My TH350 is shot, right?

[JMD]

Hi all,

I put a TH350 in my '68 K10 that was in unknown condition. Bone stock 454 in front of it, and an NP205 behind. I changed the filter, and put a B&M shift kit in it, but other than that I didn't touch it before I stuck it back in the truck. For all I know it could be the original trans. I put about 1200 trouble free miles on it and it worked flawlessly. It would chirp the tires going into 2nd, and the 2-3 shift was pretty crisp as well.

Recently the 2-3 shift became pretty soft, even under light throttle. It got to the point where it would spend over 2 seconds between gears, so I knew something was up. I checked the fluid, it was full and didn't look or smell burnt. The kickdown cable was also adjusted properly, so I just kept driving it hoping it was all in my head.

Fast forward to earlier this week and the truck wanted to hold onto 2nd gear way too long, and I had to let off the throttle completely to get it to shift. Once in 3rd it would hold it no problem. Once in awhile it would shift fine though, and it seemed to shift better under hard acceleration (firm, quick shift)

Today it refused to go into 3rd unless I really played with the throttle and revved the engine quite a bit. If it did shift, any throttle at all would cause it to slip out of third. I also noticed that reverse is slipping quite a bit. From what I've read online, these symptoms together indicate that one of the clutch packs is shot.

Is it time for a rebuild? I was planning on rebuilding the whole thing eventually, but I wanted to make it through fall and work on it over the winter when I can't be driving it anyway. Anyone have some an idea of what else it could be?

[hjewell2]

Originally Posted by JMD

Hi all,

I put a TH350 in my '68 K10 that was in unknown condition. Bone stock 454 in front of it, and an NP205 behind. I changed the filter, and put a B&M shift kit in it, but other than that I didn't touch it before I stuck it back in the truck. For all I know it could be the original trans. I put about 1200 trouble free miles on it and it worked flawlessly. It would chirp the tires going into 2nd, and the 2-3 shift was pretty crisp as well.

Recently the 2-3 shift became pretty soft, even under light throttle. It got to the point where it would spend over 2 seconds between gears, so I knew something was up. I checked the fluid, it was full and didn't look or smell burnt. The kickdown cable was also adjusted properly, so I just kept driving it hoping it was all in my head.

Fast forward to earlier this week and the truck wanted to hold onto 2nd gear way too long, and I had to let off the throttle completely to get it to shift. Once in 3rd it would hold it no problem. Once in awhile it would shift fine though, and it seemed to shift better under hard acceleration (firm, quick shift)

Today it refused to go into 3rd unless I really played with the throttle and revved the engine quite a bit. If it did shift, any throttle at all would cause it to slip out of third. I also noticed that reverse is slipping quite a bit. From what I've read online, these symptoms together indicate that one of the clutch packs is shot.

Is it time for a rebuild? I was planning on rebuilding the whole thing eventually, but I wanted to make it through fall and work on it over the winter when I can't be driving it anyway. Anyone have some an idea of what else it could be?

..

[Bigdav160]

The same clutch is used in third and reverse. Yes, i think it's time for a rebuild. Easy to overwork a TH350 with a big block. They were not intended for that application.

[Steeveedee]

Check your vacuum modulator for leaks. It can give similar conditions.

[AussieinNC]

TH350 in a 4WD environment behind a big block is a nice recipe for failure...3rd / reverse pack failure is very common...

Time for a TH400 to be put in where the 350 currently resides...

TH400 clutch packs should be maxed out with thin steels and Red plates...HD band...and a good shift kit...make sure vacuum modulator is correctly plumbed and ported to full engine vacuum, not timed vacuum on carb.

You will need to put in an electric kick down switch system...

Most important is an aftermarket trans oil cooler that bypasses the engine radiator totally...

Have fun with Winter mate...

[JMD]

Vacuum modulator is fine, that was one of the first things I checked. No leaks, no fluid in the line, and turning the screw out has no affect on the 2-3 shift.

I agree that a TH350 behind a big block isn't ideal, but I know they can be built to withstand upwards of 450hp and 400ft/lbs. From the research I've done, parts to rebuild this transmission stronger will be around $400. My 454 is pretty weak, I'd be surprised if it were putting out more than 270 horses.

My other option is a TH400, but I would need to find an adaptor for my NP205 and those cost upwards of $700 (If I can find one). Or I can buy an NP208, but I'd rather not pay $300 for a weaker part. Plus I'd need to shell out another $100 to get the driveshaft shortened.

Third option, and my goal for the future is a 4l80e, but I haven't found a way to do it under $2000

At the moment, rebuilding the TH350 is the best option to get me on the road within a week, and who knows, it might even last through next summer. Thoughts?

[JMD]

Things would be so much easier if I ditched the transfercase and just made it a 2wd truck...

[AussieinNC]

The output shaft is different between TH350 4WD and TH 350 2WD...plus the rear housing is totally different...

Driveshaft is different between 4WD and 2WD...

TH 400 4WD is still my suggestion....