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Discussion Starter · #1 ·
Im swapping out my th350 and putting in a 700r4 OD transmission, i dont have the money to go out and buy a cross member to support the 700r4 and have seen some people use there exsisting TH350 crossmember to support their 700r4 swap. Im wondering what it will take to get this to work together, i do know i need to shorten my drive shaft around 2 3/8 inches to make a nice fit but i cant narrow down which direction to go with the crossmember. Sliding the crossmember back and drilling new holes in the subframe seems to be an easy route to go but theres not alot of room inbetween the floor and the subframe to bolt it up, and if i keep the crossmember in it's original spot the tranny mount will not bolt up. Can somebody please do me a favor, if youve gone through the steps of a 700r4 swap in a 75 to 79 xbody, let me know how and what you did to complete the swap?
 

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Welcome to the Team Richard!

You can take your existing TH350 cross-member and leave it bolted where it is. Cut it into three pieces (this is best done by crawling under the car and looking to see where you have the most room as you still have to clear the emergency brake cable and the exhaust), and then marking the cross member (permanently, as it is probably pretty greasy).

Once you know where to section it you can make some STRAIGHT cuts using a band saw, Saz-All, or even a hand held hack saw. You will then weld in two eighth inch thick steel steel plates to move the section you cut back far enough to meet the tranny mount position and tack it in place. You then take the cross-member out and fully weld the three pieces back together, and gusset the cut so that it can not bend with more pieces of eighth inch steel plate.

All of this is made a lot easier on a car lift in a mechanic's garage that has the ability to fabricate the parts and help you with the tranny install (someone in the racing community probably can direct you to a fabricator to help you). Of course if you can weld and have a welder it will just be you wrestling the tranny in place and fabricating a new mount out of the old.

Big Dave
 

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Discussion Starter · #3 ·
Thank you for the warm welcome,
As of right now everything under the car is pretty cleaned up. The tranny is installed but its only resting on the TH350 crossmember. I dont have the ebrake lines or exhaust hooked up as of right now but will be soon. I dont have a welder nor have I welded before but I can bring it down to my local machine shop to have it welded up.

One thing I noticed is that even when the tranny is sitting on the TH350 crossmember with out a tranny mount is that the tranny sits to high in the tranny tunnel. Now I was told that you can just cut the tab on the croosmember off and extend that with a couple inches of steel. But that doesnt seem to be the case.

It seems like I am going to have to like you said cut the actual crossmember into three sections to extend it back to put the mount in the correct place for the tranny, but also lower the actual mounts position compared to where the TH350 sat so that the tranny doesn't sit to high in the tranny tunnel.

I also noticed I can mount my tranny in two spots on the tranny. I Can mount it to the tailshaft housing or the tranny case. I just think with the laws of physics that it would be better to mount it to the tranny case because it seems like it will be less likely to crack or have any issue's. Even though im only running a mild 307 small block as of right now. Does this seem like the best way i can go to you, between the crossmember sectioning and where to actually mount it to the tranny?
 

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Regardless of power level the transmission tail shaft mounting point will support the rear of the motor and transmission without any problem. Because it is attached at a single point with little to no torque arm to spaek of the transmission mount sees very l;ittle force that can break it.

Where you begging to break off a tail shaft housing is when you attach a torque arm to it (such as used in the third and fourth generation Camaro, or use a solid mount with flexible motor mounts (if you are going solid on your mounting, mount everything solidly so that nothing can flex or move).

The height of the tail shaft is critical. First it controls the gas level in the carb (gas should be level not tilted when the rear of the tranny is at the correct height); and it affects adversely the pinion angle which results in drive shaft vibrations if incorrect (the angle is the same front and rear, the pinion points up about three degrees, and the tranny points down about three degrees). If there isn't an angle at the pinion and the tail shaft the universal joint bearings in the drive shaft will not rotate, and as such, they quickly wear out.

Big Dave
 

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dont know if 75 frame is different than 68 but on 68 i moved the trans crossmenber back used the back hole for front hole on mount and then
drilled up threw the frame. got some long bolts alittle longer than frame
width and put lock nut on top . it was tight but worked. shortend drive shaft
close to 3 inches. used shift works linkage for colum. shift. been in car 4 years. you will need a bracket for the tv cable if you dont have one .i tryed
a factory one but didnt work good. brought holly bracket at auto zone
about 40 dollars . had it in 3 days.
 

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dont know if 75 frame is different than 68 but on 68 i moved the trans crossmenber back used the back hole for front hole on mount and then
drilled up threw the frame. got some long bolts alittle longer than frame
width and put lock nut on top . it was tight but worked. shortend drive shaft
close to 3 inches. used shift works linkage for colum. shift. been in car 4 years. you will need a bracket for the tv cable if you dont have one .i tryed
a factory one but didnt work good. brought holly bracket at auto zone
about 40 dollars . had it in 3 days.
That won't work because after GM dropped the big block engine option in 1971 they didn't sell any more cars with a TH400 transmission installed (the TH400 used those rear holes that were punched in the frame to bolt up). Second after 1973 dual exhaust was no longer offered as an option because the factory was required to install a catalytic converter under the driver's side foot well that used a diagonal cross-member (as opposed to the older straight across style) to get around the huge pancake shaped single stage cat that the factory initially installed (even though gas station's where at the time still selling cheap leaded gas up until 1978 as regular and the tetra-ethyl-lead in the gas destroyed the cat almost instantly, rendering the whole idea next to useless.

Because of these two problems 1975-'79 cars have the minor inconvience of having to buy a special transmission cross-member to install the 700R4; or to make your own. The 200R4 tranny will fit but it still requires modifying the cross-member as it uses a TH400 mounting position that was never offered as an option.

Big Dave
 

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my 68 had power glide and when i changed over to 700r i moved
the crossmenber back. mabe not clear on my previous post but only
used the back hole in the frame for the front hole in the transmember
and drilled new hole in frame for back hole in transmember.you can see
bolt it is to right and has washer on it.

 

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Discussion Starter · #9 · (Edited)
Thank you for the reminder,
I didnt even get to take the fuel level in the carb into consideration.But yes im mounting every thing with flexible mounts, I have fresh Motor mounts with the stock compound, and I have a brand new Poly-urethane tranny mount. I also have two regular compound tranny mounts but there used. I dont think mixing the Poly-Urethane tranny mount with the regular compound motor mounts will cause much of a problem after seeing what your saying.

But as far as flipping the mounting tab to the other side it looked like I would have to notch out a part of the top of the crossmember for clearance in my application. And I would have to have the tab welded up as well as the top of my crossmember. And move it back and there isn't alot of room on an xbody, if I move my crossmemeber back im almost right up against my body to frame mounts and it just seems like theres going to be alot of headaches if I go that route. And yes Ive heard that I will need a specific holley bracket for the 700r4 tv cable. I found another speed shop around me thatll sell it to me for 29 bucks so it seems like thats a good deal.

I appreciate the Pics though!

Another issue im having with this car is trying to find Front Brake lines. It seems like the only company that has a line set for my car is classic industries, and Im located on Long Island so im paying almost the same in shipping as for the part. Do you know of any companys that may have these brake lines that are located around New York?

Thank you for your time and expertise guys!
 

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I wish I would have seen this last week when I had my cross member out. Now for my little spiel the mount is where it attaches to the subframe and the tongue us where the trans mounts sits.

My subframe had two sets of holes in it. I moved the cross member back one hole. So the front hole in the crossmember lined up in the second hole of the first set of holes on the subframe. My buddy welded a tab just behind the mount on the crossmember. It was long enough to reach the first hole on the second set of holes in the subframe. I drilled the tab and bolted it all in place.

Now it's too far back for the trans mount to engage the tongue. I cut the entire tongue off in one piece and flipped it over about level to the top of the subframe and used flat stock to extend it. This is when you should start to check drive line angles. Move the tongue up and down as needed. My buddy welded it in place after I test fit it a couple of times. Be careful here because it's not as centered as one would think. I also had to extend the tongue to get the reach right. I suggest you tack stuff in place and test fit it.

Clear as mud right???
 
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