AMC 20 Differential Gear Build Thread
I thought I would put together a write up on changing the gears in an AMC 20 rear end. I did not find any write ups specific to the AMC 20 when researching to do mine, so maybe some can get some use out of this. I am by no means an expert, this is my first rear end change. So those with more experience feel free to add your comments and hints – Also be sure to tell me what I did wrong so others don’t have to go through the same drill. I’ll probably also be looking for some help on reading patterns when it gets to that time.
I used this site as a reference along with my FSM. It gives all the theory and background needed for this task.
These are the tools needed prior to getting started:
250 pound click type torque wrench
Bearing race installer (rented from AZ)
Dial indicator with magnetic base
in lb torque wrench, beam style
Bearing Separator Kit
Miscellaneous Shop tools
Almost everything above can be rented, but I chose to buy all my tools because I will be doing this several more times.
One advantage I had was that I had a spare AMC 20 to use, so I did everything on a bench and was able to keep my Jeep in commission. I decided on Yukon Gears as well as an install kit which came with associated bearings and shims. I DO NOT RECOMMEND THIS SETUP PACKAGE. The shims Yukon sends are not the OEM style shims and do not go in OEM locations. The shims they send require much more work. I ordered some additional OEM style shims from Ratech Manufacturing, thanks mcmud!
I also went through and measured all the shims and wrote down the thicknesses of each prior to getting started. (this was all for not since these shims will not be used – but this needs to be done with your shim pack)
I will be doing this again when the new shims come in.
**Unless you start with a bare housing as I did, you will need to remove the axles from your housing before getting started.
First thing that had to be done was to remove the pinion nut from the rear end. A tool of some sort is required for this. I called around town looking to rent a pinion nut removal tool, but none of the local’s had one. So I made one from some 1/8 inch plate I had laying around. I used a 1.75 inch bimetal hole saw and bimetal drill bits. The pinion nut takes a 1 1/8” socket.
The tool is on the left. I drilled a hole in the middle for the socket, then four holes to take the U-Joint brackets. Once installed, I used an 18” pull bar with a 3’ jack handle for leverage. It broke right off. I did this on the floor, not the bench just in case something gave way.
Came right off
Pulled the seal with a seal puller. I then turned it around and began to remove the carrier. Once the cover is off, the next thing to come off is the bearing caps. Make sure you mark them for side and orientation, they must go back exactly as they came off.
There are shims on both sides of the carrier bearings. These must be removed as well. Pull each one out and measure and note the thickness from each side. This can be used to assist when everything goes back together. I will probably actually start with these exact shims when going back in. The good thing about the 20 is that there are only carrier shims between the bearing races and the Housing. There are none between the bearings and carrier, which means one less set of setup bearings to make. (This is why you don’t want to use the Yukon shims)
Bearing Caps removed
One of the shims about half way out so you can see where they sit.
Once the shims are removed, be sure to hold onto the carrier, it can and will fall right out.
At this point, if you are using your same carrier, you need to remove the bearings from the carrier so your new bearings can be installed. I, however, was lucky enough to buy an Aussie Locker from another member here on the forum. It came along with the carrier and also with bearings already installed, but I want to go back with everything new so these will have to go – Later - If you are going from 2.73 to any other gear, you will need another carrier.
Everything removed, but the pinion.
I chose to use a press to remove the pinion.
It came out without any problem. There are two bearings on the pinion. The outer bearing comes off during this process and the inner bearing stays on the shaft. You could also use a hammer and drift to get the pinion out, but the press was way too easy.
Outer bearing pulls right out. The races will have to be driven out with a drift or driver. I used the brass drift for this. One thing I did not do, that I would recommend is to put the cover back on when you are driving out the inner bearing race. There is a shim behind there you need to measure. I ended up on the floor in the garage trying to find the shim because it all came out like a rocket. If the cover was on, they would have stayed in the Housing.
Inner bearing and crush sleeve (under my thumb)
There is no need to remove this bearing… Always use new bearings on the reinstall.
Use a drift to remove the races
***EDIT - Just noticed that the bearing race is in backwards here, I put it back in just so you could see where it sits, it does not go in this way - Just didn't want to confuse anyone here.
So now I began prepping my outer bearing to use as my setup bearing. Some background for those who have not done this – Setup bearings are used in order to keep from ruining your new bearings during gear set up. The gear set up will probably take several attempts where the outer bearing and inner bearing race has to be removed several times in order to change the shim stacks. I think you can buy set up bearings, but I bought a carbide bit and used a drill press in order to slightly increase the inner diameter of the outer bearing so it can easily slide on and off the pinion. Used the same carbide bit to decrease the OD of the inner bearing race. This will make life much easier during the install… Hopefully!
I used a drill press at 3100RPM and ground down the inner diameter until I got an easy, but tight fit.
Here’s the outer race
Slides on the pinion with little effort, but no play once installed.
OK, so now I need to remove the bearings from the carrier. This was not an easy task. I broke two bearing separators trying to remove them with the press. I then turned to the trusty two jaw puller – No Luck – I ended up going for advice to an engine guru down the street. We ended up cutting the outer ring and rollers off using a small dremmel tool. Then put a two jaw puller on the inner portion of the bearing and put a good amount of tension. We pulled out the trusty torch and heated up the bearing. Not too much heat, just enough to change the color of the bearing. Once it was hot, we were able to get one off with the two jaw puller. The other was not so easy. We ended up have to cut one side of the bearing with the torch and then we were barely able to get it off with a puller, what a PITA. I do not recommend cutting the bearing off. You could damage your carrier unless you are good with a torch. This guy was and there was no damage.
Another thing that can be done if you have this issue is to put a weld bead on the bearing once the outer ring and rollers are removed. This is supposed to add enough heat to the bearing for it to be pulled off with ease.
Carrier bearings removed
So now new bearings go on the Carrier. I used a press and the old bearing to assist with the install. One thing I did to make this a little easier, I put the bearings in the oven on about 300 for 20 minutes and put the carrier in the freezer. The bearings went on very easily.
Installed the new ring gear on the carrier. I put the ring gear in the oven on about 300 and the carrier in the freezer for this one as well. Make sure you have the ring gear bolts on hand so you can line up the bolt holes during install. You will not be able to rotate the ring gear in order to line things up. I did not use the ring bolts to get the ring gear on the carrier. I used a dead blow hammer once I had three bolts in so I knew it was lined up. I have read some that use the ring bolts to pull the ring gear on, I chose not to. Once you get it all lined up, put red thread locker on the ring gear bolts and torque them down to 65 ft lbs in a criss cross pattern.
Now need to press the outer race back into the Housing. I used bearing race drivers and a press.
Press on new bearing on pinion head. Oven and freezer trick here too.
OK – So I’ve got my new bearings installed on my new carrier, new pinion outer bearing race in the Housing, and new inner pinion bearing installed. Time to try setting up some gears.
One thing to note - On the face of your old and new pinion, there will be a marking. My old pinion had a +4 and the new had a 2.551. What these numbers are telling you is the optimum pinion depth location. The number is the distance the face of the pinion should be from the centerline of the axles. That’s somewhat irrelevant to us since we won’t physically be measuring that distance, but it does tell us something. This distance is controlled by the pinion depth shims located between the pinion inner bearing race and the Housing. The standard setting, centerline distance, for an AMC 20 is 2.547 inches. The +4 on my old pinion is telling me that is should be set at an additional 4 thousandths, or 2.551 inches away from the centerline of the axles. Since my new gear also reads 2.551, theoretically they should both be optimum at the same location, so the shim pack that came off will be a good place to start. It could also say +4, but for whatever reason they put the actual distance on there instead of the variance.
Example – If your old pinion read +4 and your new read +2, you would start your shim pack with 2 thousands more shims, in order to move your pinion face .002 closer to the axle centerline. There are charts available that tell you where to start, but the math is pretty simple.
Now when setting up gears, there are several things that must be correct.
Backlash – This is the play between teeth of the ring gear and the pinion gear. This is adjusted using the carrier shims. Moving the ring gear away from the pinion increases backlash and vice versa. Moving the pinion closer/farther from the ring gear also affects backlash.
Pinion Depth – This is the distance the pinion head is from the centerline of the axles. We will actually never measure this distance(though if you could it would save some time). We will read the gear pattern to help determine if we are acceptable. This is adjusted using the pinion depth shim located between the inner pinion race and the housing.
Pinion Bearing Preload – How tight the bearings are pressed into their races. Measured using the inch pound torque wrench.
Carrier Bearing Preload – How tight the carrier sits between the carrier bearing races. To determine shim thickness needed, install the carrier and enough shims on one side until there is no end play in the carrier - Then add about .008 in shim thickness. It should take several good blows from a dead blow hammer to set the carrier, it should not easily go in and out. You will not overload the bearings. Just remember, when you add shims to one side of the carrier to adjust backlash, you must remove the same thickness of shims from the other side. Keep the same total thickness of shims in the carrier.
As I go through the setup process I will be checking pinion preload, backlash, then gear pattern. In that order. Once I get backlash within spec, I will begin adjusting pinion shim depth. As you go through the process, you will get a good feel for what adjustments to make.
Since I am waiting on shims to come in, I have no choice at this point but to start back with what came out. This is a good starting point for me anyway since my old pinion was +4 and so was my new pinion (2.551 = +4).
Pinion Depth Shim = .100
Ring Gear Side Carrier Shim (RGCS) = .09125
Non Ring Gear Side Carrier Shim (NRGCS) = .085
Again, these were the shims that came out during disassembly.
The pinion is installed back in the housing using the inner bearing race setup bearing (with the pinion depth shim between it and the housing) and the outer setup bearings installed. You will need to install the yoke, but not the seal or crush sleeve at this point. I had to use the tool again to turn the pinion nut until I got a preload rotating torque of about 8 in-lbs. This is a rotating torque you are looking for! You will need a beam or dial type inch pound torque wrench in order to read this value. For old bearings on an AMC 20, preload should be 6 to 8 inch pounds. For a new bearing you are looking for 14 to 19 inch pounds according to Yukon and 15-25 according to the FSM. This reading is taken without the carrier installed. Since I am basically using old bearings as setup bearings, I will use the old bearing preload numbers until I go back with the new bearings, at which time I will aim for 15-25 inch pounds.
One thing I noticed here that I wasn’t sure about was when I was reading my rotating torque, the pinion was jumping a little, it was not a smooth rotation. According to some here that’s normal, so I kept going with it.
Anyway, got my preload rotating torque to 8 in lbs. Turned the housing around and began to install the carrier and carrier shims. Got it installed and installed my bearing caps back in their original location and orientation.
So, using my original starting shims, I had NO backlash. I couldn’t even turn the ring gear a complete revolution without binding. So, I need to increase backlash, which means moving the carrier toward the ring gear side of the carrier – Away from the pinion.
Pinion Depth = .100
RGCS = .085
NRGCS = .09125
Still no backlash
Pinion Depth = .100
RGCS = .078
NRGCS = .099
Trial 4 -
Now – Since I am out of shims to use – I just removed all shims from the RGCS, just to see where that would put me with backlash.
Finally – I have Backlash –
Now, when measuring backlash you will need a dial indicator and a magnetic base. Mount the magnetic base to the axle and get the indicator as close to perpendicular to the ring gear as possible. Rotate the ring gear all the way in one direction, ensuring the pinion is not moving. Set the indicator to zero. Rotate the indicator all the way in the other direction until resistance is met from the pinion. Note the reading on your indicator, this is your backlash. In my case here it was a whopping .053. Range for an AMC 20 is 6 to 10 thousandths with .008 preferred. I am at a standstill waiting on my shims at this point.
Upside down backlash reading.
Once my shims come in I’ll be back at it again
Here is a little better picture of the position of the dial indicator when measuring backlash. You can't read the dial, but you can see the point of the indicator much better
Thanks for this right up, it is very helpful. Quick question. If I am just replacing the seals and bearings...Is it necessary to meassure the backlash etc.... Shouldn't all that remain the same? I am not replacing the gear or pinon.
Theoretically no - but a couple things need to be done/checked. Before you remove ANYTHING - use a beam style torque wrench to measure the preload on the pinion nut with the axles removed. It may be higher than the 15-25 in-lbs mentioned above because the ring gear is still installed. When you reinstall the pinion nut, you will want to ensure you meet the pre-measured preload.
Now, if it were mine, I would go ahead and measure the backlash and pattern once I get my new bearings and all back on, just to be sure (would probably measure backlash before as well just to compare). Measuring backlash and pattern is not that difficult if you have a dial indicator and magnetic base.
BUT - If you go back with identical bearings, identical shims, and your prelaod is measured beforehand, then I don't think you would need to measure backlash and get a pattern again - assuming it was set up correctly when you disassembled it.
Some that have done this may verfiy or disagree, but from my research, this is my theory.
Just a helpful hint. When installing the ring gear, it is helpful to have guide pins made up from cheap bolts to guide the ring gear on to the pilot section of the diff case. In the case of an AMC 20, you would need a couple 7/16-20 NF bolts.
Also, after you install the ring gear and diff carrier, install the carrier in the housing, torque the cap bolts down and take a runout reading off the back side of the ring gear. I believe the spec is +/- ,002". If you have runout in excess of this, you may have a burr or foreign matter between the ring gear and diff mounting flange. Another cause for excessive runout would be a warped diff case.
Quick question. Just bought the Yukon gears/install kit as well. According to the directions it sounds like you press their .071" shim on with the carrier bearings (bearing side to carrier), then use the smaller shims to make up the difference in the stock location(race side to housing). Where is the extra work you mentioned. It seems like once the bearing/yukon shim is pressed on the carrier, the rest of the setup is as your describing,no? I'm not being a wise*ss, just don't understand. Great thread, kudos on trying this yourself...
Found it elsewhere, this is what it reads
"SK 25300- Assorted 2.285 O.D. Shims installed between the inner pinion bearing and pinion head,
COMSHIM-61 - .073 thick, 3.195 O.D. shim installed between the inner pinion bearing race and the housing.
701005X - Assorted 2.375 O.D. shims installed between the carrier bearings and the carrier,
COMSHIM-62 - .071 thick, 2.982 O.D. shims used between the carrier bearing race and the housing
Also, you would also need to create a setup bearing for your inner pinion bearing as well. So instead of needing one setup bearing and one setup race as is deisgned for OEM setup, you would need 4 setup bearings if you go back with Yukon shims. It should work, but more work on your part to get it there. The shims from Ratech were about $20 or so, worth that for me not to have to make 3 additional setup bearings - Especially since I had already pressed them on before I figured this out :)
I will chime in as well to use the oem set up on the shims. I used the Yukon set up shims and the that makes for a long process. I also had trouble with using used setup bearings and races. I used the old ones and they looked fine but when I pressed on all new bearings and new races it made my backlash 0 I also had a much harder time getting the carrier in and out of the housing. The next time I plan on using all new parts for setup. Great write up though. I am looking forward to your final set up marks to see how they compare to mine. I rebuilt mine last summer and still haven't got it on the road. JC
I have had the same experience with setup bearings also. They're good for getting you in the ball park but when you do your final setup with new bearings, you may find that adjustments may be required. More so if you make your setup bearings from old used bearings that have wear on them.
Any reason you use the old outer bearing race since it won't have to be removed to change shims? Just to keep like with like?
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