1969 Coronet surgery, transmission transplant....help!

Mocajava · Aug 16, 2016

I really could use some help with what I am attempting. I have a TCI 6 speed auto (4L80) that has been modified by TCI to be a 6 speed and want to mount that behind a 468 ci (stroked 383). When I look at how Ma Mopar has the torsion bar as well as the tranny mounts included in the one cross member, I see that I will have to cut out the tunnel as well as the cross member to make room for this. Cutting out the transmission mount and the tunnel is not that difficult but I don't want to weaken the torsion bar aspect. I wonder if anyone has attempted a transplant like this and where I could go to get some experience and assistance from a shop or individual that has made this swap. Any help would be appreciated as I don't want to start down this road if I can't make it stout for the transmission as well as the torsion bars. Thanks fellow members for reading.

mmissile · Aug 16, 2016

I'd build a new center-section of the crossmember, while the torsion bars are out. We modified mine, on the NAG install.

Mocajava · Aug 19, 2016

Good idea mmissile. It does not look like this swap is not popular or even attempted with others here as there are virtually no replies but yours. The tranny support does bolt up in a different place so building a new one vs modifying the old opens up a new way of support. The aspect I am concerned about is cutting out the tunnel AND that formed cross member piece. More room is needed for the difference in the dimensions of the trannies and I will have to enlarge the tunnel plus build a new tie-in for the crossmember so it is one piece going across the width of the under carriage. I have done all kinds of fabrication work so that actual building of it doesn't bother me, the design and positioning does. I have to provide adequate support for tranny and torsion bars without impeding linkage, exhaust or ground clearance. THAT is why I was hoping to get some pointers from members, trying to streamline the process to keep from doing it over with several attempts. Thanks for the reply and don't hesitate to jump in or PM me if you have ideas.

moper · Aug 21, 2016

You just have to replace the metal you remove with something that functions the same. Look at using 1/8th thick tubing and 1/4" plate to connect the two sides of the torsion bar crossmember. Best bet would be to remove the torsion bars and centerlink to test fit the transmission. Then cut, then test fit, then cut, and work your way towards the final opening and correct driveline angle without going overboard. Then fab up the trans mount you need, then rework the floorpan. You will have to make sure whatever design you end up with is properly supporting the torsion bar cross, can be removed, and does not crush the existing crossmembers (use sleeves where the bolts will pas through it).

Mocajava · Aug 23, 2016

After a weekend of work, my son and I have the cart built, the aluminum adapter plate cut out and mounted to both Mopar engine and GM 4L80 tranny and the K member sitting in place. Ready to try a test fit pretty soon to get the initial cut-out for the tunnel marked. I am excited because I got so much done PLUS teaching my 18 year old how to weld and having him weld it up. According to the instructions from TCI, I still have to do some "reforming" of the starter side of the GM bell housing to make the Mopar starter nose go in with some clearance. That will be for another day.....Moper, thanks for tips on steps to refabricate the cross member and especially about the use of sleeves, great ideas.

Mocajava · Sep 6, 2016

Moving ahead and finally got the guts to do some cutting. I spent days drawing pictures and making sure I had all the cutting wheels and metal to give this a shot. That first cut is nerve racking but after that you say" What the hell, I HAVE to fix it now!" Metal work and fabrication is easy for me, I just hated cutting into a pristine floor. That 4L80 E tranny is huge compared to the 727 but I finally whacked enough out to get her where I wanted her. Just a few pics for now....

Mocajava · Sep 9, 2016

Making headway to anyone that cares... Made the initial cut and did a test fit..1st photo. Second shot is the finished tunnel and the extent I had to open it up to fit the 6 speed, 1 1/2" higher and 5" wider! The last shot is the look from the engine bay prior to final sanding and primer. I will make the tranny mount bracket after she comes back from paint. BTW, the extra little off-shoot hump in the driver floorboard coming off the tunnel is for the huge electric plug that goes in the tranny right at floor level.

Mike67 · Sep 9, 2016

Nice job! I'm jealous, I want od!!!

HEMI-ITIS · Sep 9, 2016

Mad fab skillz!! :thumbsup:

moper · Sep 10, 2016

You didnt forget to add room for any fittings or stuff attaching to the case, right? You need to be able to work on it too!

Mocajava · Sep 11, 2016

Good point Moper, the hydraulic lines and shift mechanism were two aspects I had to consider. The shift cable bracket attaches to the pan bolts below floorboard level by about 1 1/2" and the hydraulic lines have plenty of room now. I actually offset the hump slightly and gave it extra room for those lines with the 5" extra width. I will build the support and fab up that when she comes back from paint. Thinking down the line, I am wondering about headers and being able to get something to fit. I will give TTI and probably Doug's a call to see if they have ever done a layout for a set up like this. That extra width on this 4L80E is probably going to put the kibash on headers! May end up going to shorties or HP manifolds to funnel the gasses back which will not look as good or give me max performance I am after. I knew and accepted that I would be limiting myself in how much room I had but wanted the over drive aspect for long runs. Thank you all for the comments and tips, I will include one more set of photos in paint of finished product before I close this thread out. Mocajava

Mocajava · Sep 13, 2016

Well fellow FBBO members, I am closing this thread out. Here is the final shots of the new tunnel with POR15 top and bottom(I love that stuff!) First shot is ground and sanded from drivers side, second is with POR15 and a spot left bare if I need to weld tabs for shifter mount and the third is the underside with grey POR15. I used a piece of 9 5/8" casing, P110 material which is 110,000 tensile yield material to support the cross member for support and formed it with torch and hammer. I thank everyone who commented for the offered help, your support in very much appreciated.

1969 Coronet surgery, transmission transplant....help!

Mocajava

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mmissile

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Mocajava

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moper

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Mocajava

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Mocajava

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Mocajava

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Mike67

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HEMI-ITIS

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moper

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Mocajava

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Mocajava

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