1969 THM400 transmission varients

[Steve Antonucci]

I have a question for the members that I'm sure can be answered rather easily.
I'm trying to learn more about the high shift point THM400's that were used
in 396-375hp cars, COPO 427-425hp cars, & ZL-1 cars. I was originally asking
these questions in Camaro forums, but someone suggested that the SHP Vettes of
the 69 model year may have also used these beefier THM400's. Therefore, I
thought you guys could possibly shed some light on this subject for me.

I'm in the process of restoring a L/78 69 Camaro that had a THM400 originally.
It is long gone and I need a correct replacement. Here are the questions:

1) What were the codes for the THM400's used during the 69 model year?

CC (396-325), CE (396-350), CY (396-375), CX (427-425 & ZL-1)?

Are the codes listed correct?

2) The '69 CX THM400 used a 6 mount-point converter. Which other THM400's used

the 6 mount-point converter?

3) Does anyone have the correct THM400 & converter for my restoration project

available for sale?

4) If the correct coded THM400 cannot be found, any recommendations on where to

get a standard THM400 rebuilt to correct original CY or CX specifications

including the correct 6-bolt converter? I would also need a correct

THM400 ID tag stamped accordingly.

I would like to know what other members here have done in this regard. There
were only but so many of these correct coded THM400's around. I would suspect
that L-88 Vettes and possibly some other SHP versions used the beefier THM400.
Assuming that these Vettes offered the THM400 option.

Steve

[Mark #28455]

430 and 435 HP Corvettes used CY

The Corvette solid lifter engines used the same CY trans as the 396/375, but good luck finding one. Even if you are lucky, the original CY tag is likely long gone. Externally, the CY trans is identical to the other short tailshaft TH400's of that year. I'm sure Joe Lucia has tons more info so hopefully he will write too!

As far as I know, the high shift point governor assembly is out of production, but recently some aftermarket racing companies have released a "tuneable" assembly. You may also want to try Heartbeat City (Jim Stubbings) he has a lot of the high perf and ZL1 camaro stuff. He's not cheap but his selection is EXCELLENT. You can usually find his ad in Hemmings.

Good luck,
Mark

[Steve Antonucci]

Re: 430 and 435 HP Corvettes used CY

Mark,

I've learned further that perhaps because the CX -or- CY coded THM400 used a
6-point converter, it may have used a special inspection cover. Any truth to
this as far as you know? It would seem odd though, unless the diameter was
greater. I did also hear that the governor assembly is no longer available.

That makes me wonder that even if I did happen to find a CX -or- CY THM400,
what are the odds that I can get the necessary parts to rebuild it properly?

Steve

[Jon S.]

Re: 430 and 435 HP Corvettes used CY

I have the original CY coded THM-400 in my '69 427/430. I don't believe there is any special inspection cover, but if you can give me an idea of what you are looking for I'd be happy to check it for you.

Jon
#10673

[Joe L.]

Re: 1969 THM400 transmission varients

Steve-----

The code used for 1969 THM-400s used with L-78 (Camaros, Chevelles, and Novas), L-72 (passenger car), and L-71, L-88, and ZL-1 Corvettes was coded "CY". This transmission carried GM PART #8626412. Unfortunately, it was discontinued from SERVICE in 1974. I cannot find any "CX" code that was used for any 1969 THM-400 transmission.

Some 1970 and 1971 Corvettes and Chevelles with 454 also used a "CY" coded transmission. While it incorporated many of the features of the 8626412, as far as I can tell, it was not exactly the same.

You have the correct applications for the "CC" and "CE" codes.

The flexplate used for the 8626412 transmission was GM #3868802. This flexplate was also used for all early 1968 Corvette with THM-400 applications as well as most 1965-68 passenger cars with THM-400. It was discontinued from SERVICE in July, 1969, though. It was replaced by flywheel GM #3937736 and, later, GM #3991403, GM #361950, and GM #471597. The latter remains available to this very day.

The torque converter used with the 8626412 transmission was GM #8625975 and, later, GM #8625982. The latter converter was discontinued in July, 1989 and replaced by GM #8656948 which is still available to this very day. Remember, though, this is a SERVICE part and may differ from the original in some ways. Nevertheless, it should be 100% functional for the application.

Where can you get one of these transmissions? I have no idea and I'll bet that an original one will be VERY hard to locate. Externally, and except for the data plate, they are identical to the THM-400s used in other 68-72 Corvettes.

If you are just after a THM-400 that can withstand the horsepower/torque loading of the engines described above, that shouldn't be too difficult to locate, at all. I'll bet that the major transmission rebuilders/modifiers like B&M and TCI could fix you up with a THM-400 that would be superior to the 8626412. I mean, a lot of technology has happened over the course fo the past 35 years and folks know a lot more about THM-400s than they did then. Then, all you'd need to obtain is a correct, reproduction data plate. I'll bet that those are out there, too.

[Steve Antonucci]

Re: 430 and 435 HP Corvettes used CY

Hi Jon,

Thanks for the offer. I don't know that I can offer any ideas as I am trying
to learn of any differences myself. Is there any possibility of you taking a
few digital image of the cover and posting it here ( or email it directly to
me )? That may help me by comparing the image(s) to my std. THM400 cover.

Is the 6-point converter a larger diameter than the standard 3-point converter?
Is the 6-point flexplate a larger diameter possibly? I guess that would then
require a special starter - wouldn't it.

427/430 - hum..... ( Do you have an L-88 ) there big guy !!!!!!!!

I would appreciate any assistance you could offer though.

Steve

[Joe L.]

Re: 430 and 435 HP Corvettes used CY

Steve----

The underpan used for the 8626412 transmission was exactly the same as that used for all other 1968-69 Corvettes (and, other Chevrolets) with THM-400 transmission. It was GM #3868810 and it's still available from GM to this very day.

The governor assembly used for the 8626412 transmission was GM #8626364. That piece was discontinued in October, 1969 and replaced by GM #8626612. The latter part was discontinued without supercession in September, 1981.

As I mentioned previously, I'll bet that B&M or TCI have available transmissions which incorporate governors every bit as capable (and then some) as these.

[Steve Antonucci]

Re: 1969 THM400 transmission varients

Joe,

As usual - THANKS A MILLION!

Much has been made to do about this "CX" ID designation. This relates to
Camaros, Chevelles & Novas. I know for a fact that the CX code exists as I
have seen an original CX coded THM400 in a 69 COPO Chevelle. I am told that
the CX & CY THM400's are probably identical. Corvette vs. all other models.
By the way, Jerry MacNeish ( you know him ) wrote of this CX THM400 in his
famed 69 SS Camaro book.

Incidently, a member from the YENKO.NET forum has the "CX" and "CY" ID tags
available as reproductions for $29.00, so that takes care of that, as they say.

Would the original 6-point converter & matching flexplate be any different than
the replacements from GM today? I am referring to the diameter of the converter
itself. Based upon your research Joe, would the CY THM400 use a different
converter cover? Someone claimed that the cover on the CX THM400 was unique.
If true, there's a hens tooth for you !

I agree totally with your assessment that todays technology could build a much
better THM400 that in 1969. But, just like almost all of us here, I have this
$*%@^&!*&@%^* disease that makes me find original ( correct ) components. If I
can find a CX -or- CY coded THM400, I'd opt for that. Now, getting that puppy
rebuilt properly has got me a little spooked based upon the unavailability of
many if the internal components. Any more thoughts Joe?

Steve

[Verle R.]

Re: 1969 THM400 transmission varients

I had one of these transmissions from a 69 Chevelle w/375-396.
The six bolt torque converter was not apparently larger diameter and a standard starter fit with no problem.

Unfortunately I sold the transmission about 1988 or so. Wish I still had it, I would like to install it in something I now have.

Verle

[Joe L.]

Re: 1969 THM400 transmission varients

Steve-----

All THM-400s used a 14" flexplate. As I mentioned, the underpan used with the "CY"-coded GM #8626412 transmission was the same as that used for all other 68-69 (and, some earlier) THM-400 installations. As far as what underpan was used for any "CX" coded transmission, if one existed, I couldn't tell you. As I say, I can find no part number for any such transmission. It MIGHT be that the code changed at some point during the model year, but I have no record of that. I can see no reason, at all, why a special underpan would have been necessary. The transmission CASE was the same for all 1969 THM-400s used with SHP/HD applications. The flexplate was the same size for all, too. So, why a special one would have been necessary for any variant is beyond me.

The flexplate and torque converter that I mentioned are the "direct descendants" of the ones used for the 8626412 transmission. GM says that they will SERVICE those applications. As I previously mentioned, that does not mean that they are in every way identical. It means that they will SERVICE those applications.

[Steve Antonucci]

Re: 1969 THM400 transmission varients

Joe,

Hoping you can respond rather quickly to this question for me.

Guess what? While on a business trip to Philly, I believe I located an original CX transmission. What luck ! I only need to know how to identify
either externally or internally that this transmission is indeed a CX ( or CY )
THM400. It does have the CX code on the tag. He told me it came out of a
69 427/425hp COPO - or the like - Nova.

It has the 6-bolt converter ( original ). But, as they were used from
1970 on, perhaps it was a replacement. Also, the owner claims that the
pump cover has 8-bolts in it. Is he referring to the circular machined
cast iron piece that mounts to the front of the transmission ( what would
be directly behind the converter when mounted )? Odd, I have a THM400
here that has an 8-bolt pattern on the machined cast iron piece I'm referring
to. Is there anything special about the modulator or any definitive proof
that this would be a CX or CY THM400?

Hope Joe or someone can assist me on this.

Steve

[Joe L.]

Re: 1969 THM400 transmission varients

Steve-----

As far as I know, all the THM variants are externally the same, except for the tag. All the differences are internal.

[Steve Antonucci]

Re: 1969 THM400 transmission varients

OK Joe,

That clouds things a little.

Steve

[Steve Antonucci]

Re: 1969 THM400 transmission varients

Joe,

Fast follow-up for you on the THM400 identification. As I mentioned, the owner
of the transmission claims that CX transmissions used a special 8-bolt front
pump cover while standard THM400's used a 6-bolt cover. I have two THM400's
in my garage. One has a 6-bolt cover & one has an 8-bolt cover. So I do know
there is at least a different front pump cover. Could this possibly be a
later design revision by GM? Or, is the 8-bolt cover a CX or CY special
feature?

Thoughts?
Steve

[Joe L.]

Re: 1969 THM400 transmission varients

Steve-----

All 68-70 THM-400 variants used the same pump, GM #8625954. I'm not sure how many bolts retained the cover to the pump body.