Originally Posted by 63GMCKid
Well if you know what the current tire size is and what RPM you were turning at a given speed, the only ratios offered in the Eaton HO52/72's were 4.11, 4.56, and 5.13, so you could plug those numbers into an online gear calculator and figure out what ratio you've got by process of elimination.
Years ago there was a company that made a 3.73 gear set for the Eaton HO52/72, can't remember the name of the company, but it was a short production run, I personally have never seen such a R&P set for sale, I learned about it from the Eaton HO72 thread on Pirate4x4, so I would assume they are very rare, but it might be worth a shot trying to track a set down.
The Eatons are excellent axles, easy to work on, extremely strong, fairly light weight when converted to disc brakes (Lighter than a 14bolt with discs) and still lighter than a 14bolt with drums if you keep the drum brakes. Only downside to them really is the pinion bearings are weird and extremely expensive, there is a taper roller pinion bearing with a dual outer race, so its like 2 bearings in 1, usually runs about $400-$450 for a new one, and there is a double row ball pinion bearing that usually runs about $300-$350 new, and the double row ball pinion bearings aren't very desirable for obvious reasons.
An easy way to tell which you have without pulling the 3rd.... if you are looking at the axle facing the pinion, looking at the 5 bolt flange that retains the pinion bearing/seal, at the lower drivers side edge of that flange, if there is a small hump that looks like it has a channel extending behind it on the 3rd, and a small hump that extends from the rib right above the flange down to the flange face, both of those things mean it has the taper roller pinion bearing (Those are the oiling and oil return channels). If it doesn't have both of those features, then it has the double row ball bearing for the pinion bearing.
In my opinion these axles are worth putting a bit of work into to keep them running strong in these old trucks. If your 3rd ends up being one with the double row ball bearing pinion bearing, I would say its worth it to go on the hunt for another 3rd with the taper roller pinion bearing, usually you can get a complete axle or a complete 3rd for $50 to $200 on CL.
Now all of this is pointless if you want to run something numerically lower than 4.11, in which case I'd say just pick up a used narrow 14 bolt dually from a cab&chassis truck.
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