I installed a THM 400 torque converter on my THM 350 transmission and it fit on the splines of the input shaft. When the engine is started
I hear a noise that sounds like the converter is loose; which leads to my
question. Is the diameter of the input shaft of a turbo 350 smaller than
that of a turbo 400? Anyone please chime in...especially 45 ACP, the
transmission guru.

 

Well, I'm not a transmission guru, though I've been around an awful lot of that sort of thing. What I did do was run a machine shop that rebuilt torque converters. Thousands of them/year...so I guess I can help you out here!

The input shafts are identical. The 400 converter will work very nicely with a 350 and vice-versa, though they (I'm talking about oem-style units here) each use different flex plates - this is a result of the different bolt patterns the converters use.

If you are hearing any rude noises out of that region, there are two things you need to do -

1. Do not run the car unless it is absolutely require to diagnose the location of the noise. You must find where the noise is coming from in order to decide how to proceed - after all, it may not be what you think it is at the moment.
2. Be prepared to tear the whole thing apart again. I know this sucks, but that is likely where you are headed here.

Rude metallic noises coming from the converter are a very, very bad thing. Do not ignore it. A converter that has puked or is in the act of puking will take the tranny with it.

Proceed with caution and find a good tranny guy that you can trust!

I have to go right now. I may post some more on this later...