Thread: TH400/208 Installation Issue

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Hi everyone,
So after all the headaches I had getting my motor in with the wrong brackets.... I guess I couldn't really expect the transmission to just go in properly...

Ok I'm not sure what transmission came in this truck when it rolled out of the factory... Its a 1986 GMC High Sierra 1/2 ton Short Bed 4x4, and it came factory with a 305. So my guess is it had a 700R4 in it originally. When I bought it, it had a 350 motor with a TH400 and a NP208. They had the transmission installed without any transmission mount in between the crossmember and the adapter plate. Yes, they "drilled" their own holes in the crossmember. When I say "drilled", I mean it looks more like they used a hammer and a punch.

Ok, long story short, I forget how I took it out, and I can't get it back in. The engine is installed, and the transmission is attached to the engine. The adapter plate is on the transmission, and the transfer case is bolted onto the back of it. There is a jack under the tranny pan holding it all up right now.
I tried installing it with the transmission mount inbetween the adapter plate and the crossmember, but the transfer case hits the floor of the cab before the crossmember can get up in place. So I took the trans mount out (and put it together the rigged up way they had it), but now my crossmember looks too low. I'm pretty sure I got the wrong crossmember (as in the crossmember was made for the original tranny, and someone just tried to rig this up), or I'm doing something very wrong here.
Does the crossmember bolt below the frame? Or inside the frame?


To sum all of this up, does anyone have a picture of a TH400 and NP208 installed in a chevy pickup truck?

Thanks!

the crossmember should be inside of the frame.

Ok. I probably bought this truck with the wrong crossmember installed then.
So I guess my question now turns into: What is wrong with these pictures (below)??

This is the best pic I could find of how it looked before I took the crossmember off. It was definitely below the frame. The problem I see with this, although I never noticed it before I took the trans out, is now the crossmember is the lowest point on the frame.
TransMountOldPic.jpg

This is the pic of how I believe it was installed before.
TransMountOldExample.png

I'm not sure that is right All the weight is on the bolts. I do not recall any cross member on the bottom of the frame...You muse have the wrong member. BUT I have been wrong before.

With all that jury rigged If I was you once you get the mount fixed,I would question the driveshaft length and how far the yoke is seated into the trans.Seems to me somewhere I saw a plate with slots welded to the cross member,but I don't remember the details.

you can see the bolt holes in the cross member that are supposed to be used. Are there holes in the frame that match them?? It looks like it would put the crossmember up about anotherinch or so.

Charlie a 86 might have that option because I think back then both trannies where offered by GM.The later models did not.

Go to the parts supply house and see if they have a detail on file or a book for that series of truck or check on a 4x4 forum. I generally buy a Hates or Chilton book for every car I own. Usually after I have taken something apart and can't remember how to get it together. Since time has caught up with me I also started taking pics with my cell phone too.

That looks like the correct cross member. But it actually go's on the inside of the frame, it looks like in the pic u have up there that it has a lift kit?. most people put the cross member on the under side of the frame to get a little bit better drive line angle for lifted trucks.

The truck is not lifted, but thats not saying it never has been.
If I put the crossmember inside frame then bolt holes wouldn't line up. On the crossmember, there are two holes on the raised part and 2 holes (further apart) on the outside part. The set of holes further apart, there is no bolt holes that match on the frame. If I were to put that crossmember inside the frame, I'd either have to turn it upside down so the bolt holes match, or there would be about a 2 inch difference between where the bolt went through the crossmember until it gets to the frame.

There has to be pictures of this on the internet somewhere, I just dont know where to look for them.

Ok, I just went out and messed with it some more. There are only 2 holes in the frame, and they only match up to those two bolt holes. There is no holes for the wider pair. Even on the top of the frame.
With the way it sits in that picture, the transfer case is very close to the bottom of the cab. I can't even get my hand in between the tcase and the floor of the cab. I don't think it will go much higher.
I guess it does have to sit down like that, because once I got to looking at it, the bottom of the crossmember is about in line with the bottom of the transfer case, and you'd want something protecting the transfer case. It just seems awful low.

That still doesn't explain why I can't get a transmission mount in. It should have one, right? The adapter plate isn't supposed to sit directly onto the crossmember, is it? I would have to put a body lift on the truck if I wanted to use the transmission mount.

I didn't realize when I posted this that I was posting in a small block forum. I'm going to try this in a 4x4 forum too.

Thanks!

On GM 4wd trucks of this era, the support is provided by the adapter. There is no location on the transmission for a mount since there is no tail housing. There are different heights of adapters and different adapters for a 700R and a manual trans. As I remember, the crossmember bolts to the underside of the frame.

Drive around and find one sitting...they are everywhere. Look under it. Solve your problem.

There is a video at <www.1aauto.com>. It shows you have it mounted right. It is shown on a 85 C10 4X4.

Ok. Thanks. The more I got to thinking about it, the more right it seemed. It just seemed like the crossmember was awful low to the ground.
I just don't get why a transmission mount wont work. These holes that were put into the crossmember for the adapter plate to fit are all hacked up and you can tell someone did them themselves, and it's not supposed to be that way.

Originally Posted by robot

On GM 4wd trucks of this era, the support is provided by the adapter. There is no location on the transmission for a mount since there is no tail housing.

I know the transmission is supported by the adapter plate, but what you're saying, is there isn't a trans mount that goes inbetween the adapter plate and the crossmember? I always thought there was...
I know it's different from the 2wd trucks, the crossmember is almost flat and bolts right to the tailshaft. I've done plenty of 2wd trucks. Maybe thats whats throwing me off so much, is that the 2wd, there is nothing that sticks down below the frame, whereas I swear I got 8 inches or more of stuff sticking down under my frame with this 4wd.