Rebuild 1977 Transmission?

[Ron M.]

I'm having seals replaced on '77 Corvette and transmission has to be pulled to do it. Mechanic inquired whether I want to rebuild transmission while it is out of car. Mileage is 50,000. I am new owner and just learning about Corvettes. I have only driven it 200 miles since buying. Given that the car is 42 years old, is it a good idea to rebuild even though no transmission problems have been observed so far?

[Patrick H.]

Re: Rebuild 1977 Transmission?

I assume it is a 4 speed?

Was it leaking?

[Ron M.]

Re: Rebuild 1977 Transmission?

Transmission is automatic. Engine is 350. Yes, significant leakage, both front and rear. Mechanic says transmission has to be removed to replace front seal. Rear seal has already been replaced.

[Jerome P.]

Re: Rebuild 1977 Transmission?

When you say significant leakage are you referring to the engine front and rear seals or are you referring to the transmission has significant leakage.

[Ron M.]

Re: Rebuild 1977 Transmission?

Leakage is at the transmission seals, front and rear.

[Joe L.]

Re: Rebuild 1977 Transmission?

Leakage is at the transmission seals, front and rear.

Ron------


Your 1977 should have a THM-400 transmission. These are pretty stout transmissions and are usually long-lived. If this car has not been abused in its life up to now, I'd say you have a lot of good miles left to go. Depending on how you plan to use the car, you might have a lot more miles left than you're ever going to use.

[Richard G.]

Re: Rebuild 1977 Transmission?

I have never seen a front seal leak on a 350THhm. I have even observed them with grooves in the torque converter snout and they still held oil.
It is possible, just not likely in my opinion. There are several other seals that have a higher probability of leaking. Seals under the bolts and a 10 inch diameter "O" ring that seals the pump to case. I have also identified converters that were cracked at the snout to converter weld.

Often it's the dipstick seal leaking and it runs down the pan ledge and drips from the front. Sure, it looks like a front seal leak. The original "O" ring was leak prone. GM replaced this seal with a large seal that looks like a top hat. Bad news is the
new seal requires a different tube. These are available aftermarket as a kit. Gauge, tube and seal in one package. Any transmission shop can get you one.




If it was me I would replace the tube and gauge with the updated parts and see if it still leaked. If the function is good on the transmission it will likely continue to work for a long time.
Rick

[Steven B.]

Re: Rebuild 1977 Transmission?

'77 L-82 th 400

[Joe L.]

Re: Rebuild 1977 Transmission?

Leakage is at the transmission seals, front and rear.

Ron------


You can easily identify which transmission you have in one of several ways. The easiest is just looking at the transmission oil pan. The THM-350 has a pan which is mostly rectangular with one corner having a 45 degree slant. The THM-400 has an "irregular" shaped pan. Both pans use 13 bolts.

You can also look at the identification plate mounted on the side of the transmission.

[Richard G.]

Re: Rebuild 1977 Transmission?

The filler tube seals were the same on both the THM350 and THM400. Both can be updated to the better seal.

One could us this:
Dipstick Filler Tube END REPAIR KIT TH350 TH400 700R4 Transmission

https://www.classictransmissionsolutions.com/dipstick-filler-tube-end-repair-kit-th350-th400-700r4-4l60-4l60e-transmission-always-free-shipping/

It basically converts the early tube to the later style with the improved seal.

I would keep the top of the tube stock. The generics are typically taller so they can also be used on trucks. When you retrofit to the cars it is possible to run into hood clearance issues.

Changing the tube is no fun. I would typically recommend having it done. If you do it yourself I recommend the Corvette be on a lift.
You can drop the transmission cross-member so you can drop the transmission tail enough to access the Bellhousing bolt you will need to remove to get the tube out.
I highly recommend on of these to access the bolt that retains the tube.

This style socket and a 28" extension will take the frustration out of the job.
Rick

[David M.]

Re: Rebuild 1977 Transmission?

Is he trying to sell you something you dont need? Maybe. Do you trust him? Clean shop? Is he ATRA certified? Is he well versed on the quirks of a TH350 or TH400 and R&R'ing a trans out of a C3?

At minimum fix the leaks and do a full flush with filter(use the good filter). A full flush is 10-11qts not 5 as with a pan drop. 42 year old fluid needs to be changed. Do yourself a favor and buy a new trans pan with a drain plug, (Jegs & Summit have them). Save the OEM pan.

Where are the leaks? Tail shaft, filler tube, pan rail, front seal, cooler lines, gov housing et al?

If the leak is anything other than the front pump or converter seal it doesn't need to come out. All that can be fixed with the trans in the car.
If it needs a front pump or converter seal different story. Out it comes. At that point why not refresh 42 year old automatic trans? Its out, its easy to do just do it.

On that caveat, install upgraded frictions/steels/seals/bearings, a shift improver kit and new adjustable modulator etc. TransGo has the best parts.

You can pick up some nice performance with a fresh upgraded trans with the TransGo shift kit and a slightly higher stall converter (TCI). Firm shifts to me feel great.

I'd use noting but a TransGo rebuild kit. A complete kit with sleeve bearings and the good band and frictions. They have the best valve body shift kit separator plate period. You can go from mild to wild with tire screeching rubber out of 1-2, 2-3 upshifts, (a few other internal mods are required for wild, which you dont need but rather want).

Which ever level of rebuild you decide on use a new TCI converter and their clear synthetic trans fluid IF you go with a rebuild.

[Richard G.]

Re: Rebuild 1977 Transmission?

"At minimum fix the leaks and do a full flush with filter(use the good filter). A full flush is 10-11qts not 5 as with a pan drop. 42 year old fluid needs to be changed. Do yourself a favor and buy a new trans pan with a drain plug, (Jegs & Summit have them). Save the OEM pan."

Good advice.

Just don't believe in spending a $1500, to rebuild it, if you don't really need it. Although he didn't mention the cost, it's a ball park figure and low if anything. Could last another 20 years with the typically mileage on our old cars typically experience. The 400THM are almost bullet proof behind stock power. The 400 was first manufactured in 1964 and by 1977 had many factory upgrades that made it a fine transmission.

I average 400 miles a year on my car. That's 8000 miles in 20 years.
And leaks drive me nuts.
Rick

[Ron M.]

Re: Rebuild 1977 Transmission?

What an awesome group. This was my first post and I am amazed at the quantity and quality of the posts. These responses encouraged me to do more reading in my maintenance and repair books and I feel a bit more knowledgable now. I even took a print-out of the posts to my mechanic and we went through them together. After all that, I decided to go ahead with the rebuild for an additional cost of $1,000.

That may be an excessive cost, but I am buying, I hope, peace of mind.

The transmission was already out for the front seal replacement. The rebuild probably is not essential right now, but I will be driving 2-3,000 miles each year and I am avoiding second-guessing whether I should have had it done.

I sincerely thank each of you who took the time to respond for sharing your experience and expertise with me. You were most helpful. By the way, if anyone knows an excellent Corvette mechanic in Orange County, CA, I would appreciate knowing his name. I think my mechanic is very good, but he does not specialize in Corvettes and, besides, a second resource is always valuable.
Ron

[Tom R.]

Re: Rebuild 1977 Transmission?

What an awesome group. This was my first post and I am amazed at the quantity and quality of the posts.

Ron
I've been following along because I have a similar problem with my 78 350 THM. A few years back I had the tranny flushed, fluid replaced and new pan gasket. But the leak persisted (another source) and I concluded it was up by the torque converter. But reading the posts, I'll have to take a look at the filler tube. I know the leak is tranny fluid...red. Good stuff and I appreciate the technical expertise posted here.

[Joe L.]

Re: Rebuild 1977 Transmission?

What an awesome group. This was my first post and I am amazed at the quantity and quality of the posts. These responses encouraged me to do more reading in my maintenance and repair books and I feel a bit more knowledgable now. I even took a print-out of the posts to my mechanic and we went through them together. After all that, I decided to go ahead with the rebuild for an additional cost of $1,000.

That may be an excessive cost, but I am buying, I hope, peace of mind.

The transmission was already out for the front seal replacement. The rebuild probably is not essential right now, but I will be driving 2-3,000 miles each year and I am avoiding second-guessing whether I should have had it done.

I sincerely thank each of you who took the time to respond for sharing your experience and expertise with me. You were most helpful. By the way, if anyone knows an excellent Corvette mechanic in Orange County, CA, I would appreciate knowing his name. I think my mechanic is very good, but he does not specialize in Corvettes and, besides, a second resource is always valuable.
Ron

Ron-----

Did you determine if it's a THM-350 or THM-400? Also, what engine do you have---L-48 or L-82?