1972 K20 rear end question

[Retro]

Hey guys, so I'm a fairly smart guy, but when it comes to rear diffs...I don't know squat.

I have an Eaton open diff in the rear of my 1972 K20. I've been told if I'm interested in a locking/posi/etc rear diff, I should consider a Dana 60. I have pretty tall gears (4.56) in the Eaton, I'd like something a little more road friendly - 4.11 or 3.86 maybe?

So, what do I look for? What year range with what gears, etc.? What are the details I need to know to buy the right thing?

Thanks in advance for any guidance! I appreciate it.

-Retro

[ElKotze]

DANA 60 is a great choice, wide range of gears and pretty much bolt in up to '72 after that they changed spring perches and shock mounts, I think, but that can be changed with a little fab work. Another option would be a 14 bolt SF out of 70's and 80's 3/4-ton and 1-ton trucks, again lots of different gears and spring perches and shock mounts need to be changed. Both are great axles and parts are readily available.

[Mike C]

4.10 were available for the Eaton but not as common as the 4.57. They were available with a Maximum Traction rear diff which was a true Locker. You can swap a Detroit into the case if you use 30 spline 14 bolt full float axle shafts.

There is an HO52/72 thread at Pirate4x4.com that is loaded with info on the Eaton.

If I was swapping, I would just get a newer 14 bolt. Full float if you really work your truck hard, but the semi-float 14 from a 3/4 ton 4x4 Suburban can be a good choice. It is rated same as the HO52 in terms of GAWR and was used in vehicles up to 8600# GVWR which was higher than any of the C20’s which maxed at 7500#.

I did this swap in my Jimmy years ago to get rid of the 12 bolt grenades I kept throwing. We just swapped another into my kid’s Jimmy project. My Suburan donor had 4.10 gears and his had 3.73. We swapped the 10 bolt front into his truck as well with a TransAm posi traction, but I just bolted the hub and brake parts from the 10 bolt onto my Dana 44.

[57taskforce]

I put a 14 bolt full floater into my 72 K20. My truck has 4:11’s so it wasn’t hard for me to find a matching rear end. If you go this route you’ll have to strip the factory perches and shock mounts from the 14 bolt and re-weld everything back in the right locations for the narrower 72 spring locations. If you can find a square body 1 ton 4x4 srw, that axle will bolt in minus one shock mount.

[collin70k/20]

I would go with a 14 bolt full floater. I have 4.10 gears in my 1970 k20. Well worth the work of cutting and remounting spring perches and shock mounts. 14 bolts are a dime a dozen and are strong rears. I work my truck really hard and prefer the 14 bolt over the Eaton. I Had 2 Eaton’s in my truck before I got smart and decided to do the swap.

[special-K]

4.10 was the best hiway ratio offered by GM in the Dana 60, same as HO52. So no advantage going to Dana 60 unless you find a Ford rear with 3.73 to swap gear set or convert housing for GM mounting. Add to that the brake parts are getting hard to find, same as HO52, I would consider going 14-bolt. There were even 3.42s available.

[Retro]

Thanks guys! Great info! I'm heading to the Portland Swap Meet on Friday morning and will start hunting (besides looking for a tailgate). I appreciate the data!

[ElKotze]

Originally Posted by special-K

4.10 was the best hiway ratio offered by GM in the Dana 60, same as HO52.

Special-K, with all due respect, but I'm pretty sure the 3/4-tons with big block and th-400 had 3.54's in the Dana 60 as standard equipment, at least in '72.

[kwmech]

Originally Posted by ElKotze

Special-K, with all due respect, but I'm pretty sure the 3/4-tons with big block and th-400 had 3.54's in the Dana 60 as standard equipment, at least in '72.

My 68 has 3.54s with the D60....... and a power loc

[Bigdav160]

Since you have a K20 you will want the front and rear axle ratios to match.

4:56 in front and 4:10 in rear no bueano

PO put a 14 bolt in my K20 but it had a 3:73. I decided to change the front to 3:73 also. I like it.

[Retro]

Thanks, Dave. Very good point.

[jocko]

While C20s indeed required a 3.54 D60 when a 402 was ordered (and a D60 was also possible when rear leafs were ordered on a C20, even with a small block), K20s came only with 4.57 standard or 4.10 optional. There were no big block K's and there are no 3.54s available for the front D44, at least not that I'm aware.
But if there are, that might be a pretty nice setup assuming tires aren't much bigger than stock and you are only planning on highway driving - in which case I'd direct you to the "C" line of 2WDs that GM offered . Generally speaking, due to the larger than most tire diameter on Ks, 4.10 is kinda the sweet spot. 3.73 or 3.54 with your average tall 4x4 tires negatively impacts low end pulling torque in 1st gear, something that is desirable in a 4x4. Best course of action for a K (my opinion only) with stock or slightly taller than stock tires is a 4.10 with either a 700R4 or GV over/under unit for 2WD use only (I prefer the latter to allow maintaining stock engine location, use of the basic but sturdy TH350, and retain reversion to bone stock configuration (except for the driveshaft, if you shortened your original - which I wouldn't - I'd order a custom one for the GV setup and stick the original in my gun safe ).

[Retro]

Thanks, Jocko. I have the standard 4.56 front/rear and the TH350. I was kinda thinking 4.10 might be my target. Right now the truck has a 2" soft ride lift from Tuff Country and 33's. I'm thinking the 4.10s might be a good compromise - street/trail.

I really appreciate all the comments - this is learning curve. Thanks everyone!

[special-K]

Yeah, I was talking about K/20s with rear ratios. The big block C/20s came with 3.54s but no 3.54 for the front axle.

[DeadheadNM]

Minor point but wasn't a 4.10 ratio standard on AC equipped K20s?

[1972RedNeck]

Originally Posted by DeadheadNM

Minor point but wasn't a 4.10 ratio standard on AC equipped K20s?

My '72 K20 came factory with 4.10's and nothing listed on the SPID pertaining to gear ratio...

[57taskforce]

Originally Posted by DeadheadNM

Minor point but wasn't a 4.10 ratio standard on AC equipped K20s?

There might be something to this Pete. My 72 has factory 4:10s and ac, with no ratio call out on the spid. Honestly I just assumed that 4:10 was standard with a small block.

[leddzepp]

Originally Posted by DeadheadNM

Minor point but wasn't a 4.10 ratio standard on AC equipped K20s?

Mine is 4:10 with a power lock, ac equipped. Power lock is called out on spid, gear ratio is not. I verified physically after checking one of sstims power team charts.

[CharlieBrown]

72 K20.....no ac 410 eatons

[Mike C]

3.54 are certainly available for the Dana 44 through aftermarket if wanting to go to lower gear. And I’ve looked at a couple of different K10 ‘67 GMC V6 powered trucks that had 3.54 gears. Not only that, they had Dana 44 in both ends.

Cheapest thing to do would be find donor K20 Suburban with 3.73 and swap both axles in to your truck. But the option of just sourcing a rear and then changing gears in front is another option.

Any problem can be solved by throwing enough $ at it!

[Bigdav160]

[Retro]

I'm probably the one clouding this up. My truck is a CE - started life as a C20 Custom Deluxe. Came to me as a 4x4 - and given the larger front axles/u-joints, appears to be a '73 or later front end. It was a small block 350, but someone dropped a small block 400 in it. Still has the TH350. So, I'm ASSUMING the rear end was the original Eaton w/ 4.56s. However, my truck is an AC cab, so the AC and 4.10 comment makes me wonder if someone swapped the rear end too. Anyway, the result is an Eaton open diff rear end w/ 4.56 and the front ratio matches. I always call it a K20, but in truth it's a C20 (for factory options considerations). Sorry if I misled.

[Bigdav160]

The chart I posted above says 350 got a 4.10 standard and 307 got 4.57 standard

[special-K]

I was sticking with K/20 information. There are all sorts of possibilities for gear ratios through gear set swaps and/or complete axle swaps from other vehicles or aftermarket.

I don't know about the A/C thing with 4.10s, but I have never seen a K/20 with 350/TH350 that didn't have 4.10s.