12 bolt to 10 bolt Driveshaft Woes

[ccz145a]

I just swaped out my old worn out 12 bolt 3.73 for my donor truck's 10 bolt 3.42 with factory limited slip. The swap included just the rear end, I kept my old brakes, brake lines, and original springs. My axle spring saddles are in the bottom spring pin. I have the two piece driveshaft.

All was going well until I went to put my driveshaft back in (with the newly installed correct u-joint) and it is about 3 inches too long now. The differential is totally un-sprung, so it is at it's furthest length of travel and it is still 1.5 inches too long and even with the drive shaft bottomed out at the center carrier shaft.

I remember when I put the 12 bolt in there in 1992, all I did was put the mixed u-joint on it and all was well. I could have sworn that the the differential I replaced was a 10 bolt because I had to change the u-joint.

If I need to get my driveshaft cut, how do I take my measurements?

Do I take disassemble the center yoke, put it on the splines at midway and pull a tape from back of each u-joint saddle with my differential totally sprung (with all weight on springs).

Or do I leave it unsprung, and place the center yoke as close to the end as I feel safe and then measure.

Or both and provide them a range.

[VileZambonie]

I think you just missed something. The length of the shaft should be unaffected. Make sure the slip yoke is all the way back.

[ccz145a]

My slip yoke at the trans is showing about 2in of metal beyond the seal, are you saying I should just sand and emory the exposed collar and put it all together a little closer to the engine?

[VileZambonie]

No, I mean at the slip yoke, where the two driveshafts come together.

[ccz145a]

The slip yoke bottoms out before I get the end u-joint in the differential saddle.

I think I need to clean up the yoke at the trans. I seem to remember moving that out years ago when I put the 12 bolt in to begin with. I might have been concerened that there was not enough yoke on the output shaft, that sounds familiar.

This action will just reverse what I did in '92 (if that's what I did, heck I don't know I drank more back then).

Vile, are there any written specs on the slip yoke travel clearance?

[Captkaos]

Technically you shouldn't have to do anything, it should fit.

See if you can let the slip yoke out.  I would pull it apart and make sure you have enough engagement, generally about 3" in/out

[ccz145a]

**Update**

Success! I got the driveshaft shortened 1" and rebalanced. Went together just fine after that.

The 10 bolt is 1" longer than the 12 bolt from the centerline of axle to bottom of the yoke saddle.

I must have misremembered what I did before. I honestly don't know why I would have thought there was a 10 bolt in it when I got it, they came with 12 bolts until 78 or so, right?

But it's fixed now and so quiet.

[VileZambonie]

I believe the 10 bolt became available in 78 and was completely phased in by 81

[SUX2BU99]

Good to hear! I've always read the 12 bolt was around until 81, and then the 10 bolt was it 82-up. Splitting hairs I guess. My 81 has a 12 bolt but I don't know if it's original. I presumed it is.

Did shortbox's ever come with 2-piece driveshafts? Mine has a 1 piece but I'm just curious.

[Captkaos]

10 bolts started in 78/9 and pretty much all 82's had them.  81's was the last year you would see a 12bolt.

[zieg85]

10 bolts started in 78/9 and pretty much all 82's had them.  81's was the last year you would see a 12bolt.

Both my 83 GMC and 86 Chev Suburban had 12 bolts, so both must have been changed prior to me owning them?

[Captkaos]

Yes anything after 1982 did not come factory with a 12bolt.