T15 to TH400 swap

[FSBOB]

Hello everyone,
So I've got a 76 J20 with a 401 that a PO put a T15 trans and Dana 20 Transfer Case. I don't like how high first and reverse gears are with the T15 but do like the Dana 20. I've got a friend with a 74 parts vehicle with the stock TH400 and Quadratrac he's willing to sell me. I was planning on putting in a T18 I bought for pretty much nothing with the Dana t case. I'm considering buying the automatic and t case though because it would be easier for future wheeling plans and ease of installation. What would you guys recommend? Also, if I go the automatic route, would the TH400 off the 74 360 bolt directly onto my 76 401? Any help in deciding is greatly appreciated.

[tgreese]

You should know - the only transfer case that will work with the Quadratrac TH400 output shaft is the Quadratrac. You can swap in a different output shaft for the Dana 20, but it's the very last thing to come out of the case when you completely disassemble the TH400. The '74 TH400 may be the Jeep-adapted version of the Buick nailhead case using a ring adapter between the engine and transmission, or it may be the later AMC-specific TH400 with a one-piece case (no adapter). You may have trouble finding a flex plate or convertor for the earlier style. The TH400 with the AMC V8s requires a thin bushing in the Torqueflight-sized crank pocket to support the front of the TH400. You need a flexplate for a 401 (or balanced for a 401). Some plumbing and wiring changes. Move the cross member. Change the driveshaft lengths. Otherwise it's a straight bolt-up.

[FSBOB]

Good to know. I don't mind using the quadratrac. When you say a thin bushing in the Torqueflight-sized crank pocket, I'm not sure what you're talking about. Do you mean where the flex plate bolts to the crankshaft? It sounds like other than buying a new flex plate and wires/cables/cooling lines it should be fairly simple?

[tgreese]

Offset of the Quadratrac is not ideal with a centered rear axle.

The front of the convertor has a "nose" that sits in the pocket in the end of the crankshaft. That pocket needs to be the right diameter for the TH400. You need a bushing to make the pocket slightly smaller. https://www.novak-adapt.com/catalog/pil ... /j5352374/

[FSBOB]

I'll have to check what's on my truck already. I believe it originally had a th400 and quadratrac but a PO converted it to the T15 manual. Maybe I'll luck out and just need a new flex plate. Would the converter off the 360 work on the 401? I'm going to buy the old drive shafts from my friend if I go this route and replace the u-joints.

[FSBOB]

Should have read your link first. That answers the bushing question.... Thank you

[FLeetFox]

You can use the TH400 and the d-20, with a novak addapter. the torque converter is specific to the trans type. I would replace the flex plate anyways. and the crank bushing for the th400 is available through Novak as well, $10 i think. But, short answer is, no cheap swap out there when replacing the trans. Novak kit is $600ish, and comes with new tailshaft and all gaskets, seals from back of trans to t case input. Like TGreese said, have to basically rebuild the th400 to replace the tailshaft. Qtrack needs offset axles, as was said. Build the drivetrain you want, but trying to cobble something together from "free" stuff often costs more. just my 2 cents

https://www.novak-adapt.com/catalog/ada ... o/kit-124/

[FSBOB]

I've got the offset axles. The truck originally had a TH400 and quadratrac based off the vin. That's why I'm thinking of buying the one off of my buddy. I can buy most of the parts off of him, it'll just be the flex plate, bushing, hoses, cables, and wires like t greese said. I've decided it's not an option to keep the T15 because of it's high gearing in 1st and reverse it won't work well for wheeling. I've got the money to go both routes and am just looking for the best bang for my buck for what I plan on doing. I've already got a T18 I can use but it may be more cost effective to go back to an automatic which might work better for the mild offroading I plan on doing. I've got access to a machine shop, all the tools needed, and the mechanical aptitude to rebuild everything myself so that's my plan regardless of which route I go. Mostly just looking for apples to oranges what works best and what's needed to get it done.

[tgreese]

The TH400/QT is a great setup if you want an automatic. Downside is parts availability for the transfer case, and the TH400 is not as fuel efficient as modern automatics. But a TH400 is strong, durable, and shifts nice. I'd go with an aux transmission oil cooler like a Hayden at time of installation. Trail driving with an automatic uses brake and gas at the same time, and makes some heat. Make sure your brakes are in good condition too. You asked about the convertor - there's only one for all Jeep engines '74-78.

[FSBOB]

Good to know about the TH400/QT. I'll probably take your advice on the aux cooler. What about the speedometer cable? Will the cable I'm currently using in the t case work on the 400/QT? The first thing I did when I started rebuilding this truck is make sure it'll stop. Everything from the master cylinder to the wheel cylinders has been replaced including the lines.