Rebuilding my pinion... need some advice.

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Aug 6, 2021
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181
Location
Washington State
I'm in the process of installing ARB lockers. To keep my 1/1980 FJ40 on the road while embarking on this adventure, I bought what I thought was a compatible rear axle based on it's general appearance, dimensions, and gear ratio (3.70, which was standard starting in 1978 according to this). I have disassembled the diff and am in the process of assembling the pinion. I noticed that the aft pinion bearings (old and new) appear to be quite different thicknesses (see attached pic). The new bearing is out of a Sierra kit (IK TOYLC-B).

Thinking that the axle I bought may be older than I thought, I took a closer look at Cruiserfaq.com and noticed that a 27 spline pinion was introduced in '78. Mine is a 10 spline (see pic). Do I have the wrong bearing set, or perhaps the difference in bearing thickness works it's self out once it's torqued?

NOTE: In the pics, the new bearing is on the pinion and the old bearing is off to the side. The races are not interchangeable between the two (e.g., the new one doesn't seat on the old roller cage and vice versa). The two races do, however, have the same OD and ID and the new one pressed onto the pinion just fine.

Any advice appreciated!

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pb4ugo

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spay-lay-wi-theepi, Ohio
IDK for sure, but I think Steamer is correct. I think you need to count the teeth on the ring and pinion to confirm the gear ratio. The ring gear may be marked with the ratio or tooth count on the side of it too. I think the 3.70:1 diffs would have fine spline pinions. The coarse spline pinions would be earlier diffs and are probably 4.11:1. IDK about sierra bearings and races, but usually the bearings and races have #s on them. The national brands use the same numbering systems or they can be cross referenced.
 
Joined
Aug 6, 2021
Messages
181
Location
Washington State
Welp! My evaluation of the gear ratio by turning the pinion flange and counting hub rotations turned out to be incorrect. After counting teeth, (9 on pinion and 37 on ring), the gear ratio is 4.11. Given that, and the coarse spline, that must mean its a pre '78. And since my truck is an '80 with stk 3.70 gears, this ring and pinion set ain't gonna work!

Looks like I need to source a new ring and pinion to match the truck. And even with the correct ring and pinion, I think this is the wrong Sierra kit/bearing. It is listed on their site as being for the Toyota 9.5" diff. I just re-measured my ring gear and it is 8.5" in diameter. Time to give the vendor a buzz and sort this out. Living and learning!

4.11 diff coming to a classifieds section near you!
 
Joined
Aug 6, 2021
Messages
181
Location
Washington State
good news! I have the correct bearings, just need to change out my ring, pinion, and flange for the 3.73 gears/fine spline. Georg at Valley Hybrids comes through again... such great customer service!
 

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