gm 12 bolt diff

[Magic1]

On my 82 Sierra k10 I've got a 12 bolt rear end and dana 44 up front. The rear has a lot of slop going in gear and I'm looking for suggestions on which route I'd be better going. Ideally I'd like a 14 bolt but don't want to change to eight lug and can't find a six lug sf rear so would I be better rebuilding my 12 bolt I've got now or try finding a good 10 bolt out of a salvage yard. 12 bolt rear ends seem to be pretty scarce. To rebuild will cost 4-750 $ depending on what has to be done

[Captkaos]

If you have to rebuild the 12, there is a possibility that you will have to rebuild a 10 or a 14 bolt.
But a 10 bolt will work also.  Finding a SF 14 in anything but a 8 lug is going to be impossible.

[Magic1]

Yeah. From what I've read they put a six lug 14 bolt in light duty 3/4 ton trucks from like 88-93 or something like that. I haven't looked for one but I'm sure they're probably in pretty high demand.

[morrow07]

My brothers 2000 GMC Sierra 1500 4x4 was ordered from the factory with a 14sf 6 lug. Very rare, but they do exist.

[Irish_Alley]

 i think in 99 is when they eliminated the 5 lugs so all ½tons 4x2 had 6 lugs(might be a good place to start looking but the only problem is with the 88+ axle idk what the width difference would be).
the sf was made iirc from 88-06 ½tons and like you said there was a select ¾ tons LD that had (86-93) i want to say they tent to have 3.73 ratio, so if yours isnt this i would look at getting both front and rear if its a solid front axle. the only problem is with the 88+ axle idk what the width difference would be

[Magic1]

i think in 99 is when they eliminated the 5 lugs so all ½tons 4x2 had 6 lugs(might be a good place to start looking but the only problem is with the 88+ axle idk what the width difference would be).
the sf was made iirc from 88-06 ½tons and like you said there was a select ¾ tons LD that had (86-93) i want to say they tent to have 3.73 ratio, so if yours isnt this i would look at getting both front and rear if its a solid front axle. the only problem is with the 88+ axle idk what the width difference would be

I'm pretty sure I've got 3:73 but ill have to check. The info I got about the rear end was from an article I read somewhere recently online in a off-road magazine although I don't remember which. It said it was a direct bolt in with the exception of the yoke being different.

[Jason S]

I don't have the links, but I've read other sites online (some with photos) about swapping the SF 14-bolt 6-lug axle from '88-up 2500 LD (and some K1500) trucks into the '87-down trucks.

I very seriously considered swapping a completely worn out 12 bolt for the SF 6-lug 14 bolt on my '74 K10. That was about 8 or 9 years ago and aside from moving spring perches, the rest could have easily been worked out. 

I rebuilt the 12 bolt because at that time the cost of rebuilding with a new limited slip and axles versus the asking prices for an open differential SF 14 bolt 6-lug axle was about the same. With the rebuild, I got new, better parts inside instead of having to modify a used axle.

[Magic1]

That sounds good Jason. I've pretty much got myself talked into just rebuilding the rear end I currently have and being done with it. I'd hate to go to all the trouble of finding/modifying a different rear diff just to have to wind up replacing seals and bearings and or having to rebuild it as well. I guess my next order of business is to see for sure which ratio I've got and whether or not this is something I can do myself or hire out.

[pholliday1]

Step one is to find out where the "slop" is coming from and go from there. You need to isolate the rear differential and figure out if you have a bearing problem or backlash issue or what's causing this slop and where's it coming from.

[Magic1]

Yeah I really haven't had time to pop off the cover and look inside. Its got new ujoints so I know they're good. With the truck in neutral and the brake applied I can grab the driveline and turn it back and forth about an inch or better. You can see the shaft slam to a stop when going from park to reverse or from reverse to drive or vice versa. It didn't have up and down play like a worn pinion bearing. I wouldn't think the ring gear and pinion could have that much play and still be meshing up and moving the truck but who knows.

[Jason S]

I wouldn't think the ring gear and pinion could have that much play and still be meshing up and moving the truck but who knows.

It could.  Loud exhaust, tires and other factors can mask any sounds indicating trouble.

pholliday1 is correct, find out what and where is the problem.  In my case, the rear end was flat worn out; including gears, bearings, worn axle shaft and dysfunctional Gov-Lok.  There should be plenty of assistance here to resolve issues you find.