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Amc 20 one piece axle and disc brake conversion |
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AMXFSTBK390
AMC Addicted Joined: Oct/22/2013 Location: SF Bay Area Status: Offline Points: 3489 |
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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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Questions are powerful tools...what's in your toolbox?
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Saz2686
AMC Apprentice Joined: Jan/03/2014 Location: Lakehurst nj Status: Offline Points: 101 |
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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.
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Scott - 73 Javelin. 401 engine. T5Z trans. 3.54 posi
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AMoCoLite
Supporter of TheAMCForum Joined: Dec/23/2015 Location: North Texas Status: Offline Points: 441 |
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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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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Online Points: 6755 |
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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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BrotherBamc
Supporter of TheAMCForum Joined: Jan/22/2009 Location: Lewiston Status: Offline Points: 2248 |
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That looks "beefy"... Nice job...!!!
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tomj
AMC Addicted Joined: Jan/27/2010 Location: earth Status: Offline Points: 7522 |
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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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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com |
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19610 |
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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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Frank Swygert
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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Online Points: 6755 |
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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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304-dude
AMC Addicted Joined: Sep/29/2008 Location: Central Illinoi Status: Offline Points: 9081 |
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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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71 Javelin SST body
390 69 crank, 70 block & heads NASCAR SB2 rods & pistons 78 Jeep TH400 w/ 2.76 Low 50/50 Ford-AMC Suspension 79 F150 rear & 8.8 axles Ford Racing 3.25 gears & 9" /w Detroit locker |
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Saz2686
AMC Apprentice Joined: Jan/03/2014 Location: Lakehurst nj Status: Offline Points: 101 |
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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.
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Scott - 73 Javelin. 401 engine. T5Z trans. 3.54 posi
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