3.08 open to 3.73 posi question

[peeblesj]

Hey Guys,

I have been researching on here for a few days and I am still not 100% clear on my question. What I have going on is a 1969 C10 with leaf springs. It has a 3.08 open rear end. I bought a 3.73 posi out of a 1968 C10. The problem is is that the posi came from a coil spring truck.

I bought new perches for the leafs to weld onto the coil spring axle housing. My plan was to weld those on, but then I came across the shock mounts on the axle housing. Bottom shock mounts on a leaf truck went on the axle tube, bottom shock mounts from a coil truck go to the trailing arms from what I can tell. So I would have to cut off the mounts and weld those on too.

My question is, is it worth the hassle to mess with the perches and angles etc. Or can I take everything from the posi and have it put in place of the 3.08 gears. I have read about the carrier series differences and that is what I am concerned about. Will the 3.73 posi be able to be put in the 3.08 rear end? I would have a professional do the install and shimming etc.

So, drop the entire rear end out and put the complete 3.73 posi in and weld in the new perches or swap out the internals? Thanks in advance guys!!

[74baggedbug]

I'd have the shop just swap internals

[Eddie H.]

Just swap the pinion gear and the whole ring gear and carrier assembly from the 3:73 into the 3:08 housing. You may have to adjust the pinion depth and backlash, but it's not a big deal.

[ryans69chevy]

If I'm understanding this right wouldn't it be easier to just change to a coil spring rear?? I think it would be cheaper and it would be something you could do.

[peeblesj]

Thanks for the replies! Sounds like I will be having the internals from the coil spring posi put into my leaf spring housing. I will have to get that lined up this week.

I considered changing to coil spring rear end, but I do not have the trailing arms and all the other parts that would go with that......

Thanks again for all the replies. I will post an update when it is all installed.

[peeblesj]

So this last weekend I was able to get the internals swapped and all new bearings installed. Everything went pretty well. The shims were a pain in the butt, but we got it all done. Backlash checked out good and there was good contact between the teeth (not shown in pics....sorry). The first three are the original gears, the last one is after the posi was installed. Thanks again for the input!

[demian5]

Nicely done. I'm swapping out a 3.73 posi for a 3.08 open...LOL I coulda used your old one but I already got one.

Any insight to doing the swap? I've never setup a ring and pinion before.

[engineer_gregh]

I didn't think the 3.08 carrier would house 3.73 gears? I thought the housings were different, i.e. there are 3 series and 4 series. I thought the biggest factory gear set for a three series carrier was 3.42.

[CUSTOM/10]

The carriers are different but the housings are the same ( as long as there both 12 bolt )as far as the carrier fitting into them. There might be slite differences in machining so shim changes could be needed. I would start by swaping the entire carrier, pinion, bearings and shims all from one case to the other ( if everything looks to be in good shape ), then check your backlash and gear contact patern. You will need new seals and crush sleave.

I basicaly did the same swap except my posi carrier was for 3:08 gears so I used a thick set of 3:73 gears to make up for the difference. Here's a link to the page on my build thread with some pics & info.

http://67-72chevytrucks.com/vboard/s...259859&page=23

[slikside]

Sub'd.

[peeblesj]

engineer_gregh, you are correct. There are the 3 series and the 4 series carriers. Just as CUSTOM/10 said, both of the housings were 12 bolt. I took the carrier, pinion, bearings and shims from the posi rear end out and swapped into the 3.08 housing. I did replace all the bearings along with the crush sleeve, pinion seal, cover seal, and wheel bearing seals.
As for the backlash, there are specs out there (cant remember them off the top of my head). There is also a torque spec for the nut on the pinion to crush the crush sleeve. Then you can make small adjustments from there to get the correct backlash. The shims where the hard part for my swap. I only had the factory shims from each rear end. Well, none of the 4 shims were a perfect fit, so I had to sand them down equally on each side of the carrier. I used 120 grit sandpaper and hand sanded each shim to keep them flat. They should go in pretty tight, so it was a game of sand, test fit, sand some more, re test fit etc. You can buy a shim set from places like Summit etc. I am by no means a professional. This was my first time doing this. I did have our shop's mechanic to help and give pointers which helped a ton.

[GR8-68]

So will a unit out of a car 12 bolt go into a truck 12 bolt housing ?

[CUSTOM/10]

Originally Posted by GR8-68

So will a unit out of a car 12 bolt go into a truck 12 bolt housing ?

NO !!! 12 bolt car units are a different animal altogether !!!

[GR8-68]

Originally Posted by CUSTOM/10

NO !!! 12 bolt car units are a different animal altogether !!!

Thanks, I wasn't sure I knew the outer housing was different

[engineer_gregh]

Great post. Thanks PeeblesJ.

[CUSTOM/10]

Originally Posted by GR8-68

Thanks, I wasn't sure I knew the outer housing was different

No problem Larry, Didn't want to see you go through a lot of work for nothing !

Gary