They were used in different models; likely with different engine setups. Possibly some were long tail, others short tail (smaller chassis). The main internal difference will be in how they are set up to shift; rpm, vacuum, harshness, etc. This is controlled by valve body & plate, pressure regulator, governor springs & weights, vacuum modulator, release springs on clutches. I have seen some with extra clutch plates for extra torque. Bruce Roe
Thanks Bruce. If you are going to do complete rebuild, does it matter which th400 you start out with? Why would one prefer the long or short chassis over the other? Is it simply a weight issue? Thanks for the quick response.
Depends on the car your intending to put the rebuilt trans in. A skylark/GS requires a short tail 400.
It would be a Skylark. So if I am rebuilding a short tail , does rhe code matter anymore. I know the bb was for a stage motors and I guess the strongest factory th40o
The valve body calibrations were different. The 1970 BB is a very sought after valve body. It gave the 70GS 455 Stage 1 it's street reputation I think, at least it was one of the things. Most 400 transmission won't down shift into 1st gear from 3rd, unless you are going very slow. The 70 BB will do it at 35 MPH and under. Imagine rolling along next to a brand X at 30 MPH, matt the gas, the transmission instantly down shifts into 1st, the motor screams to 5200 RPM, and bangs second gear. That brand X car never knew what happened as the GS just dissapears. The BA transmissions were pretty good also. Any short tail 400 can be built to take the abuse, and made to shift at WOT where you need them to, but valve body calibrations make a difference.
When the trans is apart, the output shaft can be changed. Since trans mount is on the tail housing (changed to match), a length change moves the position of the trans support. Most tails are 4" or 9", but there were a few 13". Some output shafts have an O ring to seal against a sleeve extending forward from the yoke splines. These yokes have a vent, which will leak if the output shaft doesn't have an O ring. Yokes with no vent don't have the overhanging sleeve, and can be used with either type shaft. Bruce Roe
Thanks Bruce and Larry, So what I understand is that I can take a BA of BC code th400 core ad have it built to BB specs, wit the most important thing being the valve body. I ask because I want to know if it really matter what th400 if I am going to have it rebuilt. Which leads to another question-would the rebuild be significantly cheaper by starting off with a BB Thanks,
No you can't. You can't duplicate a BB valve body. There are specific size shift valves in the valve body. Mark explains some of that in this article, http://www.buickperformanceclub.com/SPTrans.htm Yes, it doesn't really matter what THM400 you start off with, it can be built to take the intended level of abuse, but the valve body is the valve body.
A transmission builder that knows what they are doing can modify the valve body to shift the way you desire.
I have talked to Mark about this. He just built a transmission for me. The 3-1 downshift is a separate issue. Not every valve body will accept the valves to make it do that like the BB valve body (35 MPH and under). Some valve bodies are better than others, but not all can be modified.
I appreciate all the input, which helps understand and make sure I get the right parts for my intended use. thanks,
