Thinking of rear axle options

[coolwind57]

I've got some slight noise coming from what I feel is my rear D35 axle.  I think I have a minor seal leak to boot.  I'm probably on borrowed time before my axle goes out. 

 

I'm currently running 31s and I rarely go off pavement, engaging the 4x4.  I occasionally load up the bed with mostly firewood runs.   I understand that I have the stock 3:55 ratio in my stock automatic trans 1989 model.  And I'm still stock, spring under axle.  

 

So, I found a local MJ AMC20 for sale.  Its really quite appealing, but I was told the ratio is 3:73.  According to what I'm reading, the differing ratios from front to rear would be too great for proper 4x4 engagement if I were to swap her in.  

 

Also, if what I'm reading is correct, swapping out gears for either the AMC20 or the front D30 to get a match would probably be quite a bit more expensive and time-consuming than the other option I'm looking at:  a 03-07 Jeep Liberty 8.25 (with disk brakes) swap.  

 

Another option as I understand would be to just swap out the front with another D30 that has 3:73 already to match that of the AMC20.  I see this is probably more involved and time consuming as simply doing the simple mods of prepping a 3:55 ratio Liberty 8.25 and slapping on back there.  A bit of cutting/grinding of brackets and welding new perches at my leisure is what it would take, as I understand.

 

I dig any and all true, significant upgrades and the disk-braked 8.25 is really cool.  But the AMC20 is even badder-assed--both in looks/exclusiveness and stength.  Overall, I think I'd be happy with whichever makes the most sense considering costs and my very limited time.  I don't think I want to consider any D35 options.

 

Not having done much of any work with axles, are there any options I'm missing here or is anything I'm understanding above that is inaccurate or overstated?   Would love to read your constructive thoughts of my situation.  

 

The MJ AMC20 is $400.  I think the Liberty 8.25s run between $200-300 around these parts.  

[89 MJ]

I would go with the 3.73s with the AMC 20 and get a different D30. It will make the engine not need to work as hard to turn those 31s.

[Pete M]

3.73 in an AMC-20?   be sure to double check that, as 4.10s are what the dealership brochure mentions. 

[derf]

Swapping to 3.73 in the front Dana 30 is probably the best option if going that route.  Swapping out a whole axle is a pain.  And any donor probably doesn't have the right gears in it so you'd have to regear there.  The only reason I'd swap out a Dana 30 would be to get rid of any axle disconnect mechanism or go from low pinion to high pinion.

 

My question is what exactly that rear AMC 20 is.  There are two axles that use the AMC20 differential.  The CJ style (AMC20) and the Grand Wagoneer style (AMC23).  The CJ version has weaker axle tubes, two piece axle shafts, and is generally not much of an upgrade over the Dana 35.  The 23 is a full half ton axle that is comparable to the Dana 44/Chrysler 8.25/Ford 8.8.  Being new to the MJ, I'm not up on which version they actually used or whether this AMC20 was swapped in from something else.

 

If it's up to the M23 level, I'd use it.  Otherwise I'd skip it and go with something else.

[Pete M]

The MJ M-20 has one piece shafts and is on par with the 44.   I regret selling mine.  

[derf]

16 minutes ago, Pete M said:

The MJ M-20 has one piece shafts and is on par with the 44.   I regret selling mine.  

I went and did a little research and confirmed that the MJ axle is a good one.  More like the FSJ and less like the CJ.

 

The only reason I wouldn't use that axle would be parts availability on the brakes.  It appears they were somewhat unique.  If I swapped over to discs or managed to find a good source of parts, I would use it as well.

 

I had one of those axles in my J10 and it was a great axle.  The only reason I swapped it out was because I upgraded to a full float Dana 60 from a J20 (along with the front HD Dana 44).

[coolwind57]

1 hour ago, Pete M said:

3.73 in an AMC-20?   be sure to double check that, as 4.10s are what the dealership brochure mentions. 

Yes, that was my understanding too.  I had even asked the seller about it.   He said, "that is what the gears come out to.  I've counted."  I may need to ask him to look for a stamping on the internals to really confirm providing AMC did that.

 

1 hour ago, derf said:

Swapping to 3.73 in the front Dana 30 is probably the best option if going that route.  Swapping out a whole axle is a pain.

I was under the impression that swapping the D30 axle is actually cheaper than re-gearing it.  Significantly.  And not that much more of a pain to do.  Am I off on that?

 

[Pete M]

cheaper?  yes.  easier? well, that depends if you just drop the truck off and pay a guy to do the re-gear.   

 

3.73s are a common ratio in low pinion ZJ and TJ dana 30s.

[eaglescout526]

Well Pete is right. The AMC 20 only came with 4.10. So the owner probably can’t count or it isn’t an MJ AMC 20. 

 

Little known fact except to probably the older guys. Before AMC sold their axle tooling to Dana the D35 was once the AMC 35

[DesertRat1991]

Will you ever want to move to bigger tires? ...at any point...EVER?  If so, then you then you might as well plan to go 3.73 or higher now.  Plan well, and pay once.

 

Other things to consider:  

 - The carrier break on the D30 is at 3.54/3.73.  Meaning, if you want to re-gear it to match the 3.73 in the AMC20 you will need to buy a new carrier.  About an extra ~$75-100 depending on where you shop.

 

- The carrier break on the AMC20 is at 2.73/3.07 so you should not need a new carrier if you move down to 3.55

 

- When seeking quotes for your gear job, ask how much they charge to do the job with the axle installed on the vehicle and how much LESS they charge if you just drop off the stripped down axle for a bench top gear swap.  Dropping off the stripped down AMC20 for a gear swap may save you hundreds compared to driving the truck in and having the D30 gears swapped.   The D30 gear job requires more time and labor from the shops perspective.

 

- Regarding finding a completely different D30 with 3.73 already installed -- good luck with that.  Those aren't very common.

[coolwind57]

3 minutes ago, eaglescout526 said:

Well Pete is right. The AMC 20 only came with 4.10. So the owner probably can’t count or it isn’t an MJ AMC 20. 

 

Or it was regeared to 3:73.  Morris 4X4 Center sells 3:73 for AMC20.

[derf]

24 minutes ago, coolwind57 said:

I was under the impression that swapping the D30 axle is actually cheaper than re-gearing it.  Significantly.  And not that much more of a pain to do.  Am I off on that?

 

Depends on being able to find a donor axle with 3.73's.  If you can, it's pretty easy.  But if you get a non XJ donor, you're dealing with a low pinion.  If you want to keep the strength of high pinion, you're going to have a hard time finding a factory equipped 3.73 axle.  4.10 would be easier to find since that was the ratio for 4cylinder XJs.

[derf]

23 minutes ago, eaglescout526 said:

Well Pete is right. The AMC 20 only came with 4.10. So the owner probably can’t count or it isn’t an MJ AMC 20. 

 

Little known fact except to probably the older guys. Before AMC sold their axle tooling to Dana the D35 was once the AMC 35

If you look back in AMC parts books you'll actually find the axle was the AMC 15 for a long time before it was called the 35.

 

Turns out it dates back to 1962.  It became the Dana 35 in 1985 when AMC sold the tooling.  https://en.wikipedia.org/wiki/Dana_35#AMC_15

[eaglescout526]

5 minutes ago, derf said:

If you look back in AMC parts books you'll actually find the axle was the AMC 15 for a long time before it was called the 35.

Neat. 

[Pete M]

26 minutes ago, derf said:

Depends on being able to find a donor axle with 3.73's.  If you can, it's pretty easy.  But if you get a non XJ donor, you're dealing with a low pinion.  If you want to keep the strength of high pinion, you're going to have a hard time finding a factory equipped 3.73 axle.  4.10 would be easier to find since that was the ratio for 4cylinder XJs.

 

 

for his particular build specs he'll never know it was a low pinion.  

[derf]

Just now, Pete M said:

 

 

for his particular build specs he'll never know it was a low pinion.  

Fair enough.  A donor TJ axle would be perfect to match in that case.

[jeeptec1]

I did on one of my Comanche’s is use the later 8 1/4 out of a Cherokee. Yes I did have to cut the spring purches and all that. The axle was cheap $100.00 bucks drum to drum with 3.55. That was about 12 years ago. I threw in some 4.10 gears and called it a day. Oh and welded the spiders because I didn’t care


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[ghetdjc320]

I kept my dana 35 and added a tnt truss and revolution axle shafts along with 4.56 gears and a truetrac. I wanted to keep the original axles to stay true to my restomod truck. My mj came with a non-disconnect HP dana 30 (originally an abs axle). You don't want to swap in a tj front axle since they are low pinion IMHO. Also, drum brakes are really not a bad setup in a rear axle. I prefer them for the larger parking brake while towing. The advantages of disks are NOT better stopping power unless you really boost you master cylinder output (like a hydroboost setup)

[coolwind57]

Ok, so I had to owner to double check and pull the cover.  He did.  He recounted as well as found markings on the gear.  Also, checked the housing and found a "C" stamped on there.  All three of these indicate 4.10.  

 

So, the question now is this:  What ratio should I make both of my axles if I choose to buy the AMC20?  I'm pretty intrigued.  Situation once again:  1989, auto trans, 31s (which I don't foresee going any larger), mostly hard pavement, with just a little 4x4 action hauling wood on occasion.  

 

Sorry guys, I wish I knew and understood all variables to make a call that fits my truck and my needs. 

[89 MJ]

4.10s. That’s what I have in my truck and don’t regret it at all. 

[Pete M]

4.10s all day every day.    putting a set in my libby for its ~31" tires.  (mostly because that's as deep as the gears go in stock axles from the factory.  if I was paying for a regear, I'd go 4.56) 

[WahooSteeler]

I'm no expert, but I'm going to throw in my 2 cents here. I'm planning to regear my 354s soon, and I also am running an AW4 and 31" tires, and basically the same type of usage. I've debated this same question over and over. Why the push for 410s when his usage clearly is mostly on the road AND he said he doesn't not anticipate going any taller on tires? He's not towing or doing any heavy hauling. I've played around on the grimmjeeper.com gear gungalator and if IIRC 373s are closer to stock RPMs than 410s and he's running an automatic. He "should" get better MPG with 373s than 410s......marginal, yes, but every little bit counts IMO.  

[Pete M]

yes but I want 4.10s FOR the road.   offroad I want even deeper, but low range is a good enough bandaid. 

 

the rule of thumb that I've read in many places is to get at least the factory ratio back plus one more step to accommodate the extra weight of the tires.

 

either way he'll be happier than with the 3.55s.  

[75sv1]

I am going through this with my XJs. It depends on what the OP wants to do, and what area.  Georgetown IN,  he might be getting into some hill country.  My present XJ driver had 31's with 4:10s. It did well in WV, after I bored the TB.  I have 32's on it now. I think I need to regear to 4.30s. My present build will be on 30.5 (metric 215/85R16 or 245/75R16). I will be going with 3.73s on it. I am buying a D30 from a ZJ, and a Ford 8.8.  I think I'd go with the AMC20 with 4.10s and do a search for the D30 on Car-Parts.com. Try ZJ and XJs. For some reason they do not cross reference the ZJ axles to the XJ. There are rear disc upgrades for the AMC20. 

[coolwind57]

Here's an interesting chart:

https://www.revolutiongear.com/gear_ratio_chart