K20 diff question, need quick answer!

[Elpresidente]

Will the ring & pinions from factory 3/4 ton 73-87 diffs be the same size as those from a 67-72 style truck? I'm buying differentials from a '79 and was wondering if I could swap the gears over for my '70 3/4 ton.

[Irish_Alley]

the 14 bolt ff didnt come out till 73. the sf did come out in 64

[bake74]

     It depends on if you are comparing the same style axle to axle, kinda like apples to apples and not apples to oranges.  I have seen 70'ish 3/4 ton trucks with different rear axles, especially if they were 2wd.

[Elpresidente]

Right on, thanks. I assume stock K20 diffs from a '79 would have something like a 4.10 gear ratio?

[bake74]

Right on, thanks. I assume stock K20 diffs from a '79 would have something like a 4.10 gear ratio?

     I wouldn't think that a lot of them were ordered with 4.10 gears, but could be wrong.  I guess it depends really on what part of the country the truck was ordered for and what it was used for mainly.

[Irish_Alley]

Right on, thanks. I assume stock K20 diffs from a '79 would have something like a 4.10 gear ratio?

     I wouldn't think that a lot of them were ordered with 4.10 gears, but could be wrong.  I guess it depends really on what part of the country the truck was ordered for and what it was used for mainly.

all the k20s that i have touched all had 4.10s. now thats not saying most are like that just saying the handful i have opened up had that ratio

[The dream]

Mine had 3.73's.  78 K20.  14 bolt FF

[LeftysRodandCustom]

It depends on the axle housing and carrier. There are 3 carrier sizes for the 10 bolt and just as many for the 12 bolt I belive. The 10 bolt has an 8.5" ring gear, the 12 bolt uses an 8.75" one. They share axle widths, axle bearing size, and carrier bearing size. While the 10 bolt has a 28 spline axle and the 12 has a 30 spline. You'd be far better off switching out the whole unit than trying two switch gears. These break in to a wear pattern with time and wont like having the pinion depth or the backlash change. Keep em complete unless you're installing new gears altogether.