model 20 answers

no

My thrust block was a bit different. just 2 halves but no center pin. mine came out once with an axle but a long piece of 1/2 pvc pipe and grease just slid it back in since we were just replacing axle seals and didn't want o disassemble diff. 

My '70 AMX has a stock 3.54 twin-grip rear differential. In my old age, I'd really like to drive the AMX at highway speeds at fewer than 3200 RPM, so I bought a used AMC model 20 3.15 ring and pinion set, with the intention of replacing the 3.54 gears with the 3.15 gears (the 3.15 seems like a reasonable compromise between off-the-line performance and highway cruising). Two questions: 1) Is this a reasonable thing to consider having done, or is there a better way of achieving the desired result? My guess is I should have bought an entire model 20 axle assembly with a 3.15 diff instead of incorrectly thinking the ring and pinion gears could "easily" be swapped out. 2) Can such a swap be performed on my existing 3.54 axle without losing the twin-grip capability? 

PHAT69AMX wrote: I think it would be really helpful to know, and feel it has never been clearly figured out, explained, & documented, just WHY the 1970 ONLY Passenger Side AXLE is SHORTER than 1968 & 1969 ! There is some still even yet clearly figured out REASON WHY 1-Piece Axle companies WANT THE EXISTING STOCK AXLES SENT TO THEM in order to make & sell you new 1-piece axles... I would really like to know SPECIFICALLY why ONLY the passenger side Axle is different for 1970 ONLY. AND why many say the 1970 Housing is narrower... when the SAE Papers show SAME REAR TRACK WIDTH of 57.00" for all 3 years of AMX, 1968, 1969, & 1970 ! AND the Parts Book shows A DIFFERENT HOUSING PART # for 1970 only ! ???? BUT the same 2-Piece WLD Twin-Grip "Posi" Case / Carrier for ALL 3 YEARS ! Recent Parts Book "digging" appears to suggest, MAYBE... There was a SIDE GEAR CHANGE, SIDE GEAR "RING" CHANGE, CLUTCH PACK CHANGE... 1968-69, "this" Side Gear. this "Side Gear Ring", these Clutches of this thickness, these axles. 1970, "this" Side Gear. -NO- "Side Gear Ring", these Clutches of this thickness, these Axles... Man it's a bummer not to have a parts stash anymore of known vintage to do accurate "discovery" on.

Some are at 18" inside axle flange to center housing and some are at 18 1/2"
All of the axle tubes on the driver's side measure at 17 5/8" - So all the axles on the driver's side are the same.
The spring perch cc are all the same at 43 3/8"-43 1/2"

All of these rear ends have the 18" tube on the passenger side
68 AMX - Original from my car.
69 AMX - Original from a scrapped AMX
70 AMX - I measured the one in my AMX that is original
71 Hornet - original in my car.

All of these rear ends have the 18 1/2" tube on the passenger side
4.10 gear unit out of a Gremlin but unknown if original. It does have a later casting number on the center section 3212478 vs 3186891 on all of the rest.
3.54 gear unit out of I don't know what.

I am going to put a copy of this here so that I can find it again. It shows what the 1966 Tweener AMC20 ring and pinon looks like compared to the later more common one.