AMC Model 20 rear end parts swap discussion part 99/ GEAR RATIOS/CARRIERS

[Guynn <amx69@xxxxxxxxxx>]

First Marty, let's forget about the model 15 and any of the gear ratios 
offered there.In some cases the gear ratio offered may be the same for a 
6 or V8 rear, but the parts never interchange. The only parts that will 
fit in a model 20 AMC rear are parts designed to fit, not model 15, or 
35 parts.

No to explain gear ratios a bit.AMC in ''cars'' offered a certain set of 
gear ratios. Jeep offered a certain set of gear ratios as well. However 
not all Car ratios were available in the Jeep and not all Jeep ratios 
were available in the cars. This does not mean they will not 
interchange, they will. Along with the Factory Jeep/AMC gears, 
aftermarket companies also make gears. In some cases the aftermarket 
gear is the same tooth count as the Factory, sometimes not.[this is just 
a sample, other gear brands are available]
The Factory ratios we commonly know of are :
2.56, 2.87,3.08,3.15,3.31,3.54,3.73,3.91,4.10,4.44,and,5.00
Yukon gear gives us :
2.73,3.07,3.31,3.54,3.73,4.11,4.56,and,4.88
Precision gear gives us:
3.08,3.54,3.73,4.10,and,4.56

Now, imagine that if the pinion gear stays in one location, and the 
carrier Ring gear flange stays in one location[it does on same series 
carrier], then it is real easy to see that when the Pinion gear diameter 
gets larger, then in order for the Ring gear to be fitted to the exact 
same flange location, the ring gear must get thinner.At some point the 
gear gets so thin that it is weak .In order to get a numerically lower 
gear in the housing, say a 2.56, then you must design a carrier to move 
the mounting location[ring gear flange] over to allow the gear to be 
made thick enough to not break.

What may help you to understand this corelation between thick/thin ring 
gears is to get several different ratio ring gears on a bench. Set them 
flat on their mounting face, now look how much difference there is 
between the thickness of the gears.

The thing to understand is that due to one [or another] manufacturers 
designes for the parts ''They'' build, the ring gear thickness for their 
3.54 may be different than the thickness on 3.54 factory gears. My next 
post will be on Carriers and breaks and maybe when you get one of them 
the concept of the other will come on thru. Since they carrier & gear 
mount to each other the way they are built/designed, one is in direct 
relationship to the other in respect to ''how much room'' there is 
available between the bearing retainers on the AMC model 20 axel housing.

Regards,

Randy Guynn