The Stovebolt Site Forums The Shop Area Driveline 3:73 for Eaton HO72 rear

hey there Ken,
I'm new to this forum and the thread regarding the gearset for an Eaton axle was what I was looking for... but after I read through all of the postings it didn't seem you found what you were after... so , since then, have you found anywhere to get a set of 3.73s for an HO52 (or 72) ?
I'm running a 1972 C20 and would really like to find some longer gears for it...
Please let me know if you did.
Cheers,
Dan

I looked at my 3.54 Rear again. It is actually a Dana 3.54 which came out of a 1968 3/4 ton with a Big Block. engine. I think 4.10 was the fastest ratio ever built for an Eaton housing. The 4.10 are scarce and the 3.54 are even scarcer. Some years ago I read about someone trying to develop a 3.73 gearset for the HO series, but I'm not sure if any of them were ever built. I have a 55.2 GMC 3/4 ton that has 4.57 rears. My plans are to replace both axles with 1/2 ton which was 3.90. There was a 3.08 Dana behind automatics. It is a little too fast for a 3 speed unless you have a good sized motor in front of it.

I may have hit the mother-lode? Just about to swap out the HO 72 in my '48 3800 for a 14 bolt. I found a '70 2x4, 3/4 ton, small block, 4 spd. with coil springs. The truck had no hitch or whiskey bruises in the bed so I think it lived a good life. About a thousand forums ago I remember someone mention that the coil spring versions are 4:10 or even higher? It's going to take a day with a good saw clearing trees and brush just to get the truck in a position to lift it so I don't want to do this for practice.
If someone could please confirm my suspicions on the gear-set I would really appreciate it.

Pick1, you may want to read this Tech Tip: HO52/72 gearing upgrade

Per the beginning of this thread, it is my understanding that 3.73 was not an available gear set for the HO52/72 axle. The truck you are going after is likely to have a 4.10 gear set.

My only concern is that the potential donor is a 4 spd? If it has a 4:10 I'd be tickled pink, way better than the 5:14's I have now. Thanks for the help!

It seems to me that one would be better off looking for a Dana/Spicer axle as came in GMCs of the 60s and on. Or maybe a corporate 14-bolt. For one thing you are much more likely to find highway gearing. And for another, have you or anyone else on this thread gone looking for a pinion bearing for an Eaton HO52/72? The last price I saw was in the high hundreds of dollars and only one source. So I quit looking and got a boneyard HO52 axle out of a 67, 3/4 ton. 4.57 ratio. Suited my application just fine.

If you run it long enough it will need a pinion bearing and by then it will likely be unavailable. At 76, I don't have that worry but some younger guys might want to think about parts availability.

Good point, Tommy. But speaking of boneyard HO72's .... Do I assume correctly that its the same bearing regardless of the gear ratio?

So ... Grabbing a used HO72 (or 52 for that matter) just for a spare pinion bearing might not be a bad idea ... ?? Better a used one with some life left in it than none at all ...

Not a sermon, just a thought ...

So far as I know the 52 and 72 punkins are the same internally, although the '51, 5.13 had an adjustable block that rubbed on the ring gear to limit lateral travel under heavy load. The later '67, 52 with 4.57 didn't have that.
I have a punkin with what I believe is a good pinion bearing but the gearset has just too many miles on it and is noisy and has acid-etched pits on the gear faces. Someday maybe I'll run across a good 5.13 gear set and can reuse the 5.13. There are circumstances where it would be helpful.