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81 K5 Differential questions

J

Jason Thompson

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On my 81 Blazer I noticed it has different size differentials the front is 10 bolt and rear is 12 bolt. So is that a bad idea? I also need to pull the cover to find out ratio. No RPO in glove box and no tag or imprint that I can see. I rarely take off road and if I do it would only be for light 4 wheeling, however, I still frequent the freeways. The reason I ask is I'm planning on making sure the gears are 373 or 411. It has a 400 sbc with 700r4 and 35" tires on 4" lift. My second question is how difficult is it to change gears and does the front and rear need to match (in terms of 10 or 12 bolt)? Thanks for help.
 
A 10 bolt front and a 12 bolt rear would the factory combination for an 81 so they're ok. Gear ratios need to match so if there's a chance a previous owner has done any swapping or upgrading you might consider pulling the covers to verify the ratios, good opportunity for maintenance.
 
nvrenuf said:
A 10 bolt front and a 12 bolt rear would the factory combination for an 81 so they're ok. Gear ratios need to match so if there's a chance a previous owner has done any swapping or upgrading you might consider pulling the covers to verify the ratios, good opportunity for maintenance.
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Do
 
nvrenuf said:
A 10 bolt front and a 12 bolt rear would the factory combination for an 81 so they're ok. Gear ratios need to match so if there's a chance a previous owner has done any swapping or upgrading you might consider pulling the covers to verify the ratios, good opportunity for maintenance.
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Do you or anyone happen to know what the stock *81 K5 differential was? Thanks for the help!
 
Jason Thompson said:
Do you or anyone happen to know what the stock *81 K5 differential was? Thanks for the help!
Click to expand...

As I said the 10 bolt and 12 bolt were the stock axles for 81. Now if you're wanting to know about gear ratios and/or locking differentials, technically these were based on options the truck was ordered with. Most trucks of that era would have had 3.08 gears and a rear locking differential, this was the most common way they were optioned. Look in the glove box for an RPO option sticker, if the sticker doesn't have written descriptions google the codes.
 
Up to at least '83 the RPO sticker was on one of the inner fenders, visible inside the engine bay. They don't hold up as well as the ones in the glove box, but that's where it should be in 1981 I'm pretty sure.

If you are thinking you don't need stronger axles, but want a better gear ratio, then your best bet (IMO) is to look for a later 1/2 ton Suburban or K5 (1987 or so, up to 1991) being parted out, and grabbing the complete axle assemblies from it, after first popping the covers to ensure they are in good shape. The gov-lock (RPO G80) does not hold up well in the half ton variants, especially with larger tires, so be careful. I would *never* waste my time regearing a 10 bolt rear.
 
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Cheap insurance to pull the covers and just look around, clean off the magnet looking for pieces of metal, etc. Look along the edge of the ring gear for the ratio. 41/10 is 4.10, 41/11 is 3.73, 41/12 is 3.42. The G80 should be obvious if it's there. Utube is your friend. I have buried a number of 12b that have spontaneously dismantled themselves, so maintenance is key.
 
Jason Thompson said:
Do you or anyone happen to know what the stock *81 K5 differential was? Thanks for the help!
Click to expand...

It probably doesn't have the gears you're hoping for, just gonna put that out there. 3.08's were super common even with the 700R-4. 3.42's are almost as common. I know they exist, but I've never seen 1/2 ton diffs with 3.73's or 4.10's. Hen's teeth around here anyways.

Diff's are a good place to spend money, but you have to figure out if you really want to stay 6 lug (1/2 ton) or jump to 8 lug (3/4 or 1 ton). 1/2 ton diffs can be just fine, but will still eat up your money getting the gears and stuff you want/need.

Anyways, welcome to CK5!
 
tRustyK5 said:
It probably doesn't have the gears you're hoping for, just gonna put that out there. 3.08's were super common even with the 700R-4. 3.42's are almost as common. I know they exist, but I've never seen 1/2 ton diffs with 3.73's or 4.10's. Hen's teeth around here anyways.

Diff's are a good place to spend money, but you have to figure out if you really want to stay 6 lug (1/2 ton) or jump to 8 lug (3/4 or 1 ton). 1/2 ton diffs can be just fine, but will still eat up your money getting the gears and stuff you want/need.

Anyways, welcome to CK5!
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I will say that 3.73 is not that rare in 1/2ton suburban and I believe all the chalets came with 3.73.
I have never seen a 4.10 stock in a 1/2 ton.
I still think 3.73 is not good enough for 35" tires, I had 33" 3.73 and it was a dog and the 700r4 trans didn't like it, switched to 4.10 and it was perfect
 
My '85 K10 Suburban has 3:73's..with a 6.2/700R4 too...probably not a common option ?..
Most every other K5 or 1/2 ton I had came with 3:08's..

I had a 1976 K5 2WD I bought to sell to a friend who didn't care it wasn't a 4x4,that had a 4:11 non-posi rear in it,with a 350/TH350,that thing would boil the tire (one wheel peel) pretty easily!..that was the only 4:11 I've seen in a 1/2 ton truck from GM so far..
 
I've been happy 4.11s with the 700r4 and 35's. I almost got 15mpg on a road trip!
 
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