may have.

[special-K]

It's been a while since I had one apart,but everything looks the way it should from what I recall. They are wrong about the "should be a TH400". They were never used in these trucks. The NP205 transfer case came along when the first automatics went into the '69 Blazer. all trucks with automatic through '72 had TH350s. And even when GM started installing the TH400 in 4wds it was an option. TH350s went into more 4wds than TH400s all the way until o/d transmissions took over. I'm afraid you are wrong in thinking going to a national name shop is a better choice. The fact that this one knows so little about GM transmissions is proof. I used to have my automatics rebuilt by the guy who did most of the AAMCO work n this area before he retired. That's right,he rebuilt them and AAMCO just added their name to his work. Anyway,originally these transfer cases took 90 weight,as well as manual transmissions and differentials. But,at some point it became common practice to use ATF. I believe something changed in it's qualities or testing proved it lubed as well or better. I'm still not sure on that. I know it used to really matter that you used Detron in GM and not Mercron or others and now it's all the same.

[turp mcspray]

There are two seals inside of the trans to transfer case adapter, plus an o ring or two, inside of the sleeve. You will have to remove the adapter to see one of them from the transfer case side, and I think the front bearing, to see the other. When the seals go bad, you WILL get fluid transfer to the t-case. A complete seal and o-ring kit for your 205 can be had for $25.

[domeier]

Originally Posted by turp mcspray

There are two seals inside of the trans to transfer case adapter, plus an o ring or two, inside of the sleeve.

I had the same problem about a year ago. TH350/NP205. Trans fluid was low, but wasn't leaking onto the driveway. Took me some head scratching to figure out it was going into the transfer case. It's a pretty good sized PITA to repair, but there's no rocket science involved. I don't have a trans jack, so made a wood frame with a chain hoist to drop the transfer case. Napa had all the seals, about $20 if I remember right.

[mrein3]

On your NP205/th250 the part that is bolted between them is called the "NP205 to TH350 adapter". The part the couples the output shaft of the TH350 to the input shaft of the NP205 is called the drive sleeve. The drive sleeve seals the transmission fluid in the th350 with an oil seal that is just like the front oil seal on your timing chain cover on the front of your small block Chevrolet engine. And like the front oil seal, after 40+ years of spinning around, it can cut a groove in the drive sleeve.
There is the same setup on the back of the adapter. It too has an oil seal that keeps the tranny fluid or gear oil in the transfer case.
I've seen GM printed literature that states that you can use gear oil or tranny fluid in that t-case. I think they started using gear oil but later switched to transmission fluid with no changes to the t-case.
The fix is for sure to get two new oil seals. You MIGHT need a new drive sleeve if there are grooves cut in it.
If you've ever humped one of those units in and out you'll just buy a new drive sleeve if the old one is grooved BEFORE you bolt it together. This is NOT a job you want to do twice.

[JimKshortstep4x4]

I made a cradle out of two 2x8 boards and drilled the necessary holes so that it can be fastened to the transfer case using the mounting bolts. The cradle is fastened to the jack with a large bolt. This makes the job of removing/installing a NP 205 a much easier job.

Jim

[donut]

Same set-up as my '73, though I don't remember exactly how I changed them.
NAPA got me the set. Entire transmission overfilled the TC when the seals let go.

Originally Posted by turp mcspray

There are two seals inside of the trans to transfer case adapter, plus an o ring or two, inside of the sleeve. You will have to remove the adapter to see one of them from the transfer case side, and I think the front bearing, to see the other. When the seals go bad, you WILL get fluid transfer to the t-case. A complete seal and o-ring kit for your 205 can be had for $25.

[bigbadchev72]

I would take the adaptor to another tranny shop that knows what they are doing, Have them replace the seals and then deduct that cost from aamco since they have no clue what they are doing, a 350/205 is one of the most basic/common setups