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3.15 rear end

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farna View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: May/12/2022 at 5:12pm
That makes swapping gears with older models more difficult! An older carrier would need the bolt holes drilled and tapped for the larger bolts, or a shoulder bolt used. Almost impossible to use an earlier ring gear on a later carrier though. You'd have to lay out a new pattern of holes between the existing ones and drill and tap for the smaller size. Not impossible, but a lot of work. Luckily there is no real reason to do that!
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rambler Joe Quote  Post ReplyReply Direct Link To This Post Posted: May/12/2022 at 6:47pm
My '66 Classic wagon (327 4bbl) came with a lower ratio, and I swapped out the rear axle to get a better highway ratio of 3.15:1. That donor car was a '65 Classic with a 287 2bbl. Both cars, of course, came with torque tubes. 
My original question is whether or not a ring and pinion gear set for a '60 through '64 (so states the seller) will work in my '66. I'm thinking it will, since there weren't any big changes until '67.  From what you explained, it appears that I may be correct.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SC397 Quote  Post ReplyReply Direct Link To This Post Posted: May/12/2022 at 7:18pm
Originally posted by farna farna wrote:

That info is correct from 1970 up for the AMC 20, 1972 up for the AMC 15. Prior to this the axle ratio is identified by a tag on  one of the rear cover bolts with the number of teeth screen printed on as a ratio. For example, 15/43 = 2.87 (divide 43 by 15 and you get 2.8666...). The first number (smallest number) is the number of teeth on the pinion gear, the second the teeth on the ring gear.

Adding to the chart:
AMC 15 : J = 2.37 (19/45) and K = 2.53 (17/43)

There aren't any more AMC 20 ratios, at least not since 1967 (open driveshaft models). Remember, all 65+ six cylinder big cars used the AMC 20 axle. Older TSMs have the 11/39 axle listed as 3.55 instead of 3.54 (39 divided by 11 = 3.54545... Can be rounded to 3.54 or 3.55).

Older cars (torque tube drive) use lower gears in the 50s and early 60s. The 58-66 American used an open driveshaft but a torque-tube type pinion gear with a "big nut" driveshaft yoke. The nut tightened on tapered threads on the end of the yoke to clamp onto the pinion shaft. Same ring and pinion was used on the big car torque tube AMC 15 as on the open driveshaft axle those years. Since not many people swap those gears I didn't look all the ratios up. They went as low as 4.40 (8/35... actually 4.375) in the 1956 big car with 195.6 OHV.

There are actually quite a few ratios for the later AMC20 if you look at the Jeep line that used the AMC20 as well. See last page for all the ones that I could find.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: May/13/2022 at 6:55am
jpnjim pointed something out to me in a PM -- the ring gear bolts thread into the RING GEAR, NOT THE CARRIER! DUH! I knew that!! I goofed and wrote it backwards though. Thanks Jim!

So shoulder bolts would work to put the older ring gear on the newer carrier, the older carrier holes would have to be drilled out from 3/8" to 7/16" to mount a newer ring gear.

The problem then would be the pinion shaft length, diameter and splines. I don't know how they match up to the torque tube axle pinion shaft. IIRC they are the same, but I haven't had them side by side nor tried the yokes on each. Again, IIRC, AMC just machined the pinion shaft to have a threaded end for a nut to hold the yoke on -- I don't know off the top of my head if the shaft was shortened to allow for machining the threads or lengthened, nor for certain if the splines and diameter stayed the same. Someone would have to compare them. I think the bearings stayed the same... but not 100% sure of that.

SC397 -- I was just referring to factory AMC gears. Noting that there are many other non-factory options was a good idea though, I should have thought about that!
Frank Swygert
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