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70's 12 bolt questions

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snotrocket71

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hey, i have a few questions regarding a rear.... sunday i'm picking up a 12 bolt rear from what the seller says was a 70's era blazer that he had in a project truck he had. he says its a peg leg 3.08 with six lug axles, and is in working order and is complete figure for $75 plus a ride i'll take a chance (is that a fair price or am i getting ripped? seems awfully cheap to me). i plan on freshening it up in the future before any use, maybe a posi unit, new gear set, axles, bearings and anything else i can replace. i dont plan on making huge power any time soon, nor do i really drive too hard, i figure its a step up from the 8.5 i have. ...he said something about a matching front axle as well, maybe a dana 44, he wasnt sure... i just replaced the rear u-joint and was horrified to see how shot it was, it actually wore into the pinion yoke and busted the u joint strap bolts on one side so i figure it wouldnt hurt to have a spare rear. also would i need new driveshafts as well? any info would help, thank you very much
 
they are worse than 10 bolts as far as breakage goes...stick with the 10 bolt...
 
geez i didnt know they were that bad as far as breakage, i figured 12 bolt anything would/should be an improvement over a 10 bolt. i always thought that 8.5's were the weaklings as far as trucks were concerned, like 7.5's are a bad joke. i would have like the 3.08's they would have been a nice step up from the 2..73's i have currently. any way to make them more durable/reliable? or is it just a poor design and dont even bother. i wouldnt mind trying my hand at rebuilding a rear.
 
snotrocket71 said:
geez i didnt know they were that bad as far as breakage, i figured 12 bolt anything would/should be an improvement over a 10 bolt. i always thought that 8.5's were the weaklings as far as trucks were concerned, like 7.5's are a bad joke. i would have like the 3.08's they would have been a nice step up from the 2..73's i have currently. any way to make them more durable/reliable? or is it just a poor design and dont even bother. i wouldnt mind trying my hand at rebuilding a rear.
Click to expand...

A little bit bigger axle shaft (30 spline vs 28 spline) and a little bigger ring and pinion (8.875" vs. 8.5"), the truck 12 bolt uses tiny pinion bearings that are failure prone.

It's an upgrade, but not much of one. What size tire are you using?
 
it has 32" BFG AT's on it, i dont plan on lifting it, currently has a stock 305 and in the future will definetly get a mild 350 swap of some kind (maybe an oldsmobile 350). the trannys an sm465. i dont really abuse it and i dont plan on much off roading aside from going down the beach here and there. its a daily driver and not much else. i do like the fact that it would be a step up in gears especially if he has the matching front dana 44(at least thats what he seemed to get at), 2.73's suck.
 
going from 2:73 to 3:08 is not a noticeable change...you need at least a .50 (i.e. 2:73 to a 3:42...) change to make a felt difference.

I would go with 3:73's with the 32's...
 
First gen 12 bolts were a little different. Believe they had different spring perch widths and shock mounts. Might want to measure before you buy it and make sure it's a from a post 72 truck.
 
i understand your logic of putting 3.73's in and really would love to but its not feasible for me considering i daily drive it and often cruise above 60mph. i guess i'm trying to have my cake and eat it too as they say. there's kinda no winning in it, i love my sm465 and i love cruising at a low RPM on the highway so compromise is needed somewhere. to top it off the 305 doesnt exactly make the greatest low RPM torque which would help with the highway gears. I didnt know about the difference in the springs/shocks, before i go any further i'll find out what the rear(s) came out of.
 
Pass....I did a 12 bolt in my 89 just cause I had it laying around...I went 3:73, Auburn,1340H...couple years ago I ran up on a 14SF 3:42 from a van w/disc...it sat around for a few years while I waited for the 12B to break...I got offered stupid money for the 14SF, so I sold it...a year later, I gacked the pinion gear on my 12B...really wishing I had that 14SF when I rebuilt the 12B...I run 33's with a mild built 350 TBI..,
 
I noticed a BIG difference in acceleration when I swapped a 2.73 ratio rear diff in my 81 G10 Van,when it's 3.08 diff started making knocking and clunking noises around corners..it's 307 V8 is a real slug now compared to how it was!..I can get it up to 75 mph in second gear before its really tached out,I used to love downshifting it to pass slowpokes at 65 mph,their eyes would open real wide when they heard it go into second ...:D..but in normal driving its always lugging and slower than a tortise going up hills..:(

I dont think the old 12 bolts were as weak as some here claim they are,or GM wouldn't have put them in some heavier duty half tons after the 10 bolts came about..maybe when you put huge tires and rims on them and put a big block in front of one the pinion bearings mightfail sooner,but I'd rather have the bigger axles and ring gear and the diff cases on 12 bolts dont seem to get wiped out where the side gears ride in the housing like 10 bolts do..(thats why my van needed a rear end,the side gears chowed the diff case and egged it out!)...

For all the work involved swapping an diff,I'd just hold out for a 14 bolt if it were me,and upgrade the front spindles to 8 lug,and get it over with...around here junkyards get about the same prices for a dif if you want a 3/4 ton VS a 1/2 ton,the one tons and dana 60's are the ones they feel are worth a fortune..
 
i appreciate all the good information everyones put up, i think for my application and future plans with the truck i'll splurge and buy it, its only $75; in my book thats not alot, i'm more used to guys thinking they have gold with 80's GN and 442/hurstolds 8.5 rears (anywhere from $800 and up depending on condition, posi, driveshaft etc.) i figure the pinion yoke on my current rear is kinda shot too from a REALLY REALLY shot u-joint (it wore into the strap bolts through the yoke and created a nasty rattle) and it would be nice to have a back up especially if i cleaned them up and rebuilt them. i understand the durability issue that some guys have run into, but i figure i'm not out thrashing on it everyday and any offroading that i do would consist of going down the local beach to fish nor do i plan on anything bigger than my 32's. i dont plan on making huge power either so thats not a huge concern. now that i know the good and the bad of it i'll know what to expect.
 
wound up not getting it in the end. felt as though i could the money into something else instead of a part that i want but dont neccessarily need. thank you all for the information, opinions and experiences although that never seems to be a problem on this forum.
 
you chose wisely....:waytogo:
 
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