88whitemanche Posted November 20, 2014 Share Posted November 20, 2014 What's up guys Saw an amc 20 last weekend and was scared to grab it because I didn't do much research to find out what involves to get it to 4.88s and a locker.... So school me on it.... I also have a lead on a xj44 but the comanche amc 20 idk about.... Link to comment Share on other sites More sharing options...
Eagle Posted November 20, 2014 Share Posted November 20, 2014 It requires the same as any other diff -- buy new gears, buy new locker ==> install. It's a VERY heavy-duty axle, but heavy, and less ground clearance than a D44. Link to comment Share on other sites More sharing options...
dankicksass Posted November 20, 2014 Share Posted November 20, 2014 Always heard they were weak and expensive. Link to comment Share on other sites More sharing options...
dscowell Posted November 20, 2014 Share Posted November 20, 2014 What I've read is both weak or strong. Everything I've read though says that they need a truss. The weak side seems to be coming from people who had to two piece axle shaft version found on CJs. The MJ version is stronger due to not having the 2 piece. The real downfall much like the C8.25 is lack of after market support. They sound like decent axles just need a little bit put into them first. Link to comment Share on other sites More sharing options...
Incommando Posted November 20, 2014 Share Posted November 20, 2014 It depends on what you are looking at. The AMC Model 20 is actually the version used under CJ's. It has two-piece axle shafts that are problematic. The MJ & FSJ line used what is technically the AMC Model 23. It for sure has factory one piece axles and according to one photo I saw the tubes are thicker than on the CJ version. If it is an actual MJ version I would snag it in a heart beat if the price for the axle was not stupid expensive as they are getting scarce and are pretty cool. IIRC they were only available in the MJ one year but it may have been two. This MJ version is on par with the other half-ton axles like a D44. The center section internals are as good as anyone's. In fact they used the center sections in the Humvees that AM General produced for the government for 20 years after AMC dropped the axle in 1985. They do benefit from trussing and welding the axle tubes to the housing to prevent spinning. But most axles would benefit from these steps. The downside is the cost and availability of good upgrade parts for them in many cases. A D44 would probably have the edge in this area. If you just want to replace a D35 the cheapest route is usually an XJ 29-spline 8.25 but the Later Explorer 8.8 has a lot going for it. I would not pay a premium price over an 8.25 or 8.8 for an XJ or MJ D44 just because they were factory in some models. But that is for the buyer to decide. Link to comment Share on other sites More sharing options...
88whitemanche Posted November 20, 2014 Author Share Posted November 20, 2014 It is a comanche specific amc20.... Link to comment Share on other sites More sharing options...
shelbyluvv Posted November 20, 2014 Share Posted November 20, 2014 HMMWV diffs are aluminum. They share nothing in common with the AMC 20 other than a round cover. A Hummer diff is not a good comparison. They are junk and snap pinion gears like nothing. I've been a Motor T mechanic in the Marine Corps for over 20 years and see blown diffs weekly. With that said the MJ AMC 20 is a good axle. They are just as good as a D44. It is more common for the local parts store to have D44 parts in stock over a AMC 20 though. For that reason alone I'd go with the D44. Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 20, 2014 Share Posted November 20, 2014 It is a comanche specific amc20.... If you can grab it for $300 or less - snag it. Do you know what ratio it has now? Link to comment Share on other sites More sharing options...
Eagle Posted November 20, 2014 Share Posted November 20, 2014 What I've read is both weak or strong. Everything I've read though says that they need a truss. The weak side seems to be coming from people who had to two piece axle shaft version found on CJs. The MJ version is stronger due to not having the 2 piece. The real downfall much like the C8.25 is lack of after market support. They sound like decent axles just need a little bit put into them first. The AMC Model 20 in the XJ and MJ never had two-piece axle shafts. Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 20, 2014 Share Posted November 20, 2014 The AMC Model 20 in the XJ and MJ never had two-piece axle shafts. The 81-86 parts manual only shows the AMC 20 axle was available only for the 86 MJs, not for the XJs of that era. The big axle for the XJs was the Dana 44. Or is the parts manual wrong? Link to comment Share on other sites More sharing options...
gogmorgo Posted November 21, 2014 Share Posted November 21, 2014 HMMWV diffs are aluminum. They share nothing in common with the AMC 20 other than a round cover. A Hummer diff is not a good comparison. They are junk and snap pinion gears like nothing. I've been a Motor T mechanic in the Marine Corps for over 20 years and see blown diffs weekly. With that said the MJ AMC 20 is a good axle. They are just as good as a D44. It is more common for the local parts store to have D44 parts in stock over a AMC 20 though. For that reason alone I'd go with the D44. The HMMWV center sections changed about 20 years ago. They had been running an AMC20 with a Torsen T1 in it. Don't know about the replacement, as it didn't interest me beyond it not being a T1, but it was brought about by cost... Torsens aren't cheap to manufacture unless you're doing it in Eastern Europe. I've been wondering about trying to put something together with an old HMMWV T1 and an MJ AMC20, in an attempt to get a factory-esque Torsen rear diff into an MJ. Trouble is, the HMMWV achieves its final drive through the gearing in the portals, and the diff runs something like a 2.7 ratio, which is below the cut-off line for gear sizes on a regular AMC20 carrier. I don't know if it would be possible to even have reliable gears for a more practical ratio custom made for the Torsen carrier. Having a custom Torsen carrier made would be prohibitively expensive... we're talking several days worth of machine time. Link to comment Share on other sites More sharing options...
dscowell Posted November 21, 2014 Share Posted November 21, 2014 The MJ version is stronger due to not having the 2 piece. The AMC Model 20 in the XJ and MJ never had two-piece axle shafts. That's what I was saying Link to comment Share on other sites More sharing options...
Incommando Posted November 21, 2014 Share Posted November 21, 2014 HMMWV diffs are aluminum. They share nothing in common with the AMC 20 other than a round cover. A Hummer diff is not a good comparison. They are junk and snap pinion gears like nothing. I've been a Motor T mechanic in the Marine Corps for over 20 years and see blown diffs weekly. With that said the MJ AMC 20 is a good axle. They are just as good as a D44. It is more common for the local parts store to have D44 parts in stock over a AMC 20 though. For that reason alone I'd go with the D44. The HMMWV center sections changed about 20 years ago. They had been running an AMC20 with a Torsen T1 in it. Don't know about the replacement, as it didn't interest me beyond it not being a T1, but it was brought about by cost... Torsens aren't cheap to manufacture unless you're doing it in Eastern Europe. I've been wondering about trying to put something together with an old HMMWV T1 and an MJ AMC20, in an attempt to get a factory-esque Torsen rear diff into an MJ. Trouble is, the HMMWV achieves its final drive through the gearing in the portals, and the diff runs something like a 2.7 ratio, which is below the cut-off line for gear sizes on a regular AMC20 carrier. I don't know if it would be possible to even have reliable gears for a more practical ratio custom made for the Torsen carrier. Having a custom Torsen carrier made would be prohibitively expensive... we're talking several days worth of machine time. For anyone interested a new Gleason/Torsen principle limited slip for an AMC20: http://www.jegs.com/p/Detroit-Locker/Detroit-Truetrac-Differentials/1032953/10002/-1 That does suck as the Model 20 uses, as you know, one carrier for 2.73:1 and down and another for 3:08:1 and up. I could not imagine the cost of fabbing a custom carrier but I bet it would be more than the new ones listed above which run around $600. No they are not the actual Torsen T-1. I also don't know if a simple solution such as the "thick" gear sets available to run 3-series D44 carriers with 4-series gears has ever been explored. If so then perhaps a spacer would mate it all together. Who knows? The AMC Model 20 internals were supposedly used on at least the first 10 years ( starting in '84??) or so in both the front and rear of the Humvee. Although not a Humvee the AMC 20 internals were for sure used in the H1 its entire run. The housings may or may not have been aluminum that whole time. From what I have read the diff troubles started with the practice of "up-armoring" the vehicles and increasing the weight much beyond what should be asked of a 1/2 ton diff and the original design parameters spec'd to AM General in 1979. Add in that the diff was spec'd for the 6.2L non-turbo diesel that got replaced by the 6.5L turbodiesel. That would also increase the strain on the diff and possibly result in a design change for the later models and/or an increase in failure rates for a diff originally spec for the other engine and GVWR. A diff that is pretty darn good for a Cj/MJ/SJ/J10 might give a different impression in a newer Humvee. The Humvee itself has nearly doubled in curb weight since its '84 debut @ 5,400 #'s to today's 11,200 #'s on average. For Am General to have used the almost in-house M20 internals in its '79 design would not be a surprise. As the IFS/IRS design on the Humvee removes much the load from the diff by it being mounted to the chassis and therefore not part of the structure supporting the weight of the vehicle like a traditional semi-float axle assembly's diff I could see the internals being deemed sufficient ( and cost effective ) for the original Humvee roll-out. Back story: AM General was the sister corporation to AMC when the contract was bid. It was originally a division of Kaiser Motors created for government contract vehicles and stood along side of the Jeep division. Think Buick/Chevy. The Legendary M715 so often called a Jeep was not technically a Jeep but an AM General product the way a skylark is not a chevelle although both are GM. You can see that it does liberally use the same parts as the Jeep Gladiator of the time including jeep stamped parts, though. When AMC bought Jeep from Kaiser in 1971 it spun AM General off into a separate corporation also owned by AMC and not just a division of the same company as under Kaiser. AMC sold Am General to LTV in 1983 before Renault gained controlling interest in AMC so AM General was not part of either Renault, Chrysler, Mercedes, or Fiat control. And some AMC 20 info: http://www.ringpinion.com/technicalhelp/ContentFrame.aspx?FilePath=~%2FContent%2FHowTo%2FGeneralInfo%2F%2FDon%27t_Throw_Away_That_Model_20_Rearend_Yet.inc Link to comment Share on other sites More sharing options...
88whitemanche Posted November 21, 2014 Author Share Posted November 21, 2014 It is a comanche specific amc20.... If you can grab it for $300 or less - snag it. Do you know what ratio it has now? Don't know gear ratio But will go next Friday to grab it... And I'll inform you guys on it... Link to comment Share on other sites More sharing options...
MuddFoot Posted November 21, 2014 Share Posted November 21, 2014 yeah if you don't use it. i might have to make a trip down to get it from you if its cheap enough. make them think its a dana 35 so it be 100 bucks or less Link to comment Share on other sites More sharing options...
jpnjim Posted November 22, 2014 Share Posted November 22, 2014 It is a comanche specific amc20.... If you can grab it for $300 or less - snag it. Do you know what ratio it has now? Don't know gear ratioBut will go next Friday to grab it... And I'll inform you guys on it... The gear ratio is 4.10 all MJ M20's are 4.10. the only variation is whether or not it has a limited slip if yes, its just a trac-lok, not a great LSD anyway. Axle shafts are one piece, just like the FSJ's Good luck Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 22, 2014 Share Posted November 22, 2014 The gear ratio is 4.10 all MJ M20's are 4.10. the only variation is whether or not it has a limited slip if yes, its just a trac-lok, not a great LSD anyway. True True, but I don't believe it until I peek inside. You never know what previous owners have done. Link to comment Share on other sites More sharing options...
gogmorgo Posted November 22, 2014 Share Posted November 22, 2014 I could not imagine the cost of fabbing a custom carrier but I bet it would be more than the new ones listed above which run around $600. No they are not the actual Torsen T-1. I also don't know if a simple solution such as the "thick" gear sets available to run 3-series D44 carriers with 4-series gears has ever been explored. If so then perhaps a spacer would mate it all together. Who knows? I've never really looked into thick cut gears. I know they've been used with success in other applications, but I'd be concerned with the strength of the carrier itself when increasing the ratio enough to make it practical. As far as cost for a custom carrier is concerned, machining a tiny one out of billet aluminum for a 500lb FSAE car that puts out 90hp at best (for the gears from the T1 out of the Audi Quattro center diff) took almost 36 hours of machine time. We were sponsored by the shop in question and only paid material cost. 36 hours at the shop's regular fees would be over $5000. Granted, the shop in question deals mostly with large industrial stuff (they have a lathe that can turn 17' long 62" diameter "bar stock"!), but that's still more machining than a differential is worth to me, especially when, as you point out, there are other ones already available that work almost as well. I suppose if you could get a custom casting that only needed machining for finishing, you could significantly reduce cost of production but it's still of questionable value in comparison to mass-produced differentials which are pretty well equivalent. But this is very quickly getting very off-topic. Oops. Link to comment Share on other sites More sharing options...
Incommando Posted November 22, 2014 Share Posted November 22, 2014 On topic would be a recent Four Wheeler/JP article ranking the best axles for a jeep swap. They ranked the AMC M20 as used in the FSJs and MJs ahead of the vast majority of D44 rears and among the best of the 1/2 ton axles. Link to comment Share on other sites More sharing options...
88whitemanche Posted November 28, 2014 Author Share Posted November 28, 2014 It's out...now to drag it back it home... Eagle- you weren't lying about it being heavy.... Link to comment Share on other sites More sharing options...
Incommando Posted November 28, 2014 Share Posted November 28, 2014 Nice score Link to comment Share on other sites More sharing options...
MuddFoot Posted November 28, 2014 Share Posted November 28, 2014 I'm on my way to get it from you lol along with my tailgate.....just if i didn't have all these bills Link to comment Share on other sites More sharing options...
88whitemanche Posted November 28, 2014 Author Share Posted November 28, 2014 lol... Well it does have the c stamped on the axle unfortunately it doesn't have the double c... Outlaw met me out there because he wanted the sending unit and noticed when we opened it at the yard that the gear oil had goldish shavings... So it needs to be rebuilt anyways.... So I'll be saving up for gears/rebuild and a Aussie locker.... I'll get more pics of it tomrrow... Link to comment Share on other sites More sharing options...
MuddFoot Posted November 28, 2014 Share Posted November 28, 2014 if it has the limited slip could be the plates in it because my 8.8 had gold looking crap and i said the same thing pulled it apart and nothing wrong just needed new clutches and plates in it. if you pull it out and use the limited slip you should be able to get a few more disk than stock so it will lock up better...i put 2 more in my buddys 8.8 in his f150 made big differance with stockish tires. Link to comment Share on other sites More sharing options...
88whitemanche Posted November 30, 2014 Author Share Posted November 30, 2014 It doesn't have the limited slip....if it had the double c it would've been considered limited slip... But it only has the 1-c stamped on the axle Link to comment Share on other sites More sharing options...
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