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

Hi, it's been a while since I've been on here. I have a C10 with a SBC350 and TH350 with totally unknown history. I've had this truck for four years and basically drive the truck 500 miles a year or less. When I first had it it seemed to shift funny from 2nd to 3rd gear but that somehow went away on it's own so I ignored it. Maybe it had too much fluid in it?

The trans pan has always been leaking a bit of fluid so I thought maybe I should change the gasket with a cork one, replace the filter and replace the fluid. I'm not sure if that may be asking for trouble and I should just leave it alone. I've read some say not to change the fluid unless its been routinely changed and I don't know the history of this thing. I've attached photos to show what the fluid that's leaked looks like.

Also, if I want to drop the pan I can't get to the two back bolts because the crossmember is in the way. Would I need to remove the two inner transmission mount bolts and ever so slightly raise with a floorjack after removing the cross member bolts so I can slide the cross member horizontally out of the way to access the rear pan bolts while I leave the floorjack in place?

 

350s are a "dirty" trans so benefit from oil changes, especially as
you'll only get 1/2 or less out. Looks like your rear mount has collapsed,
common on a leaker, so you probably have gear whine in 1st gear
that disappears in 2nd. Take out the 2 lower mount bolts, lift up the
trans with a jack by the tailshaft, and you should be able to get the
2 pan bolts with a 3/8 drive non-impact flexsocket. Use a rubber/cork
gasket, bang all the dimples out of the pan where every bolt has been
overtorqued using a 1" or so round bar in a vise and a ballpein and go
3 times around when tightening the pan bolts, do not over tighten. And
get an OE quality rear mount. Put a retainer and bolt on your speedo
adapter and if if is leaking by the cable end, it is probably worn oval
inside [if it is aluminum] replace with a cast iron one from a powerglide.

 

I see what looks like sealer oozing out around the gasket so it has been changed before. good time to instal a new dipstick and kick down o-rings.

 

Leave it. At 500 miles a year it's sitting way more than moving. You say it shifts okay now. Your catch pans don't indicate much of a fluid loss between drives, so just check it, top it off and forget it. When it stops leaking is when you should be worried.

 

That is some nasty looking fluid

 

Why worry about something driven less than 42 miles a month.

 

Thanks, the reason why, because it's annoying to leave it in my driveway leaking trans fluid on occasion when I don't want to have it inside my garage. I was going to replace the gasket with a cork one but if I do that then I was wondering if I'm asking for trouble replacing the fluid or not.

 

I think! I don't know the history of that thing.. the fluid looks not too bad on my finger in the picture but that might be deceiving?

 

Thanks, I'm tempted to leave it. What is annoying are the stains on my driveway when I want to leave it outside and not have any catch pans. I'm using old plastic bin lids when it's inside the garage. I was mainly looking to fix the leaks so I don't have a mess in my driveway and that's where I wasn't sure if replacing the fluid might be a bad idea with an unknown transmission. It does seem to shift ok.

 

Thanks for the advice. Will there be any risk in changing 5qt of fluid with the transmission history being completely unknown?

What do you mean by the rear mount collapsed? The actual rubber transmission mount that bolts up to the crossmember?

 

Thanks! It's a rubber gasket so someone must have changed the previous gasket and I think the orange markings are sealer but hard to tell. I do know for sure the gasket is rubber.

 

Also I forgot to mention that one of the pan bolts on the corner is stripped and when I looked at the threads on the bolt itself it looked fine but the problem appears to be in the transmission housing. It tightens a little and then spins and not sure I can use something to make it grip, like teflon tape. I don't know if that indicates I'll likely waste my time adding a cork gasket to fix the pan leak? This truck is useful for some stuff but not worthy of a trans rebuild.

 

I don’t think there is any “risk” in changing transmission fluid. If you don’t know the history, I would definitely change it. Either a rubber or a cork gasket will work. Using a LIGHT coating of silicone or Aviation Permatex will make it completely leak free. Straighten out the transmission pan sealing surface as someone else mentioned.

As bundoc bob mentioned, your speedometer cable retainer is missing. I would recommend installing one of those, and replacing that o-ring while you are at it. That is a very simple job, and will likely help with tho oil leaks.

When you have the transmission pan off, you should repair the stripped out bolt hole. Buy yourself a heli-coil kit. You need one 5/16 inch, coarse thread. They are pretty simple to use, and it will make the stripped out bolt hole as good as new.

 

Like a few have mentioned if your going to do your pan gasket, check the other external gaskets at the same time and do those as well.

the fluid does not look horrible on your finger, but if your concerned save and reuse your old fluid and just add new to get it back to the correct level.

an easy but “dirty” way to fix stripped pan bolts into aluminum trans housing is get a small thin cotter pin, open it up flat and put it into the bolt hole with the pan bolt . When you tighten the bolt the cotter pun acts like a Heli coil and allows the bolt to torque down in place.

I’d fix the leak and possibly the mount if it’s collapsed, somewhat common on these units
And drive on.

the th350 is a good strong trans and cheap to build and maintain.

 

My recommendation would be not touch anything and sell the truck to me. LOL


Sent from my iPad using The H.A.M.B. mobile app

 

I have heard lots of guys say "don't change trans fluid, it will make it slip" their logic is that on a trans that has not been serviced for a long time the new fluid will wash the varnish off of the valve spools and cause internal leakage. I say BS, anytime you can get new oil with all new friction additives that come with the new oil, it will be a good thing. I say change the fluid and try to get that leak stopped. Hard to tell but that fluid looks dark, like it is burnt.
And I bet you could get the rear pan bolts with a 1/4" drive socket and U joint. If not unbolt the mount and replace it while your at it.

 

I'm with Driver50x and 46international Drain the oil, change the gasket, filter, put new o-ring on dip stick tube, fix speedo cable retainer, put help-coil (check size) in and change rear trans mount. One days work and a few bucks. Transmissions leak when they sit for a long time because the converter is full when running and drains out by half when shut off. That level of fluid is up higher than trans pan gasket so there is always fluid trying to seep out. Good Luck.

Pat

 

@dundoc bob, referring to the whine, what causes this? I have a th350 with a whiney 1st gear and the trans mount was failed when I bought the car - the misaligned crossmember has been reworked and the mount replaced. It's been running well for 4 years in my ownership, but the whine is a concern and I'm wondering what damage has been caused and when it's likely to fail on me.

Chris

 

Don't get a 3/8 helicoil kit. All turbo 350 pans are held on by 5/16" 18 (coarse) bolts about 3/4 inch long. Many times just a tap and a longer bolt will solve the problem on a trans pan. Either way, it's 5/16 coarse for the correct bolt size.

As others have said, trans mount, speedo drive seal and clamp, and pan gasket are at most a 1-2 hour job. Give it a go.

 

Buy a plastic spray bottle and fill it with diesel fuel (or kerosene) and keep it handy. Whenever something drips or leaks on the concrete, spray it with the diesel fuel and in a day or two it will evaporate into thin air without a trace. It helps if you don't wait too long or let it rain on the area before spraying. Try it............its like magic

But I would still fix the leak. Are you sure its not the front seal and the wind just blows it rearward?

 

Change it out , fix it and be done with it, Its part of reg service not wives tales.

 

Transmission fluid, like engine oil, will break down over time and with use. Do you change the oil in your engine or just leave it in there forever?

 

I’d grown up hearing that as well “ don’t change the fluid on old trans”. Well, I picked up a 67 Chevy in the late 70’s My dad asked if I was going to change the tranny fluid. Told him no and what I’d heard. He replied along the lines of “better to know now when your close to home “. Never did have issues with it after changing the mucky fluid

 

Thanks everybody for the suggestions. I'll move the cross member, drop the pan, replace the filter, replace gasket with cork and add new fluid and take care of the speedometer seal, trans mount, and add a helicoil for the stripped bolt hole. I'll report back once I do all that!

 

Absolutely change the fluid . there are o rings on the shift shaft , seal or o ring on the kickdown cable , both internal & external seal,/ o ring on the speedo bullet . heli coil is quick & simple . cork gasket , rubberised cork , no gasket at all ( right stuff or silicone) are all effective , IF USED PROPERLY , straightening the pan rail is probably the most difficult thing for can novice to do correctly.

 

Thanks, I looked underneath and I don't see any leaking from the rear drive shaft seal, nor the dipstick cable. The leaks seem to be mostly from the pan gasket, the speedometer cable and from the side, I'm not sure what the cover/retaining clip has inside and what that is called but there is a drip of fluid at the bottom of the clip.

What is that called (I've attached a photo)?






Does Napa carry most of the seals? I'm looking up a 1973 Chevy Chevelle which would have the TH350 trans. I couldn't find a speedometer retainer on Napa's site but found one on ebay.

 

I take my coupe (350turbo) to a transmission shop for oil change and checkup
cost $120 easy

 

That's the governor cover and there is an o ring behind it. Any transmission shop near you will have the speedo drive tab, its the same as almost every GM trans auto or manual. While there, see if they will sell you the seals you need.

 

Thanks! I thought that might be the governor but I've never taken one of those covers off. I found the speedo retainer on ebay for $10 so I just order it. I'll get the seals from Napa if they have them otherwise I'll check in with a local trans shop.

 

I bought an O/T Impala from a guy who never changed his trans fluid when I worked at the Grand Canyon.
He drove it over from L.A. and got up to the Canyon and it quit pulling. I gave him a 100 bucks for the car. I pushed it into the shop that night after work and pulled the pan. the fluid was almost yellow and the filter was all but stopped up.
After an oil and filter change and a shot glass full of brake fluid the car started to pull itself.
I drove it around for about a month before I would venture to Flagstaff. Then I drove it to Vegas and back. Ran great. Sold it to another employee at the Canyon and they drove it for at least 4 years before trading it in on a new car.
So I would highly recommend regular service on your transmission.