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How to ID a Twin Grip

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rsrguy3 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rsrguy3 Quote  Post ReplyReply Direct Link To This Post Topic: How to ID a Twin Grip
    Posted: Feb/03/2020 at 12:20am
Sounds pretty cool.. just like 31 flavors lots of choices every one delicious but different.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1970390amx Quote  Post ReplyReply Direct Link To This Post Posted: Feb/02/2020 at 11:51pm
You need to google "true trac" its a style of torque limiting differential carrier that uses gears not clutches. They are often used in road race cars because of the handling. The carrier housing drives both axles not just one. but one axle can drive the other so it can go around corners. AMC diffs use a thrust block in the carrier so that a single tapered bearing can be used on each side. I believe there is no provision to use a thrust block in this type of carrier. You would also need to convert to a one piece axle.
1970 390 4speed Bittersweet shadow mask AMX
1970 Amx missing most everything, or in a box
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mchaggis View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mchaggis Quote  Post ReplyReply Direct Link To This Post Posted: Feb/02/2020 at 11:07pm
Just the same as a normal diff. That’s the advantage over a locker. 
I think it was a member here that helped me?..  rouge_66 I think. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rsrguy3 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/02/2020 at 10:18pm
And the handling in the turns? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mchaggis Quote  Post ReplyReply Direct Link To This Post Posted: Feb/02/2020 at 9:56pm
The only way I got better MPG was the OD Trans and Lock-up Converter.  Going from 2.87's to 3.73's will reduce your MPG and the weight of your wallet but will increase your acceleration and smile.  I wouldn't be in a hurry to change a 2.87 to a 3.73 without an OD trans.

I went with a recommendation from this forum and bought the Truetrac and Ring & Pinion from someone who's name escapes me right now.  I needed it shipped to New Zealand and he was great to deal with.  He supplied Yukon Gears and the install kit.  Maybe other members will chime in here????

Yes it all fits into the original housing.  Doubt I have pics now as I'm on a new phone.
TWO black marks are always better then one!  

Yukon part number looks to be YG M20-373  (24523)
Truetrac One part number I can find is M20-DET-TT.  

Hunt around as some of the early Jeeps ran M20's so most of the part are listed for them.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rsrguy3 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/02/2020 at 3:07pm
Holy cow that rocks do you have a part number? Am I to understand that it fit in the existing rear-end housing? Do you have installation pics? Pricy? As a kid I always wanted to put down two solid tire marks.... so maybe not the most logical rational for posi... My last question is, do I need it or do I want it? I love the thought of increasing fuel efficiency, beyond fuel efficiency and laying two black marks in a burnout.... are there other benefits I'm missing, does it handle better? 

Edited by rsrguy3 - Feb/02/2020 at 3:11pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mchaggis Quote  Post ReplyReply Direct Link To This Post Posted: Jan/31/2020 at 3:10pm
I went with the truetrac because the side gears on the twin grip were fretted and I didn’t want to wreck the new Moser axles.  I researched for sometime and it became obvious the truetrac’s were well liked by anyone that installed one.  Going to 3.73’s sure woke things up somewhat but I also installed a TH2004R with a 3600 stall 9.5” lock-up converter at the same time.  Mileage went from about 16mpg to just over 26mpg on a 150 mile trip. These are mpg values in imperial so the 26 works out about 23 us mpg.  A huge improvement and the car is now a real pleasure to drive.  With a small 265 50 15 tyre I’m doing about 2250rpm at 65mph so a 60 series tyre would improve things a lot.

Edited by mchaggis - Jan/31/2020 at 3:19pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rsrguy3 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/31/2020 at 12:06am
Why true track? Did you see any fuel economy change with the different gearing?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mchaggis Quote  Post ReplyReply Direct Link To This Post Posted: Aug/14/2014 at 3:00pm
Boy time flies!  My last post was almost 4 years ago!  Since then I've replaced my 2.87's with 3.73 gears.  I also replaced the axles with Moser ones and changed out the twin grip and installed a Truetrac.  I got lucky as at some stage in the distant past one of my axles had spun but was rusted on so well I never knew.
So I can confirm that the 2.87 carriers in the Javelins are the same and all the "good" gears will fit just fine.  And they're a whole lot more fun too!

Oh.  And I never did find any letters stamped anywhere on this particular rear end.


Edited by mchaggis - Aug/14/2014 at 4:42pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Aug/14/2014 at 11:38am

OK, I read the whole thread, info is still good, but the original poster probably doesn't care anymore Smile


and I believe the case break for our M20's is

2.73 (a CJ ratio in the 1980's) and taller is one case

2.87 and deeper  is the other.



71 P-code 4spd Javelin/AMX
some Jeeps and some Fords
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