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Unread 10-31-2013, 07:29 PM   #1
epwildcat
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1981 CJ7 
 
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Help me identify what I got!

Just bought a 1981 cj7 and have some questions about what's what...from the pics can you all tell me what I've got at my rear end?

Pic 1- rear diff. Numbers on large gear is F8 k M20-411 10x41. On the pinion gear (I think) 2.55 etched in the metal. On top of the rear diff case N 891, with 324 written under it.

The cover is perfectly round.

Help this new guy out!

Thanks

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Unread 10-31-2013, 07:32 PM   #2
Newbjeep
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Amc 20 if its perfectly round like you said
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Unread 10-31-2013, 07:33 PM   #3
epwildcat
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Second pic appears to be fuel pump and inline filter? The pump is making a vibration and buzzing sound
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Unread 10-31-2013, 07:34 PM   #4
LumpyGrits
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Std. Model 20 rear end.
Not well known for strength.
You REALLY need to look at getting a copy of the REAL Factory Service Manual(FSM)and I don't mean a Chilton or Haynes either.
The pump and filter are NOT, OEM.

LG
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Unread 10-31-2013, 07:36 PM   #5
skizriz
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http://www.ringpinion.com/Calculators/Calc_GR.aspx Acording to this, you have 4.10 gears. Check the front to make sure they match.
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Unread 10-31-2013, 07:57 PM   #6
G-FOURce
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Quote:
Originally Posted by LumpyGrits View Post
Std. Model 20 rear end.
Not well known for strength.
You REALLY need to look at getting a copy of the REAL Factory Service Manual(FSM)and I don't mean a Chilton or Haynes either.
The pump and filter are NOT, OEM.

LG
Where do you get that manual?

Doh! A little quick on the trigger there.... I found it.
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Unread 10-31-2013, 08:02 PM   #7
epwildcat
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My gears were eaten up. Want to off-road with this. What type of rear end do u suggest? I have 10factory chromaloy axles?
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Unread 10-31-2013, 08:03 PM   #8
LumpyGrits
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These dayz, try E-bay for a copy.
LG
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Unread 11-01-2013, 12:30 AM   #9
TRUVOL
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Do you have throttle body injection under the hood?
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Unread 11-01-2013, 02:39 AM   #10
BagusJeep
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You have 35" tyres so the axle is on the margins where it MAY be strong enough for you. I would recommend welding the axle tubes on so they cannot spin and then with the one piece axles it should hold up to moderate off roading. If you want it to last longer consider 33" tyres.

The 4.1:1 ratio is a bit low for the size of tyres, they would suit 31-32" tyres. You may find it struggling on highway to keep up with the traffic an doff road not have a very deep first gear.

Overall it looks like you have an overtired CJ, not uncommon.
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Unread 11-01-2013, 08:01 AM   #11
epwildcat
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TRUVOL = YES...I believe it does have throttle body injection under the hood. There is a pressure gauge mounted on the engine as well.

BagusJeep = Thanks for the heads up. Since the tires are were pretty much new when we got the CJ, those are going to stay. Rear axle has Ten Factory Chromoly in it if that helps me any.

With that being said, the differential wheel and pinion got destroyed over the weekend. My dad said he was climbing a hill with some small boulders and the rear diff basically dropped out. So now is the opportunity to put in it whatever gears I need to support the current lift/tires we've got. I'd prefer to stick with the current AMC 20, but need some help identifying a good setup for what my jeep has on her.


Thanks,
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Unread 11-01-2013, 08:48 AM   #12
jeepwhore
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Do you have the know how and tools to do a R&P swap? If you change ratios you'll be spending twice as much in parts and labour as the front end will need to be changed to match the rear.

I'd just replace them with the same ratio (provided the front also has 4.10 in it already) unless you have an O/D transmission and do alot of highway driving.
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Unread 11-01-2013, 09:22 AM   #13
epwildcat
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I personally don't have the know how, but I where I work has a large mechanics shop (trucks, heavy equipment, cranes, etc) that I can get stuff done for basically nothing....I just gotta provide the parts. Would it make sense to go ahead and upgrade while they've got it up in the air working on the rear end?
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Unread 11-01-2013, 09:28 AM   #14
jeepwhore
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Quote:
Originally Posted by epwildcat View Post
I personally don't have the know how, but I where I work has a large mechanics shop (trucks, heavy equipment, cranes, etc) that I can get stuff done for basically nothing....I just gotta provide the parts. Would it make sense to go ahead and upgrade while they've got it up in the air working on the rear end?
If you have the money to spend, sure, why not? Mind you, to upgrade a M20 rear end it usually involves swapping it for something else. You already have 1 pc axle shafts so if you are going to regear and keep it have them stitch weld the tubes to the center section. Thats about all you can do for upgrades other than adding a truss.

Just make sure whoever does it for you knows what they are doing. Gears aren't something to be slapped together. Tolerances and specs must be met or you'll be spending more money and doing it over in short order.
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'84 CJ-7: Yota axles (spartan locked front, e-locked rear) 4" BDS, 1" Daystar & 3/8" shackle lifts, 35" X-Terrains on steelies, YJ Tub & family roll bar, heater blower upgrade, 4.2L w/MC2100 & Team Rush, Warn 8274. Rebuilt 4.2/4.0 hybrid, AX15 and twin sticks/clocking ring waiting to go in.

The advertised position for a topless, dirty jeep girl has been filled......by my wife. Thanks to all that applied.
<------------and yes, that is her.
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Unread 11-01-2013, 10:29 AM   #15
JeepHammer
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Quote:
Originally Posted by epwildcat View Post
Numbers on large gear is F8 k M20-411 10x41. On the pinion gear (I think) 2.55 etched in the metal. On top of the rear diff case N 891, with 324 written under it.
Future Reference.
M20 is M-20 differential housing, the center section/gear set. Also known as AMC 20.

-411 is gear ratio (4.10 and 4.11 are interchangeable, especially when sold in front/rear sets). This usually isn't stamped into factory gear sets, so someone has probably changed the gears at one point.
Just for safety sake, I'd check the front to make sure BOTH got changed...

10X41. 10 teeth on pinion gear, 41 teeth on ring gear.
Divide 10 into 41 and you get 4.1 which is the gear ratio.
2.55 etched on the pinion gear head is the case depth to tell the installer how many shims the case/gear need to contact ring gear correctly.

[quote=epwildcatThe cover is perfectly round.[/quote]

Round with 12 bolts, AMC 20/M-20 differential. Standard issue in Jeeps from '76 forward until '86 when a Dana 44 was an option.
*IF* The bolts were factory, they would have been 'Torx' or 'Star' head bolts, make sure you use some 'Never-Sieze' on them when you put them back in, they like to lock up,

The gasket is an 'O' Ring type compression gasket, so DO NOT over torque or it will leak.
Overtorque will crush the gasket and it will not expand/contract with the housing.

Use a side to side tightening pattern when you put the cover back on, this gasket needs even compression to work.

Quote:
Help this new guy out!

Thanks
You can only help out those that will ask questions and take advice... Looks like you are off to a good start.
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