can I swap 12-bolt internals?

[BarryV]

Hi Guys,

Can I take the 3.07 gears out of a 1971 12-bolt leaf spring rear end and put them into a 1970 12-bolt rear trailing arm rear end?

They are both 12-bolt, one is 1970 and one is 1971 ... I don't see why not, but I know that things change sometimes.

My goal is a 3.07 rear end on a 1970 SWB, but mine has a 3.73 rear end, and I stumbled across a 3.07 rear end that is designed for leaf springs. I think I just need to swap ring gear and pinion gear (and have an expert get the gears meshed correctly) ... right?


Thanks!


Barry

[70blackfish]

I think so ..only difference is 12 bolt truck, and 12 bolt car are not the same.

[68shortwide]

Yeah as 70blackfish stated the housings on a truck and car 12 bolt are different so that's a huge factor.

[YBNORML]

Yes. I have done this in the past. I took a 3:73 posi unit out of a 72' rear and put them in the narrower 68' rear. No issues.

[mechanicalman]

Originally Posted by BarryV

Hi Guys,

Can I take the 3.07 gears out of a 1971 12-bolt leaf spring rear end and put them into a 1970 12-bolt rear trailing arm rear end?

They are both 12-bolt, one is 1970 and one is 1971 ... I don't see why not, but I know that things change sometimes.

My goal is a 3.07 rear end on a 1970 SWB, but mine has a 3.73 rear end, and I stumbled across a 3.07 rear end that is designed for leaf springs. I think I just need to swap ring gear and pinion gear (and have an expert get the gears meshed correctly) ... right?


Thanks!


Barry

As long as you use the carrier with the 3.07 ring gear you can do it. You cannot put 3.07 ring gear on a 3.73 carrier on a 12 bolt truck (truck carriers are different from car carriers regarding series).

[BarryV]

Thanks everyone!

I left out something rather important ... I would like to have posi-trac (or some sort of limited slip). Ooops!

Would I be throwing away the carrier and spider gears if I want to go with limited slip?

Barry

[doubleb's69]

Originally Posted by BarryV

Thanks everyone!

I left out something rather important ... I would like to have posi-trac (or some sort of limited slip). Ooops!

Would I be throwing away the carrier and spider gears if I want to go with limited slip?

Barry

Yes unless you put in a powertrax unit that replaces your spider gears. They are alright but can cause problems on turns as they are more of a locker but will slip if you aren't in the pedal while making turns. Here is a link to them at Summit.

http://www.summitracing.com/search/b...ew=ProductName

[Bigdav160]

Also, the pinion shim under the large pinion bearing should stay with the carrier (or axle housing as most people call it)

[Mike C]

The best way to do this is to measure pinion depth and backlash in the donor axle before disassembling it. When you reassemble into the new housing, first verify pinion depth is the same. If it has changed, shim to reestablish original pinion depth. Then put the ring gear in on the carrier and measure back lash. If it has changed over the baseline, re-shim to correct. This will most likely require an aftermarket carrier shim set because the OEM ones are cast and not adjustable for width.

Basically, this involves all of the same tools and processes of setting up a rear from scratch. The other option is to take 'em apart, put 'em back together, and run 'em. We did that with one of the 12 bolts in my Jimmy, and I think that cobbled together setup lasted longer than the two factory axles it had in it. Go figure.

[BarryV]

Lots of great info ... thanks everyone.

A slightly off-topic question is this:

Do I do a rear-end gear ratio swap or get an OD tranny?

1) 350 / TH400 / 3.73 -> current setup, requires no new purchases
2) 350 / TH400 / 3.07 -> setup discussed in this thread, requires rear end work
3) 350 / 700R4 / 3.73 -> requires a transmission swap
4) 350 / 200-4R / 3.73 .....

You get the point.

This truck has been driven ZERO miles, and is about ready for the engine and tranny to be installed. I had not thought of getting a different tranny, but it might be a better bet as it requires less work.

Opinions?


Barry

[motorcritter]

If you're at that juncture- an OD transmission is a good idea, for regular driving. It's lo easier to do it now, than do it later. Or wonder 'what if?...'.

[Eddie H.]

A 700R4 or 2004R will work well with 3:73 gears. As far as cost goes, the OD trans set up properly will be considerably more expensive than changing your existing rear end to 3:07.

I have one of each. my 69 has a TH 400 and 3:07 and my 71 has a 700R4 with 3:73.

I prefer the 69 with the TH 400 myself. It's a much tougher trans, and I can tow or haul anything with it without any worries.

[BarryV]

Originally Posted by Eddie H.

A 700R4 or 2004R will work well with 3:73 gears. As far as cost goes, the OD trans set up properly will be considerably more expensive than changing your existing rear end to 3:07.

I have one of each. my 69 has a TH 400 and 3:07 and my 71 has a 700R4 with 3:73.

I prefer the 69 with the TH 400 myself. It's a much tougher trans, and I can tow or haul anything with it without any worries.

Thanks for the info.

This will be my son's truck and he'd prefer a 4spd (or so he told me last night).


Barry