Technical TH350 Replace Fluid, Filter and Gasket or leave alone?

There's absolutely no downside to changing the fluid. The trans obviously has had some damage internally based on the metal in the bottom of the pan. You can't hurt anything by cleaning it all out and replacing it. On the other hand, keeping that crap in there will only accelerate the wear on your already damaged clutches and internal parts.

 

That was once your clutch & band friction material , looks pretty much worn out .

 

Thanks. I'll keep on with the original plan and hope I can get some more time with the truck before a trans rebuild especially since it's just my home depot truck or get down to the bay with my rowboat.

 

Thanks, yeah, it could be a completely original trans. It shifted fine the last times I drove it but shifted a little weird the very first few times I drove it when I had bought it 4 years ago. It will be interesting to see what happens when I put it all back together and fill with new fluid and it mixes with what's in the torque converter, etc.

 

You can do as you wish, but in my book there is no need to replace the stock transmission pan. The factory pans work just fine. But for Gods sake replace the fluid, don’t reuse the old trans fluid.

 

Yea will change the fluid thanks! For sure, I probably reacted too quickly when I bought the cast one on ebay.. it came out to $50 total shipped. not the end of the world, and maybe could have saved that but I often spend more time at night overthinking stuff and I might have lost some sleep over it so I'll keep that in mind for future projects. This truck surely won't be my last old car.

 

Here's an update finally after getting this pup back in action.

• Replaced the trans mount
• Replaced the governor seal
• Replaced the kickdown cable seal (not too bad)
• Replaced the shif-shaft seal (the tool had a hard time biting into the old seal so I took the assembly apart including the rooster comb since I had the pan out)
• Tried to replace the fill tube o-ring but there was none and the one I ordered wouldn't fit so had to use high temp red RTV like someone had done before since I didn't have patience to order a new fill tube.
• Replaced the accumulator seal (kinda of a PITA with the snap-ring and pushing spring back in).
• Replaced the filter.
• Drilled/tapped and installed a helicoil on the stripped bolt.
• Installed cork/rubber combination gasket onto pan without any adhesives/glues
• Installed new cast aluminum pan (what a pain, I wanted to use hex bolts but the draft on the pan didn't allow enough space to put a socket on them so I had to use the socket head cap screw and the ones that came with the pan require a metric size. I got annoyed and managed to obtain some new 5/18-16 socket head cap screw that take a 1/4" allen drive.
• Filled with new fluid (5 quarts) same amount that came out.
• Torqued bolts by hand snug but not too tight. I had a ft-lb torque wrench and read some articles online that they are not too good for the lower end and the typical 12 ft-lb maybe not applicable to cast aluminum. I didn't have patience to go search for an in-lb torque wrench so snugged by hand carefully but not too tight. Hopefully I don't have to retighten them because the cross-member definitely blocks the socket head cap screws in the back.

Once I finished all that work the damn thing wouldn't go in gear and I was thinking maybe I screwed up something with the rooster comb assembly but I remember double checking that. I looked at the dipstick and it was dry. I put in another 3 quarts of fluid after checking the dipstick tube each quart I added and then it went in gear. The new pan must be a little bigger than the old one.

I took it for a drive and oh my god it shifts so incredibly smooth its hard to believe. I hope that's not a sign the trans is about to blow or something but I couldn't believe how well it shifted! After I got back I tried to read the dipstick level and it looks like it's a little high but hard to tell because the fluid is so clear, even after I took it for a drive. I didn't observe any leaks at all once I had the engine running and looked under, and I took it for a drive and didn't see any leaking after I got back but I couldn't get close with everything still hot. I'll re-check tomorrow morning.

Thanks to everyone that has helped me with advice and answering questions I had asked!

 

Nice job, looks great and glad it's shifting better. Way to dive into it

 

and you got a nice drain plug!

 

Good deal ,drive that sucker anywhere now.

 

better to keep the fluid level a little LOW than high. I always keep them just below the full line to allow for expansion when the tranny gets hot.

 

Thought I had it! I checked again earlier today when I brought it back in the garage and it has some fluid drops collected on the extension housing in two/three spots.

The only thing I didn't mess with is the rear seal and the extension housing to case seal. I guess I should work on those. Does anyone know if I need to buy a bushing? I know there are some kits that come with a bushing, or I can buy the rear seal and housing o-ring from NAPA and skip the bushing.

I'm not sure how to take the driveshaft out, I don't see any U-bolts like I've seen on other cars anywhere at the trans yoke connection, the middle connection of the two driveshafts or on the rear axle. Is that a press fit assembly?

 

Looks like seeping from the rear transmission seal. After you pull the driveshaft out, be prepared with a pan to catch some fluid, then after changing seal, add as needed (more will leak will seal is out).

Also, that sure looks like the driveshaft is too long, unless I'm missing something or not seeing it properly?

You should be able to take the drive shaft loose from the rear, move it towards the transmission, lower it down (after taping the Ujoints together) then pull it out of the transmission.

But sure looks like the shaft is too long.

 

You unbolt the driveshaft at the rear end , take the bolts holding the carrier bearing out and slide the shaft back . Before you slide the shaft back , take ahold of the front yoke and " feel" if its excessively loose in the tailstock ,that & what the bearing surface of the front yoke looks like will tell you if you need a bushing . The rear seal is a simple install .

 

Mark the shaft and yoke and return in the same order. good time to change the joints too.

 

I'm not sure who did the mods to the truck to put in the 350/350 drivetrain and whether the rear end was kept stock and who would have shortened the driveshaft (or even what driveshaft is in it). It seems to drive ok, and from what I can tell it isn't rubbing against the transmission.

 

Thanks for the advice, I'll start with unbolting the driveshaft and checking out the play and what the yoke surface looks like.

 

I got the driveshaft out. When I checked for play on the yoke it didn't move all that much in either direction along the seal but when I pulled it all out and inspected the surface there are subtle ridges where the old seal or seals have ridden over time but it does look smooth over where the bearing rides. The ridges are subtle when I glide my finger over the entire surface. Speedi sleeves aren't used for this application are they?

Does it seem like I should replace the bushing? I'm also thinking might as well take the tail shaft out and replace the o-ring where it attaches to the case. I bought the o-ring, rear seal, and a rear seal and bushing kit in case I should do the bushing.

Also, I likely had the trans overfilled. When I checked this morning I only saw a drip at the rear seal not by the case to extension connection like last time I posted. Maybe it found its way out the vent and through there. I'm going to replace that seal anyways. The pan had a bit of fluid in it after it dripped out once the driveshaft came out. Hard to estimate but maybe 1/2 qt....

 

Update, I went ahead and changed the bushing and rear tailshaft seal and the tailshaft extension to case o-ring.

It was difficult to find the bushing at NAPA after looking up all sorts of 70s Chevys with TH350. I got the kit (bushing and seal) from O'reilly Auto, Part 5200 which is a National Brand (Federal Mogul) containing the bushing and the correct 9613S seal.

I used a 30mm socket to pop the old bushing out and a 32mm to drive the new one in after letting the bushing get cold in the freezer for a couple hours. I had to clean it up a little with a file afterwards just at the entrance so the driveshaft would slide in without getting scratched up.

I put it all back together and filled with 2.25 quarts to get it to the right level. 3 quarts came out when I took the tailshaft out so I was 3/4 of a quart over last time I had driven it. It drove fine yesterday and I checked for leaks today and it appears I am TH350 leak free (for now!).

One funny thing I observed, the speedo gear on the output shaft is purple while the speedo bullet has the red gear. I had bought the red to replace what was already in there and didn't know the output shaft had the purple one. Hmmm....... Not sure if that has any influence on my intermittent speedometer bouncing problem.

Thanks again everyone and Happy Thanksgiving.

 

If your new gasket and chrome pan leaks go back to the steel one and use a black neoprene gasket. Do not use sealer . Make sure bolt holes are flat . Torque bolts to 13 ft lbs . It will not leak- guaranteed .

 

Thanks, the new cork/rubber hybrid gasket and cast aluminum pan do not leak. I've been checking for the past 11 days and so far no leak. I didn't use any sealer. I torqued by hand because I didn't have an in-lb torque wrench and didn't want to trust the ft-lb torque wrench down that low. I went with a snug approach but not overly tight basically just to where I could see the gasket getting squeezed a little with the snug feeling. So far no leaking even after driving twice. If it leaks eventually, I'll buy an in-lb torque wrench and redo.

 

Only 2 1/4 qts to refill? You know you’re supposed to get it warmed up then put in drive/reverse a few times, then check while running.

 

There's more to this story. After initially pulling the pan and draining and working on all the seals (minus tailshaft) I put it all back together and filled with 4 quarts, it didn't move then I added about another 2 and a half or so. This was all 11 days ago. Definitely overfilled because it was really hard to see the fluid level, it was quite clear and the dipstick is shiny smooth chrome making it hard to see the contrast.

After seeing a leak from the rear seal, then I went back and pulled the tailshaft extension housing off and 3 quarts drained. Once I finished with the tailshaft seal, bushing, etc. I put it all back together and it took 2 and 1/4 quarts to get to the proper level with the trans hot. So in other words in the first round of work I had it 3/4 overfilled. Each time I added about half a quart I would go back and put it in each gear and go back and recheck after maybe 1-2 minutes to let the fluid all drain down the tube and I checked the dipstick on both sides at various angles in the sunlight and wiped before inserting to get a proper read. I did this all after the truck was heating up for maybe 30 min... I was cooking food and didn't want to tempt fate to go check the fluid level and burn my food so I let it really warm up fully until I was done eating!

 

I gotcha!

 

Thank you again for all the helpful advice, and also to all the others who have helped!

 

If it is 10 teeth, drive gear may be red or purple in color.

Bouncing speedo is usually related to the cable itself whipping.
This can be caused from a faulty cable, you can pull it out make a 'U' shape and slowly turn it by both ends, if it rotates smoothly it is fine. If it whips the cable is whipping inside the housing which will cause irregular cable speed and speedo bouncing.

If the cable is fine, verify it is not covered in ATF fluid from a leak. ATF will wash out the cable grease inside the speedo housing, which will cause the cable to hang up and whip = speedo bounce.
If the cable housing is not damaged, clean it out if it was ATF contaminated, dry it out, lube up the cable with light grease, reinsert and go for a drive.

 

The speedometer bullet I bought has a red gear with 21 tooth. I didn't count how many teeth are on the drive gear (the purple on the output shaft)

Thanks I'll give that a try next!

 

Shake the cable housing when you have it out. You might find a trace of a black powder, if so it’s graphite lube. Housing then needs to be flushed out real well, can of brakekleen will work now days.
I’ve never used any kind of oil or grease on them, just the powdered graphite.

My gut says the issue is a dry, in need of lube, Speedo assembly though., unless things were smooth before the gear change?

 

Thanks, I get a grinding noise simultaneous to the needle bounce which I can hear while driving the truck. It sounds like its coming from the dash when I hear it.

I read in some other forums it maybe be the speedo head needing to be lubed as well. I will take the cable out tomorrow and take a look and lube it and take it for a drive or alternatively use a drill to test.

It always did this since I've owned it even before I replaced with a new speedo bullet assembly.

 

If the inner cable is lubricated, and not dry, I have found speedo needle bounce is quite often caused by the square drive end (either gauge or g/box end) of the inner drive cable is slightly bent, straighten with pliers usually fixes the problem.. If it is still bouncing, as mentioned before, the clock workings itself might need a lube..