71 K-20 rear

[roy c]

Hello I'm new to this site and I am restoring a 71 K-20 My question is how do I tell the difference between an HO 72 or HO 52 rear Its a FF I also wanted
to see if a locker of some sort is available for this rear Thanks Roy

[Stocker]

Welcome to the forum!

Pretty sure HO72 has larger brakes than HO52.... can't recall the measurements but the info is here somewhere. If I find it, I'll post it in this thread.

Also, all HO72s have the load bolt. Some HO52 have it and some do not.

There were Detroit Lockers for Eaton diffs but apparently not many, so they can be hard to find.

[Stocker]

OK, just found this info in a post by SS Tim:

HO52 has 11 x 2 3/4" drums, and HO72 (SRW) has 13 x 2 1/2" drums.

[roy c]

Thanks for your help I have the HO72. Would you think that the drum set up would be better than disc in the rear I've seen from a outfit called Ruffstuff Specialties.com a conversion bracket. Where you can use calipers and rotors
off a 1975 3/4 ton. to replace the drum set-up thanks Roy

[SS Tim]

Since HO52 were the standard axle in K20 trucks I doubt it is a HO72 as it would have had to have been swapped. Did you actually measure the drums internal size or is this a guess? The outside diameter of the 11" brake drums is right at 13" (see picture). Did you check the axle code stamped into the carrier on top (see picture)?

The No-Spin locker is interchangable between either axle as are most parts including the drop out as a whole and is the only drop-in locker/limited slip I have been able to find for the HO52/72 axles.

A later 14-bolt Detroit Locker can be fitted to the earler HO52/72 differential case assembleis but modified 14-bolt axle shafts must also be used due to side gear spline count differences.

On the brakes a couple of things to note.
11x2-3/4 drums are availabe from Centric with two different p/n depending on year due to stud size changes.
13x2-1/2 drums are very hard to find at all in usable condition. If you find a souce for them new please let me know.

While a 14-bolt intended disc brake kit can be used/adapted to the HO52/72 axles the problem of a parking brake comes up. Some kits do not have one and others use mechanical calipers intended for a much lighter car. While this might be acceptable for some it is not for a loaded truck the weight of a K20 and even less so if it has larger than stock tires.

[special-K]

Not all HO52s had the same brakes

[DeadheadNM]

No problem rebuilding my HO52 - so glad I didn't swap it out. Post a WTB ad on the parts board for an Eaton locker and go...

[Bigdav160]

Question for the group. The service manual list those two RPO axles as SERIES 20-30 5200 AND 7200 LB capacity axles. Then they list a Series 30 11,000 lb axle.

The 11,000 lb axles has the pinion supported by ball bearings instead of tapered bearings.

Am I to presume the H072 is the 11,000 lb and the H052 is the lesser?

The service manual also shows threaded holes for removing the axles in the 5,200-7,200 and a slide hammer necessary for the 11,000 axle shafts.

My original axle is long gone but the door tag list the GVRW as 7500lbs. This Suburban would have had a H052 or H072?

[SS Tim]

Originally Posted by Bigdav160

Question for the group. The service manual list those two RPO axles as SERIES 20-30 5200 AND 7200 LB capacity axles. Then they list a Series 30 11,000 lb axle.

The 11,000 lb axles has the pinion supported by ball bearings instead of tapered bearings.

Am I to presume the H072 is the 11,000 lb and the H052 is the lesser?

The service manual also shows threaded holes for removing the axles in the 5,200-7,200 and a slide hammer necessary for the 11,000 axle shafts.

My original axle is long gone but the door tag list the GVRW as 7500lbs. This Suburban would have had a H052 or H072?

Thought we were talking about pickup trucks, but here goes.
H052 5200lb axles were the standard axle for the C20 and K20 series trucks with a GVW of up to 7500lb. The HO72 7200lb axles were standard for the C30 trucks with GVW up to 9000lb on pickups and 10000lb as dual wheel chassis cabs. An optional axle the HO110 11000lb was offered on C30 dual rear wheel trucks with a 14000lb GVW rating (15x4 brakes and ten stud wheels).

On your 72 K20 Suburban, it could have had a maximun GVW of 6000, 6400, 7200 or 7500lb as built the left column. All would have stated maximum capabilty as 7500 in the right column. That means with the correct added springs and tires as required, any vehicle could be brought up to the 7500lb limit. This is charted in your driver's handbook and the chassis service manual.

Now the rear axle load limit is the variable here as the front axle limit at all GVW ratings for your K20 are 3300lb. That means your rear axle, depending on options and equipment, would be rated at 3900, 4700 or 5000lb. Now this is the maximum design weight on the rear axle, at the ground. All are well within the limits of the standard HO52 it would have been built with in most cases. Remember the GVW rating of your vehicle is less than the combined axle rating limits.

Overall there are some Dana 60 (5500lb) applications I am still trying to work out but generally they were big block four speed series 20 full floating applications and series 10 semi-floating applications.

Added a chart for those that want to poke into it.
To keep things clear the HO52/72/110 are described as banjo style axles. The Chevy 12-bolt and the Dana 60 are salisbury style axles.

[SS Tim]

Originally Posted by special-K

Not all HO52s had the same brakes

Tim, Please expand on that if you will. Every CK20 aplication I have looked up seems to be the 11x2-3/4" brakes. Are you refering to a rarely seen vehicle like an earlier C20 DRW or something more prevalent?

[special-K]

'71 K/20s used a one year only rear brake. IIRC they were 2 x 13? That's a guess,but I could look to see if I have the exact info. I just recall they are narrower and taller. I had a hard time finding drums in the early '80s and had to use a national parts locator to find them. They knew of 9 existing in stock back then! Not sure about all the other applications,but know this on '71 K/20s through personal experience.

[roy c]

well I took a better look (A little hard when your grand kid is hanging from your neck) and the over all diameter of the drum is right around 13". Thanks for the wakeup call Tim. I was told that a rear disc brake set up from a 75 3/4 ton would work on this housing your thoughts

[SS Tim]

Glad to hear things are clearing up. As to rear disc brakes, well I gave a good reason (#5) not to run them due to the park brake issue. Some people don't care at all and most people don't load their truck to its rated limit. Depending on a parking pawl is not a good way to go either. However they are easy to work on and as I understand about 75lb lighter.
Properly set up stock rear drum brakes are fine most of the time though.

[Longhorn 70]

This may also help.

Rear end and brake information.