Overdrive options 401 and D20

[Grand_Wag_85]

Looking to pick up a 77 J10 that originally had the 360 swapped up to a 401 for a current drivetrain of 401 T15 D20.

What manual trans conversions are out there for a solid 5spd swap that will survive towing stuff?

[KJ Ryu]

77 should have a T18. I can't answer your question though, sadly.

[Grand_Wag_85]

That's what I thought too......
I'll have to take a better look.

[Cecil14]

Three and four speeds were available for most of the 70s, if I'm not mistaken. A T-15 would be the three speed option.

As for 5 speeds, how much are you planning to tow? I've towed things heavier than I care to admit with the NV3550, and never had a problem. It's not something I do often, though. If you're looking for something on par with T-18 strength, your only real manual option for a 5 speed is the NV4500. If you swap out transmissions, do not waste money keeping the D20. You can drop in a D300, if you really want to keep a passenger drop gear drive case, or swap to any number of NP/NV chain drive cases in either driver or passenger drop.


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[tgreese]

The T-15 was standard in 1977. It's a very rugged transmission. I'd only replace it if I wanted to change the final drive ratio.

[derf]

The bigger issue than finding a transmission to work with the engine is finding one to work with the Dana 20 case. The Dana 20 was phased out before anyone put it behind an overdrive manual and adapters are hard to find.

One alternative to the Dana 20 that makes life a lot easier is a Dana 300 from a CJ (1980-1986). It will match the passenger drop of your Dana 20 and is a well regarded transfer case. It's superior to the Dana 20 in most respects. And there are a whole bunch of upgrades available. It has the round NP2xx bolt pattern and 23 spline input shaft.

If you go with the 300, you can easily adapt the AX-15 and NV3550, both from TJ/XJ/YJ donors. I'd give a little bit of preference to the AX-15. As they are behind a 4.0 already, the have an AMC bellhousing. And you can upgrade to a hydraulic clutch while you're at it. The Dana 300 will bolt right up with no mods (assuming that you don't end up with a 21 spline output on the transmission). However, if you plan on heavy towing, I might skip those as they are medium duty transmissions. Good for up to a 1/2 ton pickup for regular driving but towing heavy loads (i.e. cars on trailers) really is more than they are designed to take. They can probably last for a good while but would maybe develop worn bearings and such earlier than usual.

If I was going to pick one behind a 401 for towing, I'd go with the NV4500. You can easily adapt the Dana 300 behind it with an input shaft swap and trans-to-tcase adapter from somewhere like Novak or Advance Adapters. You would have to get a bellhousing that works with the transmission. I seem to remember that the NV4500 will bolt up with only slight mods to the T15 bellhousing you currently have. But don't quote me on that. The 4500 was a strong transmission used in 1/2 through 1 ton pickups behind big blocks and early generation diesels from GM and Dodge. It will hold up to a 401 and heavy trailers for a long time.

[Grand_Wag_85]

The T-15 was standard in 1977. It's a very rugged transmission. I'd only replace it if I wanted to change the final drive ratio.

That's the only reason I aim to replace it, just for the overdrive.

[Grand_Wag_85]

The bigger issue than finding a transmission to work with the engine is finding one to work with the Dana 20 case. The Dana 20 was phased out before anyone put it behind an overdrive manual and adapters are hard to find.

One alternative to the Dana 20 that makes life a lot easier is a Dana 300 from a CJ (1980-1986). It will match the passenger drop of your Dana 20 and is a well regarded transfer case. It's superior to the Dana 20 in most respects. And there are a whole bunch of upgrades available. It has the round NP2xx bolt pattern and 23 spline input shaft.

If you go with the 300, you can easily adapt the AX-15 and NV3550, both from TJ/XJ/YJ donors. I'd give a little bit of preference to the AX-15. As they are behind a 4.0 already, the have an AMC bellhousing. And you can upgrade to a hydraulic clutch while you're at it. The Dana 300 will bolt right up with no mods (assuming that you don't end up with a 21 spline output on the transmission). However, if you plan on heavy towing, I might skip those as they are medium duty transmissions. Good for up to a 1/2 ton pickup for regular driving but towing heavy loads (i.e. cars on trailers) really is more than they are designed to take. They can probably last for a good while but would maybe develop worn bearings and such earlier than usual.

If I was going to pick one behind a 401 for towing, I'd go with the NV4500. You can easily adapt the Dana 300 behind it with an input shaft swap and trans-to-tcase adapter from somewhere like Novak or Advance Adapters. You would have to get a bellhousing that works with the transmission. I seem to remember that the NV4500 will bolt up with only slight mods to the T15 bellhousing you currently have. But don't quote me on that. The 4500 was a strong transmission used in 1/2 through 1 ton pickups behind big blocks and early generation diesels from GM and Dodge. It will hold up to a 401 and heavy trailers for a long time.

Didn't realize the D20 was such an oddball to adapt things to. I completely forgot about the D300 working with the AX15 and 3550!

I don't plan on towing very often but when I do it will be a Jeep on a trailer so it will be heavy.

[derf]

Didn't realize the D20 was such an oddball to adapt things to. I completely forgot about the D300 working with the AX15 and 3550!

I don't plan on towing very often but when I do it will be a Jeep on a trailer so it will be heavy.

It's not so much that the D20 is an odball. It's just that it is an old case that hasn't been sold new in a vehicle in nearly 40 years. With it's unique mounting pattern, one has to make a dedicated adapter. But the D300, which is stronger than a D20, is so much easier to work with since it shares the NP2xx mounting pattern and input shaft. So that's what everyone uses now.

If you aren't going to tow real often, and if you take it easy when you do, the AX15/D300 would probably be the easiest choice. No expensive adapters. Just make sure the pilot bushing is the right ID for the transmission. Get the right diameter clutch disk with the right splines and keep the 401 pressure plate. The hydraulic clutch slave *should* fit without too much trouble, assuming the depth is right. The shifter on the AX15 is towards the rear so it, in theory, should come up near where the spaced back T15 shifter comes up. You can reuse the stock D300 shifter or swap over to a twin stick if you like.

[Cecil14]

The NV3550 offers the same benefits as the AX-15, and is a bit stronger from everything I've read. Ultimately I would give it the same cautions, though: don't try to abuse it, it's not a T-18/NV4500. Look for them in 2000-2005 TJs with the 4.0L.

If you decide to go AX-15, grab one from a later 90s Jeep, that will have an external slave cylinder. Keep the bellhousing with it, and everything bolts up directly. Drill two holes for the master cylinder to bolt to the firewall and you're good. I would keep the whole clutch package from the transmission together, though. That pressure plate is designed for a hydraulic clutch, the stock pressure plate is not. It'll work, but may not be as comfortable and might be harder on the hydraulics. If you want to go 11" clutch, there are options out there for the newer style.


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[Grand_Wag_85]

That was going to be my next question....Anything funky to watch out for with the flywheel?

[Cecil14]

I6 to I6, they're both neutrally balanced and swap directly. You will need to keep the starter from whichever generation flywheel you run.

If you want to run the 4.0L flywheel on a V8, you need to get it match balanced to an existing V8 flywheel, or get the entire rotating assembly balanced with the new flywheel included.

You can use either flywheel, they should all stick out the same distance, provided they haven't been resurfaced. If you run the 4.0L flywheel and bellhousing, it sets you up nicely for a CPS should you ever run FI.


aa

[derf]

I6 to I6, they're both neutrally balanced and swap directly. You will need to keep the starter from whichever generation flywheel you run.

If you want to run the 4.0L flywheel on a V8, you need to get it match balanced to an existing V8 flywheel, or get the entire rotating assembly balanced with the new flywheel included.

You can use either flywheel, they should all stick out the same distance, provided they haven't been resurfaced. If you run the 4.0L flywheel and bellhousing, it sets you up nicely for a CPS should you ever run FI.


aa

They don't all stick out the same distance. I learned that the hard way on the 401. Modern replacement flywheels are 360 spec balanced to a 401 but they stick back too far and push the pressure plate into the shift fork. Stick with the 401 specific flywheel and pressure plate that works now. Put in a clutch disk the right diameter with the right spline count and it will all work.

[Cecil14]

If you're using the throwing bearing from the AX-15/NV3550, would you not want the flywheel that matches that? Seems like you'd want the whole package to match.

I didn't know the new flywheels were different, but I guess I'm not surprised. Nothing is manufactured to the right specs anymore.

Originally all the flywheels were the same, the only difference was the balance. I believe the casting numbers on the older (Gen 3 AMC days) flywheels were all the same, even.


aa

[derf]

The 401 was always a different depth. I tried switching over to a manual behind a 401 in my old CJ and had the depth issue with the only flywheel I could buy. I had to abort that whole project. I also had a problem with my Novak AMC to 700R4 adapter. The flex plate they gave me was balanced correctly but it wouldn't fit the 401. They sent me a second one and it was the same as the first. I worked with them to get a TH400 flex plate from a stock 401 so they could get their flex plates built properly. I took another stock 401/TH400 flex plate and had a machine shop drill it for the smaller diameter GM torque converter in order to get it bolted up and working right.

I did some research and managed to come up with some measurements off the back of the block with the depth of the crankshaft in relation to the transmission mount face. The 401 does indeed stick back farther by a little bit.

And honestly, the relationship between the flywheel and the throwout bearing doesn't matter in the slightest. They never come into contact. It's all about getting the pressure plate at the right depth. The throwout bearing contacting the pressure plate is what really matters.

[Cecil14]

The flywheel depth is going to directly influence the pressure plate location. Are the clutch kits for the 401 also different than the 258/360 kits?

Never really had a 401 to play with, odd that they would make it that much different than everything else. AMC didn't have money to throw around, and making something like that unique adds a ton of cost. Not saying it isn't accurate, it's just something odd for them to do.


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[derf]

The mounting surface at the back of the crank ultimately determines where the pressure plate lands. The pressure plate is the same between the 360 and the 401. But with the crank being off between the two, a modified 360 flywheel on a 401 crank will sit too close to the trans and interfere with the clutch fork.

Thing is, AMC did the 401 on the cheap. They needed an engine that started with a 4 because people care about numbers. And there was no way a 390 was enough compared to the 426/427/428/429/440/454 crowd. But a 401 at least was in the 400+ club.

So AMC squeezed 401 ci out of their block with a forged crank (they couldn't afford casting dies given their budget and low production numbers). The crank was made to work, they fit the flywheel, and chucked them out the door. It was fine being off a little because they specified the flywheels they installed.

Over time, the low numbers of AMC parts, the 401 in particular, meant that manufacturers of aftermarket parts dropped their production and cheaped out with stuff thats out of spec.

[Cecil14]

And there's more reason AMC is out of business. You don't introduce unique parts in an "on the cheap" project unless absolutely necessary. There was no reason to change the stick out length of the crank face, or use a different flywheel (other than balance).

Does the 401 flywheel place the friction surface at the same distance from the block as a 360 or 258 would have?


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[derf]

The 401 flywheel ends up putting the mounting surface for the clutch disk and pressure plate (along with the starter ring) in the same place.

So as long as you match flywheel to crank properly, everything else is the same.

[Cecil14]

Gotcha, noted for the future. Guess you don't really have much choice but to run the 401 flywheel then. Still mind-boggling that they would change something like that. Ugh.


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