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AMC 20 changing ratio 2:87 to 3:54 |
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captscot 
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Joined: Feb/05/2013 Location: N Brunswick, NJ Status: Offline Points: 241 |
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 Topic: AMC 20 changing ratio 2:87 to 3:54Posted: Oct/05/2022 at 9:01pm |
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Orig owner of a '72 Jav AMX, 727 auto trans. My car has an AMC 20 differential, a 2.87 gear ratio, I want to buy a pinion and ring gear to change the ratio to 3.54, they're available from RockAuto and other sites. I thought this would be a simple swap, however, on RockAuto's site, under "Information" for this item it states "Requires a 3.08 and Numerically Higher Ratio Carrier". What exactly does this mean? All the AMC 20 differential gear ratios used by AMC for V8 engines with auto trans, none came with a 3:08 ratio, the 3.08 was only used in differentials with 232 and 258 6 cyl engines.
V8 cars with a 727 auto trans came standard with 2.87 ratio, optional were 3.15 and 3.54, therefore swapping out a 2.87 for either a 3.15 or 3.54 should be a simple exchange? Any one ever have a problem doing this? Thanks, Scott |
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1972 Javelin AMX - orig owner
1985 Grand Wagoneer 1967 Jeepster 1968 Jeep Gladiator 1959 Jeep CJ5 1929 Marmon model 68 |
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1970390amx
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Joined: Jul/11/2008 Location: colorado Status: Offline Points: 3314 |
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Posted: Oct/05/2022 at 9:13pm |
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Your 2.87 original carrier is fine to swap to 3.54 gears. I have done it several times.
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1970 390 4speed Bittersweet shadow mask AMX
1970 Amx missing most everything, or in a box |
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WesternRed
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Posted: Oct/05/2022 at 11:20pm |
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^+1 on this, probably a good time for some fresh bearings while you are in there.
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I've finally given up drinking for good...........now I only drink for evil.
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FSJunkie
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Posted: Oct/05/2022 at 11:48pm |
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You might want to make absolutely sure that the kit you're looking at is for the AMC 20, because the AMC 15 could have a 3.08.
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1955 Packard
1966 Marlin 1972 Wagoneer 1973 Ambassador 1977 Hornet 1982 Concord D/L 1984 Eagle Limited |
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Mopar_guy
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Posted: Oct/06/2022 at 5:18am |
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Also beware that there was an issue with aftermarket pinion gears that weren't made right. There's some old threads a couple of years ago about that on here.
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WesternRed
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Posted: Oct/06/2022 at 6:14am |
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The 3.08 carrier break relates to Jeep applications of the AMC 20, Jeep never used the 2.87 gear set.
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I've finally given up drinking for good...........now I only drink for evil.
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tufcj
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Posted: Oct/06/2022 at 8:06am |
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Buy your master kit from Ratech. They're the only company that sells AMC specific shims. All the other master kits I've seen use repurposed GM shims that don't quite fit right. Bob tufcj
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69 AMX
74 Javelin AMX 67 Rogue If you need a tool and don't buy it... you'll eventually pay for it... and not have it. Henry Ford |
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sweatlock
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Posted: Oct/06/2022 at 8:26am |
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Just an observation/personal opinion -
If you’re sticking w/ the 727 auto you’d probably prefer the 3.15 gears over the 3.54 for purely street driving. If you’re ever on the highway/freeway with the 3.54’s it’s going to rev like a mo’ fo’ looking for another gear. I have 3.15’s with my BW M11 3-speed auto and at 70 MPH it’s….interesting. I can’t imagine 3.54’s Just depends on what you’re doing with the car, your expectations, and your personal preference, of course
Edited by sweatlock - Oct/06/2022 at 8:29am |
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mark09
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Posted: Oct/06/2022 at 9:00am |
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I 2nd that, with my S/C 3.54 @70 IS 3200 RPM AND I'M WANTING ANOTHER GEAR. BUT THAT'S NOT AS BAD AS WHEN I HAD 4.44 GEARS
THAT WAS 4200 @70 DID THAT FOR 10 YEARS BUT GAS WAS $0.32 A GALLON.
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captscot
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Joined: Feb/05/2013 Location: N Brunswick, NJ Status: Offline Points: 241 |
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Posted: Oct/06/2022 at 9:05am |
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Thank you everyone for your suggestions and experience. I did wonder about what it would be like driving on highways with the 3.54, I'd be using the car for causal road trips and going to the occasional show. I think now I'll stay with the 2.87 gears, just freshen up with new bearings and seals.
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1972 Javelin AMX - orig owner
1985 Grand Wagoneer 1967 Jeepster 1968 Jeep Gladiator 1959 Jeep CJ5 1929 Marmon model 68 |
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