C-3, TH400

[D S.]

Are there any mechanical or cosmetic differences between the small block and big block TH400s besides the letter codes in the 1970-1972 Corvettes?

Thanks,
Scott

[Michael H.]

Re: C-3, TH400

Scott,

There are several differences, internally, between small and big block TH400's, but none that can't be easily changed. Most have to do with clutch disk count and the use of a wave or cushion disk in the three clutch assemblies. Some big block 400's use a six disk pack while small block 400's typically use five with a wave plate. Somewhere, I have a complete list of the clutch pack build up, with associated part numbers, for all 65-72 400's. There may have been a different accumulator spring also but I can't remember for sure. If you want to build to exact specs for your car, I can dig it out and post or send. If your only interested in external appearance and not too concerned with exact function, any 400 should work, even though the internals haven't been changed to the correct Parts for your application.

As far as I know, there were no external differences between small and big block 400's.

[Brandon Metzger]

Re: C-3, TH400

Would the performance be effected by these differences?

Also if you have a TH 400 from a truck I would assume this is geared up totaly different than the 400s for a VEtte....or am I wrong.

Scott and I have been emailing about this subject wich is what prompted the post.

I have a TH 400 that runs great. I have it cleaned and ready to go The number on the tag are 73 c j 99103. Now, I have a '72 Feb built BB Vette. If there is realy no difference should I just find a tag and stick it on? I hate to be shady about it but in the end if there realy isn't a difference ....who would care. My thinking is If I have to pay a few hundred dollars for a "correct" trans and then pay a hundred to ship it is it worth the effort. Sticking a tag in an envelope is a bit easier and cheaper. I should also mention I am in the middle of a frame off restoration, trans and engine are off the frame.

Latly, are the numbers judged or not. Seems that the JG says yes but I have had members tell me no.

All the best and thank you,
Brandon

[Dick W.]

Re: C-3, TH400

Once the body is on the frame and the car is on the ground it is impossible to see the data tag on the transmission

[Brandon Metzger]

Re: C-3, TH400

So if I am understanding this correctly people are suggesting not to worry about the tag.

If the car is judged, would they be able to see the tag--or even look for that matter.

Lastly, Is there a performance difference between all the different uses for the TH 400s. Is a light truck TH different than a heavy duty truck's TH wich might be different than a Vette TH? Or is it all symantics and driving/performance wise I will never be able to tell.

Thanks,
Brandon

[Michael H.]

Re: C-3, TH400

Brandon,

Most of the changes between these units had to do with the firmness of the shift. The TH400 for a basic low perf small block pass car would have had more cushion, or "wave" clutch plates in the packs, instead of the heavier duty "flat" plates, which would produce a smoother engagement for each shift. As engine size/torque increased, as with a big block, the design and number of clutch plates changed. If you install an original base small block transmission in a car with a big block, the 1-2 2-3 shifts will probably feel spongy or weak instead of the typical positive shift feel when all components are correctly matched. The "CJ" code unit looks like a slight step above a basic unit but definitely not for a big block. The good news is, it can easily be converted to big block specs by changing the clutch drive/driven plate stacks. I can provide the numbers and info if someone can give us the letter code for the correct big block transmission that you want to build. I found the chart and have all of the part numbers per code but it's dated 1973 so I'm sure some may have changed over the years.

Not 100% sure but I believe the gear ratios for all 400's, including truck app's, was the same @ 2.5-1, 1.5-1 and 1-1.

I agre with Dick on the number judging. Most likely not judged.

[Norris W.]

Re: C-3, TH400

Let's not forget that there are at least two different big block turbo 400's. The Special High Performance and HD 427's (L71 & L88) had the high shift point turbo 400 with the 6 bolt torque converter. I assume the 390 and 400 horse engines didn't, since the shift point would be considerably past their redline.

[Michael H.]

Re: C-3, TH400

Actually, There were a lot more than that. There were twelve different control bodies for Corvette from 68-74, six of which were for 427-454, seven different governors and a host of other misc components too numerious to mention. That's why there were so many different TH400 transmission codes over the years. In the years 1964 through 1973 (all GM lines) there were 84 different TH400's. Trying to sort through all the various different components can be very confusing, to say the least. Most transmission shops in the 60's and 70's were clueless and mixed parts between units which resulted in a 400 that never again had that correct feel. My all time favorite automatic transmission.

[Norris W.]

Re: C-3, TH400

Yep........... that's why I said "at least 2". Of the 5 69 big blocks that I presently own (all for over 15 years) only one is an automatic (an L89) and I was hopeful that the transmission was unmolested until I noticed that the 6 bolt converter had been removed. The good news is that the transmission is serial numbered to the car and the tag is still in place showing the high shift point transmission (can't remember the code), but the bad news is there's no way to know who was in it or why or when.

[Mark #28455]

Some applications will not interchange

Don't get a long tailshaft trans.
Some truck transmissions use a bolt on style yoke, I learned this the hard way when the pass car yoke bottomed out on the output shaft - not enough engagement to use it - had to swap for a correct slip type output shaft.
Mark

[Mark #28455]

code CY *NM*

[Michael H.]

Re: C-3, TH400

L89 W/TH400? That's got to be a rare piece. I have all of the correct part numbers and descriptions/quantity for all of the CY clutch disks and steel plates, pistons and springs if you plan on a rebuild some day. The parts book is not only very confusing, it's also very incorrect in several applications. This info is an easy to read chart direct from Hydra-Matic division and covers all TH400 from 64-73. Actually, I considered scaning and posting it one day because it would be handy for everyone but it's six pages.