Rebuild 1977 Transmission?

[Richard G.]

Re: Rebuild 1977 Transmission?

Ron
Thanks for posting your solution. Everyone has one that works for them.
Good conservation.
Rick

[Jimmy G.]

Re: Rebuild 1977 Transmission?

With the difficulty of removing a transmission and the Murphys Law that says it will go at the worst time I would rebuild it if it were mine If for nothing else "Peace of Mind"

[Joe L.]

Re: Rebuild 1977 Transmission?

With the difficulty of removing a transmission and the Murphys Law that says it will go at the worst time I would rebuild it if it were mine If for nothing else "Peace of Mind"

Jimmy-----

Here's the conundrum: when it comes to automatic transmissions, there's nothing better than a factory-assembled unit. In my opinion, it's superior to field overhaul. The question is if a 50,000 mile old factory-assembled transmission is superior to freshly field overhauled transmission. I don't know the answer to that. However, if the transmission is a THM-400 which has never been abused, my opinion would be the factory-assembled transmission is better. If it's actually, THM-350, then I don't have an opinion.

[Dick W.]

Re: Rebuild 1977 Transmission?

I subscribe to the same train of thought as Jimmy Gregg. If you have the transmission out, it is better to overhaul it now rather than a few miles later. Lot of the time the replacement seals and clutches are better than the 1977 vintage OEM parts.

[Mark E.]

Re: Rebuild 1977 Transmission?

Jimmy-----

Here's the conundrum: when it comes to automatic transmissions, there's nothing better than a factory-assembled unit. In my opinion, it's superior to field overhaul. The question is if a 50,000 mile old factory-assembled transmission is superior to freshly field overhauled transmission. I don't know the answer to that. However, if the transmission is a THM-400 which has never been abused, my opinion would be the factory-assembled transmission is better. If it's actually, THM-350, then I don't have an opinion.

It's water under the bridge for the OP since he made the decision to rebuild, but my thinking is the same as Joe's: A low mileage factory built TH400 that's operating normally trumps a fresh rebuild at a local shop... unless the shop has a known stellar reputation.

When did Corvette transition to the TH350? From this thread it sounds like it happened during MY1977? Was it a running change?

[Tom R.]

Re: Rebuild 1977 Transmission?

When did Corvette transition to the TH350? From this thread it sounds like it happened during MY1977? Was it a running change?

Yes, partially. L82s go the THM 400 while L48s left with a 350 THM. Thereafter (1978) all M38s were THM 350s. Often, the broadcast code can be seen on the passenger side but one must get up under and look up.

The revision is about to get started for the 75-77 TIMJG but we'll keep an eye out for any L48s with THM 400s. Perhaps we'll get some indication whether it was a model change or running change.

[Gary R.]

Re: Rebuild 1977 Transmission?

my 02
When I had my 72 TH400 out it had about 55k miles on it. I personally went through it and added some modifications to it like a 69 2nd gear drum and higher element sprag, modified the pump and valve body. It shifts normally unless I nail it then it shifts much firmer and will catch 2nd gear. The only thing about the mods I did that I don't like is the pump pressure changed so an off idle drive, like in a parking lot, will cause it to clunk. It's annoying but I know what it is.

If you find a good "old time" trans guy, a TH400 shouldn't be a problem. If good parts are used it should go a long time in street car. The mods I made are for more power and abuse then I really need so you should speak to your tech. If he knows these he will guide you in the build.

[James G.]

Re: Rebuild 1977 Transmission?

STEVE LuVISI at AUTOMOTIVE EXPERTISE. in 17862 SAMPSON LANE, HUNTINGTON BEACH. Phone 714-596-5034. He has done BOD OFF RESTORATIONS on Corvettes since 1988.

[Steve G.]

Re: Rebuild 1977 Transmission?

In the years that I did automatics I can say that I never once saw a trans "wear out". They failed usually for one of two reasons, abuse or internal seal failure. Abuse covers a lot of territory, but where I was a lot of it was rocking a stuck vehicle between forward and reverse until they burnt one of the clutch packs out. But the internal seal failure was the most common. Just as your external neoprene rubber seals have gotten old, hard and brittle, so have the ones that seal the pistons and servos on the inside.

In the GM transmissions the most common seal failure was the direct clutch drum. It's used in reverse as well as direct and line pressure is elevated in reverse. On a cold morning with pressure high you put it in reverse and the lip blows off the seal. What the driver notices first is slower than usual engagement in reverse, especially cold. They operate it like that until the slow application burns the clutch pack up. On transmissions that use square crosscut seals with no lip to fail, the seal shrinks away over time and looses it seal that way.

All this is a function of age, not miles. I've seen guys question whether their odometer had been turned back because their 40 year old car had a transmission failure with only 40K on the clock.

Summary, reseal means reseal. Not just the ones that you can see the leak, but all the ones that are leaking that you can't see, the internal leakages. I'll maybe get yelled at for this, but if it were my trans and I were doing it myself, I wouldn't do the blanket blind overhaul. If this is a good working trans in all probability all those clutches and the band are reuseable, and I would reuse them. If the linings pass the visible and fingernail check, I would reuse them. Ensure your endplays and clearances are all good.

I used the SK brand shift improver kits on a lot of trans's, almost all TH350's. These kits aren't competition shift kits, they cure some of the inherent traits that some people find annoying. I personally didn't like the way the TH's want to hang on WOT shift. The engine runs out of rpm and the trans hangs there for what seems like forever before it shifts. The SK kit does away with that and a few other things.

FWIW

[Tom R.]

Re: Rebuild 1977 Transmission?

In the years that I did automatics I can say that I never once saw a trans "wear out". They failed usually for one of two reasons, abuse or internal seal failure. Abuse covers a lot of territory, but where I was a lot of it was rocking a stuck vehicle between forward and reverse until they burnt one of the clutch packs out. But the internal seal failure was the most common. Just as your external neoprene rubber seals have gotten old, hard and brittle, so have the ones that seal the pistons and servos on the inside. FWIW

I must say that having followed this thread, and an owner of a 350 THM, this has been the most coherent, factual based post...thus far. Thx