How to replace the Iron Duke with a 4.2 T5

[captspillane]

If you own an Iron Duke AMC Eagle you're in great position to begin restoration of the vehicle. If you are going to rebuild an Eagle from top to bottom, it’s an Iron Duke you will want to start out with. Most of us are not willing to repair or rebuild the original engine when there are so many 4.0 and 4.2 engines available, so when it comes time to restore your car or swap engines you will likely be upgrading to a 6 cylinder.

This thread is intended to discuss the differences between a stock 1982 Iron Duke four speed and a stock 1983 258 five speed. I happen to be restoring one of each. If anyone else has information about this conversion please post it here.

The standard four speed Iron Duke was the only stock Eagle with 3.54 gears and absence of a catalytic feedback system. Those are the biggest reasons that Iron Duke Eagles are so desirable. I’ll discuss gear ratios in a separate posting, but for now I’ll conclude by saying that your 3.54 gears are a perfect match to the T5 transmission and yet no factory AMC Eagle ever had 3.54 gears, a 258, and a T5 married together. That’s the ideal combination you’re able to achieve.

First off the SR4 is junk. It should be discarded immediately because the shift lever is prone to breaking where the fork is notched. The last picture compares the shift levers of a T5 and an SR4. If you get a T5 from a CJ you should expect to spend about 150 dollars. It’s an easy transmission to rebuild for the cost of a kit ($110) and a rebuild book ($30 or downloaded from this site). A professionally rebuilt unit is readily available for about $600.

The SR4 and T5 are direct bolt on interchangeable units. The first picture shows the transmission removed with the entire clutch system and bellhousing forward of the transmission left alone. The T5 will slide right into the same space and even have the shifter come out in the same hole. The only difference between the two is shown in the second picture. It’s a plate that a metal rod bridging the front axle and transmission bolts to. On the SR4 and on the opposite side of both the T5 and the SR4 this plate attaches to the transmission with two bolts. On the passenger side of the T5 only the bulge of the 5th gear demands a different plate that only attaches to one bolt. In the third picture you can see the SR4 version of that same support bridging two holes above the rubber mount.

The two biggest differences between a 258 and an Iron Duke Eagle are the motor mounts and the transmission mount. The crossmember is actually identical except for the mounts welded to it, so you just need to get a 258 crossmember along with the engine. Those are very easy to come by. At the transmission mount, it’s actually the exact same transmission crossmember and even the exact same rubber mount, but its installed differently. In the third picture you will see the rubber mount bolted to the bottom of the Iron Duke SR4. It’s not symmetrical. To install behind a 258, you rotate the mount and bolt it on with the oval hole on the other side of the round hole. This makes the mounting holes an inch further to one side or the other. Drilled into the stock crossmember you will also find six holes. The rubber mount uses the forward four holes behind an Iron Duke and it uses the rearmost four holes behind a 258. I think its genius that they used the same rubber mount and even two mutual holes in the crossmember. It reminds me why I love AMC so much. They put alot of effort into making things interchangeable.

In conclusion the transmission sits exactly an inch rearward and an inch over in a 258 Eagle than in an Iron Duke Eagle. This means you must modify the hole in the floor to do an engine conversion. The modification of the floor is not necessary when upgrading from the SR4 to a T5, only when switching engines. The next two photos shows the floor with the SR4 hole and then the same floor with a new hole cut further back and over and a patch placed over the old SR4 hole.













 

[captspillane]

The hydraulic clutch system between a 258 and Iron Duke Eagle is identical except for one part. The Clutch Slave cylinder is made up of the metal piston assembly, a rubber boot, a smooth rod, and a plastic cap at the end of the rod. The smooth rod is longer on an Iron Duke than a 258. If you switch engines you can retain everything else as is and just shorten the rod.

[j2sax]

Since you are knowledgeable in this matter, maybe you can help answer this question:

If I swap a T5 for an SR4 in a 258 car, do I need to modify the Prop shafts (driveshafts)? 

If I swap the 258/T5 into an Iron Duke Eagle, will the driveshafts work? 

Thanks and looking forward to your progress!  I have 2 Kammbacks, one a 258 Auto and the other a Duke Standard.  I also have an SX4/4 spd car that I was going to use the parts to swap one of the T5's I have sitting around into one of the vehicles.  I think I am going to leave the 258 Auto alone and put a 258/T5 in the 4 cyl car with the 4.0 head/EFI and a cam. 

Have a great weekend,

[carnuck]

My question: Do the Iron Dukes (in Eagles) only come with SBC pattern or are they 60 degree like the AMC 2.5 and Chev 2.8 V6?

Oh, a little craziness for someone to ponder. The 60 degree pattern includes Caddy FWD motors like the aluminum Northstar (too bad the trans wouldn't handle the power) We can move this part to a "crazy ideas" forum if need be.

[captspillane]

The Iron Duke started off in 1977 and 1978 with the intake on the drivers side. In 1979 they changed it to have the intake on the passenger side. It is a different casting with the distributor moved forward and a different head casting. The motor mounts and bellhousing pattern is identical from 1977 to 1979 however. In 1980 they changed the casting again to be 60 degree bell housing pattern like the Chevy 2.8 V6. At the same time the tooling and rights to make the 1979 motor was sold to AMC.

Every Iron Duke that AMC made from 1980 through mid 1983 is identical to the 1979 GM Iron Duke.

Every Iron Duke made by Chevy from 1980 on is a different bell housing pattern. All of them have the intake opposite of the exhaust manifold. Every Iron Duke marine engine is based on the 1977 block so it has the same motor mount and bellhousing as our Iron Dukes. The marine engines have impressive power ratings and two different displacements. The marine engines have the intake on the same side as the exhaust like a 1977 Iron Duke would be.

[carnuck]

good info! I always wondered why there was so much disparity in parts fitting on those!

[captspillane]

If I swap a T5 for an SR4 in a 258 car, do I need to modify the Prop shafts (driveshafts)?  

If I swap the 258/T5 into an Iron Duke Eagle, will the driveshafts work?  

I think I am going to leave the 258 Auto alone and put a 258/T5 in the 4 cyl car with the 4.0 head/EFI and a cam.  

The T5 and SR4 are externally identical in every way except for the overdrive hump that blocks one of two bolt holes on the passenger side. I have pictures of that on the first post. Your transfer case will be in the exact same spot with the exact same driveshafts.

If you put a 258 T5 into an Iron Duke Eagle, the transmission will be one inch back and one inch to the passenger side than it used to be with the old engine. It’s true that the driveshaft will need to be slightly longer but the original one will still work just fine. The slip joint in the front is not as critical as the slip joint in the rear driveshaft because the front axle is bolted to the engine. It only needs about a quarter inch of travel to compensate for vibrations. The front driveshaft on the Iron duke has several inches of potential travel, however, so it can easily adjust for an extra inch. There will be no problem using your original Iron Duke driveshafts with the new 258 engine.

The only thing you will need to do is remove the old driveshaft and make sure it has not rusted into place. The older driveshafts did not come with a grease fitting and most people have never removed them to lubricate. Because the front axle is bolted to the engine you will not notice if the slip joint has rusted fast until the vibrations have loosened the front axle bolts and made the front axle fall free of the engine. This is exactly what happened to motivate the previous owner to sell my 1981 Kammback to me. The front slip joint was so rusted solid that I needed to pull it apart with an engine hoist while heating it with a torch. After removing the rust and adding grease it slips effortlessly like it should.

If you have a fuel injected 258 you will not need a pulse air system. With a car titled as an Iron Duke you will also not need a pulse air system. I live in PA so it matters. I would have the 258 Auto titled as an Iron Duke and the pulse air eliminated. I would have the EFI 258 T5 titled as a 258 Eagle because it doesn’t need the pulse air with fuel injection.  

[68AMXGOPAC]

My question: Do the Iron Dukes (in Eagles) only come with SBC pattern or are they 60 degree like the AMC 2.5 and Chev 2.8 V6?

Oh, a little craziness for someone to ponder. The 60 degree pattern includes Caddy FWD motors like the aluminum Northstar (too bad the trans wouldn't handle the power) We can move this part to a "crazy ideas" forum if need be.

O O OH - I have one of those !! but I been thinking of trying to fit the whole front clip under the Eagle : ) at 310hp. weeeewhooooooooooooo

[j2sax]

You know, I also have a AMC/Jeep 2.5 with 5 speed and D300 setup from a CJ... always considered putting the Jeep EFI on that little puppy and dropping it in in place of the Irron Duke, though I doubt it would be that big an improvement. 

Hmmm, I need to see what model of 5 speed Tranny that is.  Never realy through about as it is before the BA10 or AX15... maybe a T5 Also? 

I have WAY too much unused junk sitting around! 

Have sold/scrapped 8 AMC/FSJ's in the last 8 months and still have 14 spread around... DANG! 

Thanks for the good info!


Jesse

[carnuck]

I know that problem. I have a
'71 IH 5 ton flatbed (about to go away. I'm going to post it for $1100)
'85 Datsun king cab 2wd (parts. Good NAP-Z24 with 5 speed)
'82 Spirit (pre-runner project. 4.2L 998 about to swap to AW4 with Eagle axles)
'82 Eagle wagon (just put the 727 in)
'82 Chev C20 454 TH400 5th wheel hauler (about to go up for sale)
'03 E250 van with shelves and headache rack 4.6L and near new trans (up for sale for $6,000 obo)
'12 Kia Soul ! (wife's car)
'68 Travelall (currently 6 cyl 4 speed, which are for sale and 304 IH V8 auto going in with O/D)
'84 J10 (mini monster truck 4.0L/AW4/NV241 that I hope to finish this summer)
'82 Honcho shortbox stepside (needs rear fenders. Selling running 360/T18A/NP208 combo for $500. I have a 290/AX-15 NP208 to go in)
'87 Comanche shortbox lifted 6" on 33" tires with 4.0L AW4 (need to finish hooking up the wiring and shifter)
'89? Comanche longbox 4.0L auto (2wd) Converting to 4x4, extending cab and putting on shortbox

[captspillane]

In my first post in this thread I promised to discuss axle ratios. Today I finally wrote it for a different thread and can post it here. This thread is intended for someone with an Iron Duke and four speed transmission who is considering converting to a 258 five speed transmission.

I set up a spreadsheet to do the math a little differently than most people. The table at the bottom is in MPH instead of an obscure multiplication factor. My spreadsheet has this same information for just about every possible combination of tire size and type of Jeep, Scout, and Eagle transmission available. I'm focusing here on the T5 because it is the ideal swap for your original SR4 chunk of junk.
 
I consider 2500 RPM's to be the minimum RPM needed when the engine is under full load going up a hill or trying to maintain a speed higher than 70 MPH. Sometimes it’s more efficient to coast at a higher gear at idle, but then you would have to downshift to get efficient power from the engine under load. 2500 RPM is then considered the minimum RPM necessary to stay in that gear under load. Likewise it is approximately the RPM you'll have the engine at in the new gear after you've upshifted while accelerating up a hill. Keep in mind that most of us memorize shift points in mph because we don’t have working tachometers.

This table shows shift speeds corresponding to each gear of a T5. Your original four speed is nearly identical except that it missing the fifth. The table shows that you need to be going at least 64 mph to stay in fifth with 3.54 gears. It says you need to be going at least 74 mph to consistently stay in fifth with 3.07 gears. That works pretty well but it plagues the trip with many more annoying downshifts to fourth along the highway. It says that you need to be going at least 97 mph to shift into fifth with 2.35 gears. Good f rickin lord. That's outrageous. It also says that first gear with 2.35 gears is almost exactly like starting in second with 3.54 gears.

This is why the Iron Duke Eagles are so desireable. The perfect gear ratio to match a T5 is 3.54 yet the only Eagles to get 3.54 from the factory were the Iron Dukes. Some moron at AMC made 2.35 the standard ratio for all 6 cylinder manual cars. It is worthwhile to start your restoration with a four cylinder even if you intend to install a fuel injected 6 cylinder because of how rare and desireable your gear set is.

The minimum speed per gear in mph while at 2500 RPM with a CJ T5 (4.03 first/ 0.86 fifth) and 205/75R15 tires:

Gear- 3.54- 3.07- 2.72- 2.35
1- 13.66- 15.80- 17.83- 20.64
2- 23.23- 26.86- 30.32- 35.09
3- 36.70- 42.44- 47.90- 55.44
4- 55.05- 63.66- 71.85- 83.17
5- 64.02- 74.02- 83.55- 96.70