Th350 from heck

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Obwonkonobe

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Ethan
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C10 custom deluxe
Engine Size
350
I have a stock th350 in my 78 c10, 350 350 combo. And it leaks pretty bad. It leaks from the pan, rear driveshaft seal, and speedo. Ive replaced the pan gasket, Speedo bullit and oring, tail housing large oring to the trans, tail housing bushing, driveshaft seal, and the yoke with one that doesn't have a weep hole. I've actualy replaced the bushing and main seal a few times, as well as the speedo bullit and oring. Im considering rebuilding a better craigalist tranny in the future, and would like to keep my driveshaft, and have it fit my trans tunnel. The trans tunnel has no hole and is the slimmer one not for a th400. Any advice for an easy swap appreciated, and any advice on how to get this damn thing to quit crying is greatly appreciated
 

MikeB

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A pan leak is typically due to over-torqued bolts or bent/wavy pan gasket surface. In my experience, a cast aluminum pan like the one below will fix the problem forever. Typically less than $40 at Amazon or eBay with bolts and gasket.

Seems like new seals and o-ring should fix the other leaks.

Any chance the vent tube is obstructed causing pressure build-up?

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Obwonkonobe

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@MikeB never thought of a cast pan, sounds solid thanks. And no i haven't looked at the vent at all, how would i chech it without removing the trans?
 

78C10BigTen

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@MikeB never thought of a cast pan, sounds solid thanks. And no i haven't looked at the vent at all, how would i chech it without removing the trans?
Its not hard to get to, its on the driver side up high on the trans. Should have a little plastic cup like cap, pop that off and stick a wire in and see if any crap falls out. I had to check the same with my 350. (My trucks on its 4th th350)
 

78C10BigTen

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Most look like the first one but some have a metal tube like the second. The second one shows your location.

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SkinnyG

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G
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C10 Silverado
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Everything will show up leaking onto the pan gasket seam.

Also check:

Shifter shaft seal

Front pump o-ring and front seal (change the front pump bushing and gasket while you're at it, use a skim of Permatix Black on both sides of the pump bolt washers).

Governor cover o-ring

Intermediate Accumulator o-ring (on the passenger outside side of the case).

I use a new dipstick tube o-ring, but I also use a slightly larger one to seal the flange where the tube hits the trans case.

I put a slightly larger diameter o-ring around the speedo bullet (took a bit of squeezing), which seems to have stopped leaking (for now).

My TH350 is finally leak free!
 

Obwonkonobe

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Everything will show up leaking onto the pan gasket seam.

Also check:

Shifter shaft seal

Front pump o-ring and front seal (change the front pump bushing and gasket while you're at it, use a skim of Permatix Black on both sides of the pump bolt washers).

Governor cover o-ring

Intermediate Accumulator o-ring (on the passenger outside side of the case).

I use a new dipstick tube o-ring, but I also use a slightly larger one to seal the flange where the tube hits the trans case.

I put a slightly larger diameter o-ring around the speedo bullet (took a bit of squeezing), which seems to have stopped leaking (for now).

My TH350 is finally leak free!
I've been looking for an oversized oring for the Speedo, where'd you get yours?
 

Obwonkonobe

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Also, i looked at the vent, theres nothing in the hole and its pretty dirty, any suggestions on how to clean it or what to put in it?
 

Jason Deckard

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Try to stay away from a cork gasket for the pan. Find a good hard rubber gasket. I am always free for questions if anyone wants to call. I don't always have tI'm to catch up on these threads. 317-410-9918
 

QBuff02

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Any possibility it's simply overfull? Long story short, back when I bought my '82 and made the deal and drove it home, it had been sitting for several years with only a few short drives right before me buying it.. So needless to say I got several miles away from the guys house where I bought it and it started puking tranny fluid.. You could see it, smell it and it was blowing smoke to the point that my brother was following behind me and thought it had caught fire before I'd gotten it pulled off on a wide spot on the shoulder to investigate. Like oil slick under the truck leak. Thought from the heat and pressure it knocked a hole in a dry rotted trans cooler line and sent tranny oil spewing everywhere (best guess while laying on the side of the road) so we cut em both off to good rubber, re hose clamped them, and went on our way. couple miles down the road, same thing! Oil blowing everywhere, smoking and burning off the headers and exhaust making it look worse than it was! Got out and checked everything again, couldn't find a definitive source when stopped on the side of the road.. Took off again to more smoke and oil a mile or so later, My brother called asking if I wanted to abort and go get the trailer and come back for it, My response was "I'm not stopping at this point, we've got 10 more miles to go and it's either going to make it or grenade the sonofabitch!" I hung up the phone and kept driving, It carried on the whole rest of the way home, and even though it would very nicely BANG second and drive, it acted like it was half slipping at both part throttle and speed so I thought "If it makes it, it's probably Gonna need a trans rebuild".. Well, long story short, guy I bought it from had knocked the dipstick out, didn't get it seated all the way when he put it back in and overfilled the trans, by a lot! And basically, while I was driving home it was finding it's own level by puking out the excess from the vent tube! Once I got it home and cooled off to where I could crawl around and look at it, I could see the dipstick grommet was torn and the tube wasn't bolted up on the bellhousing. I'd guess he had about 3-4 quarts too much in it. But even after I made the repairs and cleaned it up, it still looked like it was leaking from about 3 other places for awhile after the fact.. And the slipping part of the trans, there's a difference between ported vacuum and manifold vacuum and that makes a huge difference for the modulator on the trans as to where it's hooked up, and apparently the previous guy didn't know that.. Lol switching the hose location solved that problem, and gave way to no slipping and happy, firm shifts! But I questioned it still for the next few months (mind you I only put about 500 miles on it last year) whether it was still leaking somewhere, because I'd drive it and it would leave drips, off the pan, off the rail, around the shift linkage seal, around the modulator, around the cooler lines and on the driveway.. basically anywhere tranny oil had penetrated from that vent blowout ordeal, it would leech out of when it got hot while driving, and it would make it look like a repeat or new leak.. Finally I said the heck with it and just drove it, and had fun with it, and after enough time and heat cycles, it doesn't "leak" anymore.. I know, long story, but I like stories, and I like trying to help..
:favorites13:
 

Obwonkonobe

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Ethan
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C10 custom deluxe
Engine Size
350
Any possibility it's simply overfull? Long story short, back when I bought my '82 and made the deal and drove it home, it had been sitting for several years with only a few short drives right before me buying it.. So needless to say I got several miles away from the guys house where I bought it and it started puking tranny fluid.. You could see it, smell it and it was blowing smoke to the point that my brother was following behind me and thought it had caught fire before I'd gotten it pulled off on a wide spot on the shoulder to investigate. Like oil slick under the truck leak. Thought from the heat and pressure it knocked a hole in a dry rotted trans cooler line and sent tranny oil spewing everywhere (best guess while laying on the side of the road) so we cut em both off to good rubber, re hose clamped them, and went on our way. couple miles down the road, same thing! Oil blowing everywhere, smoking and burning off the headers and exhaust making it look worse than it was! Got out and checked everything again, couldn't find a definitive source when stopped on the side of the road.. Took off again to more smoke and oil a mile or so later, My brother called asking if I wanted to abort and go get the trailer and come back for it, My response was "I'm not stopping at this point, we've got 10 more miles to go and it's either going to make it or grenade the sonofabitch!" I hung up the phone and kept driving, It carried on the whole rest of the way home, and even though it would very nicely BANG second and drive, it acted like it was half slipping at both part throttle and speed so I thought "If it makes it, it's probably Gonna need a trans rebuild".. Well, long story short, guy I bought it from had knocked the dipstick out, didn't get it seated all the way when he put it back in and overfilled the trans, by a lot! And basically, while I was driving home it was finding it's own level by puking out the excess from the vent tube! Once I got it home and cooled off to where I could crawl around and look at it, I could see the dipstick grommet was torn and the tube wasn't bolted up on the bellhousing. I'd guess he had about 3-4 quarts too much in it. But even after I made the repairs and cleaned it up, it still looked like it was leaking from about 3 other places for awhile after the fact.. And the slipping part of the trans, there's a difference between ported vacuum and manifold vacuum and that makes a huge difference for the modulator on the trans as to where it's hooked up, and apparently the previous guy didn't know that.. Lol switching the hose location solved that problem, and gave way to no slipping and happy, firm shifts! But I questioned it still for the next few months (mind you I only put about 500 miles on it last year) whether it was still leaking somewhere, because I'd drive it and it would leave drips, off the pan, off the rail, around the shift linkage seal, around the modulator, around the cooler lines and on the driveway.. basically anywhere tranny oil had penetrated from that vent blowout ordeal, it would leech out of when it got hot while driving, and it would make it look like a repeat or new leak.. Finally I said the heck with it and just drove it, and had fun with it, and after enough time and heat cycles, it doesn't "leak" anymore.. I know, long story, but I like stories, and I like trying to help..
:favorites13:
Thats pretty much what im hoping for lol, that one day it'll just quit prob gonna sell the trans and toss in a 4l60 or something of the sory
 

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