regular rear dif to posi-trac

[KyFarm]

Ok, I have a 72 c-10 4x4 lwb that has a crap body and a real loose motor. It has a good TH350, NP205, tilt steering column, and good axels and brakes. It was advertised as having 3.08 gears and has 32" tires.

My '69 K-10 swb stepside has a good body, good power steering and a decent (for now) motor. It however has the 3 speed on the column and 4 drums with no power assist. It was advertised as having 4.11 gears and has 32" tires.

My plans are very soon to put the TH350/NP205, steering column and the front axel from the 72 with all the power brake parts in the '69. Then I need to either also swap the rear axels to make the gears match OR

Here are my questions finally. Is a posi-trac rear a completely different axel or just the gears in the rear diff. And if just the gears about how much are a set and can they be bought new or do i have to find a used set?

Both trucks have the standard 12 bolt rear axels. Thanks for any help

[76stepsidechevy]

Theres a carrier break at 3.42 ? I believe so 4.10s wont fit on the the posi carrier with 3.08s. If it were me I'd probably want the 3.08s with 32s and no overdrive anyways. That motor will be screaming with 4.10s 32s and no overdrive. If its an offroad only truck it would be fun though. I'd probably swap the axles to get the posi and disk brakes and swap gears down the road if I had to. Depends on what tire size you plan to run and if the tuck will ever see the highway also.

[KyFarm]

The '69 will live on my farm but not be a mudder truck. Just my light truck to pick-up supplies and do some firewood duty. Thing is I bought it and was told that it had 4.11's but it doesn't scream like you said and I also thought it would on the highway.

I am definitely gonna swap the axels, which will put the (advertised)3.08's from the '72 in the '69 so I can have the power disc brakes, and change over the trans and TC. I guess I'll run it like that for a little while and then when I upgrade the motor I'll think about changing the rear to posi.

Is there anyway to identify the gear ratio's in these axels?

[Elkaholic11]

Take the diff cover off and count ring gear teeth and divide by the number of pinion gear teeth for gear ratio's. Or you can jack up the rear put it in neutral, rotate the tire on complete turn and count the number of times the driveline makes a complete rotation(this way is crude and only gives you a good idea of what it is) counting gear teeth is the best way.

[Elkaholic11]

Also, posi is just glorified limited slip and you would need a whole new carrier, you are better of spending a little bit more money and getting a quality locker that you can lock and unlock like ARB or OX

[KyFarm]

Thanks E...11

that is exactly what I will do

[Mike C]

If the front axle is original and not messed with, it should have a ratio tag on one of the bolts that will show what the r&p ratio is. If 4x4 works, then the rear will be same. Does it say on the SPID in the glove box what gear is? 3.73 most likely would be original. I'd just swap the disk parts onto the open knuckle if it was me and keep the gears you have. If the '72 axle has a posi on the 3.08, you could always use a ring gear spacer to put the 3.73 on it. I ran a thick 4.10 gear in my Jimmy's 12 bolt on the 3.08 posi for a long time.