1978 Transmission Repair

[78Avanti]

I have adopted a 1978 Avanti II for a friend who took an overseas assignment with the State Department. I need to pull the transmission (turbo 400?) for repair. Is it possible to do so with the engine in the car or does that need to come out as well?

Thanks, Tom

[Avanti83]

I have adopted a 1978 Avanti II for a friend who took an overseas assignment with the State Department. I need to pull the transmission (turbo 400?) for repair. Is it possible to do so with the engine in the car or does that need to come out as well?

Thanks, Tom

Tom

I pulled a 700R4 out of my 83 with the engine in place. They are about 30" long so I think your TH400 should also come out. Most of them run from 28 to 33 inches long.

Be sure to use a Jack as these things are incredibly heavy. You will need to support the tranny, pull the rear crossmember, loosen the torque converter from the flex plate and remove the trans to engine bolts. Be sure you have enough height to let it down and roll it out. I assume you've pulled the drive shaft and speedometer cable already.

Bob

[78Avanti]

Tom

I pulled a 700R4 out of my 83 with the engine in place. They are about 30" long so I think your TH400 should also come out. Most of them run from 28 to 33 inches long.

Be sure to use a Jack as these things are incredibly heavy. You will need to support the tranny, pull the rear crossmember, loosen the torque converter from the flex plate and remove the trans to engine bolts. Be sure you have enough height to let it down and roll it out. I assume you've pulled the drive shaft and speedometer cable already.

Bob

[78Avanti]

Thanks Bob,

The fram appears to include a welded 'x' beam supporting the rear of the transmission with the rear mount at the cross. Does this whole assembly come out?

[Avanti83]

Thanks Bob,

The fram appears to include a welded 'x' beam supporting the rear of the transmission with the rear mount at the cross. Does this whole assembly come out?

Tom

On both my 74 and 83 there is a plate that spans the X-member in front of the meeting point of the X. It's bolted in and removes readily. The trans is bolted to the plate.

Here's a shot of the 74 with the cross member in the yellow box. Sorry it's not great but you can see the bolts holding it to the crossmember.

I've not seen one bolted directly to the X-member.

Hope this helps.

Bob

Edited January 17, 2012 by Avanti83

[78Avanti]

Tom

On both my 74 and 83 there is a plate that spans the X-member in front of the meeting point of the X. It's bolted in and removes readily. The trans is bolted to the plate.

Here's a shot of the 74 with the cross member in the yellow box. Sorry it's not great but you can see the bolts holding it to the crossmember.

I've not seen one bolted directly to the X-member.

Hope this helps.

Bob

[78Avanti]

Thanks Bob,

Yes, that looks like it. I appreciate your help.

Tom

[78Avanti]

I have adopted a 1978 Avanti II for a friend who took an overseas assignment with the State Department. I need to pull the transmission (turbo 400?) for repair. Is it possible to do so with the engine in the car or does that need to come out as well?

Thanks, Tom

[78Avanti]

Bob, the transmission removal was smooth. Thanks,

I thought you might be interested in the following:

The parts tag reads C 78 B 585, which would imply a Turbo 400 built in 1978 on August 8^th (equal to 585 in a Julian date format?).

The 'CB' identifies it as a Turbo Hyda-Matic 400, which is confirmed by the position of the modulator on the right front of the tranny (as opposed to the rear, which would be a 350).

Interestingly, Corvette stopped installing 400s in 1977 and moved to 350s in 78.

The serial number 78-CB-1485 says Turbo 400 from 1978 unit 1485.

The Turbo 400s were build for a lot of vehicles, including trucks, which probably explains the 'GMC' logo on the oil pan.

The only difference is the internal spring that defines the firmness of the shift with soft for cars and firm for trucks.

Avanti II: stands for Auto of Very Arcane and Non-Traditional parts Integration… II

Turbo Hydra-Matic 400 Decoding

Code/Engine

HP/Vehicle

CA - 350,396,400,402 (145,160,170,240,255,265hp) 2bbl, Chevrolet, Monte Carlo

CB - 400,427,454 (180,345,390hp) Chevrolet

CC - 396 (325,350hp) Chevelle, Camaro

CD - 327,350,400,402,454 (215,235,240,250,300hp) Chevrolet, Chevelle, Monte Carlo

CE - 396 (350hp) Chevelle, Camaro, Nova

CF - 402,427,454 (215,235,270,335,350hp) Chevrolet, Chevelle, Monte Carlo

CG - 396,454 (265,365,390hp) Chevrolet, police

CH - 427 (390hp) Chevrolet

CJ - 396,402 (???,240hp) Chevy Truck 10,20,30 series

CK - 350 (165,190,195,200,300hp) Corvette

CL - 427 (400hp) Corvette

CP - ??? (???hp) Chevy Truck C-30 series

CQ - 427 (335hp) Chevrolet

CR - 400,402,454 (170,215,235,240,270,360,390hp) Chevrolet, Monte Carlo, Chevelle, police

CS - 454 (245,270,360,365hp) Corvette, Chevelle, Monte Carlo

CT - xxx (???hp) Chevy Truck C10 & C20 series

CW - 402 (375hp)...Chevelle, Camaro, Nova

CX - 396,427 (375,425,430,435hp) COPO

CY - 350,396,427,454 (high shift point) (245,375,425,430,435,450hp) Chevrolet, Corvette, Chevelle, Camaro, Nova.

CZ - 350 (205,250,255hp) Corvette

Edited January 18, 2012 by 78Avanti

[Avanti83]

Tom

Glad all went well with the removal.

Your summary of the transmission data is interesting. It continues to add to the data stream that says that Avanti had to buy ahead to get the parts from GM and we need to think along the lines of one year earlier replacement parts on our cars.

Bob

[78Avanti]

Tom

Glad all went well with the removal.

Your summary of the transmission data is interesting. It continues to add to the data stream that says that Avanti had to buy ahead to get the parts from GM and we need to think along the lines of one year earlier replacement parts on our cars.

Bob

[Gunslinger]

It continues to add to the data stream that says that Avanti had to buy ahead to get the parts from GM and we need to think along the lines of one year earlier replacement parts on our cars.

Bob

I agree. My own observations over time made me conclude that many parts purchased from GM by Avanti Motors were often prior year GM production parts. I found on my '70 that it was originally equipped with the external voltage regulator when GM had been using the internally regulated alternator in most, if not all their production for the '69 model year. Other things seem to be the same. Whether it was done because prior year parts may have been less expensive in and of themselves or from making a large purchase (like a six month supply) to get a quantity discount remains to be seen.

Nate Altman had the reputation of a conservative businessman and it's easy to see him making purchases that way. Just a change from an externally regulated alternator to an internal unit also would require the design and fabrication of a new wiring harness...probably not an insignificant design change for such a small company.

I think when trying to source GM parts for Avanti II's we should not rule out looking for previous model year parts first as the correct part to ask for.

[78Avanti]

This story keeps getting stranger. The torque converter in this car is from a diesel truck and the pan has GMC stamped in it. Can anyone confirm that this was an original configuration?

I am sure it pulls strong but it sure seems strange. My inclination is to rebuild it to 1977 Corvette specs.

[Gunslinger]

That doesn't sound like it's original to me. The transmission may well have been swapped out at some point. At some point the Feds required that all transmissions and engines be stamped with the car's VIN, but I don't know when that was. If your transmission case has a VIN on it that doesn't correspond to the car in any way, it probably is a transplant from a GMC, or at least has parts from a GMC.

I can believe Avanti Motors could have purchased from GM a GMC marked transmission as long as it's the correct unit for the engine...after all, the only difference would be the markings...but the diesel torque converter sure doesn't sound right. I can see why you say it has plenty of pull...a diesel engine would use a transmission with lots of down low go.

Probably the best way to determine if it's the original transmission is to get a copy of the build sheet from Nostalgic Motors. It would list the numbers from the engine and transmission that were installed.