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Amc 20 one piece axle and disc brake conversion

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AMXFSTBK390 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXFSTBK390 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2018 at 11:50am
 My 68 MSTG FSTBK390, 4spd, 9" rear end had a large letter "N" cast into the third member. This "N" designated nodular iron = ductile iron = spheroidal graphite iron. Nodular iron has a lot of carbon added to it making it much stronger than grey cast iron. I believe AMC used Nodular iron in casting certain crankshafts that were not forged steel to get close to the strength of forged steel cranks. Maybe the M20 differential housings were cast from nodular iron.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Saz2686 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2018 at 12:00pm
I knew it would quickly turn into a welding thread. Here is one more pic. I used a Yukon brand diff cover with a drain.


Scott - 73 Javelin. 401 engine. T5Z trans. 3.54 posi

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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMoCoLite Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2018 at 4:52pm
Originally posted by Saz2686 Saz2686 wrote:

I knew it would quickly turn into a welding thread. Here is one more pic. I used a Yukon brand diff cover with a drain.




I’m interested in the swap (axles/discs)...not the welding.

I’ll be changing from a 3.15 to a 3.73 (3.91 if I can find it)...this is fantastic stuff...and very timely.

Great thread topic...thank you for posting!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2018 at 9:47pm
I have a WC T5 and 3.73 and find first gear is just too low. Just in and out to roll.
Staring in second is not good either, bogs down unless you rev and pop the clutch.
I've been thinking a 3.55 or 3.27 would be a better combination for my build.
I just don't know if it's worth swapping out and were first gear will actually not only roll but be useful acceleration.
Could just try changing the rear tire diameter to get it right and work from there.

Your work looks real good and I like the cover! 


Edited by Trader - Sep/03/2018 at 9:50pm
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Post Options Post Options   Thanks (1) Thanks(1)   Quote BrotherBamc Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2018 at 10:21pm
That looks "beefy"... 
Nice job...!!! 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2018 at 10:38pm
the place that narrowed my (mustang) axle shortens the tubes about a foot in from the bearing flange -- avoids messing with the casting and bearing stuff. they jig it up and told me they get them straighter/more parallel than factory.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2018 at 5:31am
I think Mustangs used a 3.23 axle with OD, many cars use a 3.55... Jeep XJs used 3.08. Depends on first gear. Too much gear and you have what you discovered -- a pretty useless first gear. Too little gear and you have a fifth (or OD fourth) gear that is useless unless you're running 70+ on a road with average hills -- have to down-shift to pull anything sorta big. I tried running a 3.08 with my 0.70 OF AW4 and was in the later situation. 3.55 (what XJs ran with the 4.0L/AW4 combo) solved it. In the car I think I could run 3.31 and be good, but hard to find. Had plenty power to hold hills at 70+, but you could tell the motor was working harder (but not straining). Picked up 2 mpg on average with the lower gear! Part of that was easier take-offs.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2018 at 9:20am
Yes, thinking the 3.31 myself.
Shortening by cutting the tubes is fine, but if you run 400+ HP they recommend welding tubes to center as the three plug welds have been known to let go on hard launches. Welding the tubes to center is supposed to be good to 800 HP without any other modifications. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2018 at 9:35am
Agree with 3.31 gearing... I chose a 3.25 gear on my Ford rear.

Since my gearing will be lower than stock, around 3.33 for 1st.

And yes the welding needs to be done to reinforce the factory welds. Not all axles are perfect. It more for trouble free service than finding out later.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Saz2686 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2018 at 6:59pm
I think a 3.31 might suit me perfect. I think a 3.54 will be better than 2.73. About the welded tubes, I figured my wheels will break loose before the tubes will twist. Or, my trans will break first. I am sure to find out at the expense of a hobby. I will be sure to tell my opinion on the t5z and 3.54.
Scott - 73 Javelin. 401 engine. T5Z trans. 3.54 posi

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