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I’m in the process of installing a 350 T400 in a 4-speed frame. I have a T400 crossmember to make the switch.
Will the T400 crossmember be welded in the exact location that the 4-speed crossmember sat or is the location different? If so, how?
The automatic transmission crossmember is a bolt in application.
The bolts holes may already be in the frame at the correct locations.
It is a big advantage having the bolt in crossmember over the welded in manual transmission crossmember.
If the holes are in the frame everything should bolt in and be in the proper location.
There are other members that may have more experience in this than I have so I’f I’m incorrect about the holes being in the frame, hopefully they will chime in.
I checked out the 4-speed frame on the 72 C3.
There are no holes on the bottom of the frame rails 6” in either direction of the existing crossmember.
So, now back to my original question: does the automatic transmission crossmember fit in the same exact location as the welded 4-speed crossmember? If not, where on the frame does it get bolted in?
I checked out the 4-speed frame on the 72 C3.
There are no holes on the bottom of the frame rails 6” in either direction of the existing crossmember.
So, now back to my original question: does the automatic transmission crossmember fit in the same exact location as the welded 4-speed crossmember? If not, where on the frame does it get bolted in?
I've read a lot of articles, watched a lot of YouTube videos, about the interchange of manual and automatic transmissions for C3 Corvettes. I've never read/heard any conversation indicating that the transmission crossmember location varied depending upon whether the transmission was manual or automatic. There's a C3 Corvette frame/FAQ document on the internet....Ive since lost the www location....that has the dimensions for three dimension drawings of the frame. I don't recall that the transmission crossmember locations on the frame varied for automatic and manual transmissions. So I think the transmission crossmember is located in the same position for either manual or automatic transmissions.
If you have an option, I'd vote for making the transmission crossmember removable when a manual transmission is going to be installed. Makes servicing the clutch so much easier.
Escue
Well. Until the turbo 400 was replaced by the turbo 350 you could bolt in a T-400 onto a manual crossmember vehicle. Of course you could NOT put it in from underneath.
The manual crossmember came with the 2 little holes already drilled to bolt on the turbo-400.
Let me get a picture.
What year is the frame?
What year is the crossmember?
Your profiles i blank so no idea. Asking about years because there are some subtle difference in the later year C3s
I would NOT weld in the new crossmember. If you have the body removed just transfer everything over and bolt the crossmember in.... there are other options for using the bolt-in crossmember ..... for example you can use the stubs of the old manual crossmember to fabricate your own mounting of the removable crossmember.
You can see my original crossmember after I modified to bolt it in....in the yellow circle you can see the pre-existing 2 holes for bolting on the Turbo-400 (that's where I bolted on my 200-4R which uses the same mounting location). I later made my own crossmember and you can follow this thread if you want to see that for mounting ideas. https://www.corvetteforum.com/forums...post1607005557
Originally Posted by Escue
I checked out the 4-speed frame on the 72 C3.
There are no holes on the bottom of the frame rails 6” in either direction of the existing crossmember.
So, now back to my original question: does the automatic transmission crossmember fit in the same exact location as the welded 4-speed crossmember? If not, where on the frame does it get bolted in?
You could always just mod your existing manual crossmember with four vertical flanges and make it a bolt together affair.
This is my original manual frame 72.
This makes trans and clutch work so much easier on manuals.
But IIRC it is absolutely required for removal of the large TH400 trans. With the body on.
Last edited by leigh1322; Dec 10, 2025 at 11:45 PM.
Escue
I concur....I modified mine in stages... you can go follow the thread but essentially:
1. I first made my crossmember removable so I could install a richmond 6-speed.
2. Later I modified that thing to allow the exhaust to "pass under" the crossmember when I was installing a 200-4R (which is bulkier -- passing under allowed easier exhaust routing).
3. And even later I just made my own pass-under crossmember because the original I modified ended up really heavy.
If I was starting now I would make it pass-under to begin with--- it allows easier routing and you can drop the exhaust a lot easier and you can locate the exhaust as you wish instead of being forced through those holes really close to the tranny...... or I'd buy a pass-under crossmember and modify the outboard ends to bolt it onto the old manual stubs (similar to factory style bolt on).
Originally Posted by leigh1322
You could always just mod your existing manual crossmember with four vertical flanges and make it a bolt together affair.
This is my original manual frame 72.
This makes trans and clutch work so much easier on manuals.
But IIRC it is absolutely required for removal of the large TH400 trans. With the body on.
Here's what I did to make the manual crossmember removable on my 68. Notice it's the same concept as that shown in post 7.
The flanges shown here were made in house by J&D Corvette in Bellflower, CA. I have recently had the same conversion made to my 70, except here the flanges were bought online. Both of my cars have Tremac 5 speeds and there's little space left in the tranny tunnel. In the event of a need to service the clutch, the removable crossmember allows the tranny to be replaced without the need to move the engine forward.
Comparing my conversion with post 7, I think my implementation may be easier to make, given that the body is on the frame. Easier from the standpoint that the body presents less of an interference with the crossmember cutting procedure. Anyhow, I really don't know since I didn't physically do either conversion.
.............
Notice the rusty headers and inlet tube to the factory style sidepipe mufflers. I now have an all stainless steel set of headers (Sanderson) and stainless steel mufflers (Hi Performance Mufflers). Two types of headers: rusty and stainless steel.
C3 Crossmember
