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Old 07-22-2021, 09:45 PM   #251
Short 3/4
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Join Date: Jan 2017
Location: Los Osos Ca
Posts: 417
Re: Eaton HO72/HO52 vs. 14 Bolt FF

Quote:
Originally Posted by Dave View Post
I suspect that you may have a gear set that was made by aftermarket company some time ago to fit into the Eaton 3rds.
Or it could be a GM Canada thing, whereby there were definitely differences in production and equipment.
Having a GM part numbers would clear up any confusion on being original or aftermarket.
Thanks
Read in this forum somewhere (maybe even early in this thread) that the 3.90's were made for the NAPCO 1/2 ton conversions. If you have ever seen one, they are really strange looking as they retain the small 1/2 ton rear, with stock 3.90 gears, but use a modified Eaton for the front, so you have this rear that looks like it came out of a car, then a massive HO52 up front with 6 lug wheels. The 3/4 ton conversions look a lot more normal as the front and rears are both matching Eatons (with the usual 4.56 gears)
So that really makes sense why the 3.90 exists, and is so rare. I don't think anybody has ever found evidence of Chevy offering it as a RPO (I of course could be wrong on that point). NAPCO must have had them custom made for their conversions, and then probably they or the manufacturer had some others they sold as parts for the aftermarket.
This is the best theory on the subject I have come across, and it makes sense to me. What do you all think? Love to hear other opinions on this.
Tom
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Old 07-31-2021, 09:58 PM   #252
Captainfab
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Join Date: Mar 2007
Location: Coeur d'Alene, Idaho
Posts: 23,130
Re: Eaton HO72/HO52 vs. 14 Bolt FF

The 1/2 ton NAPCO conversions used the Eaton HO32/33 both front and rear. That is what came stock in the '55 second design thru 1962 in the 1/2 tons They look just like the HO52/72, just smaller. The 3.90 gears were the most common gear found in the HO32/33's.

Quote:
Originally Posted by Short 3/4 View Post
Read in this forum somewhere (maybe even early in this thread) that the 3.90's were made for the NAPCO 1/2 ton conversions. If you have ever seen one, they are really strange looking as they retain the small 1/2 ton rear, with stock 3.90 gears, but use a modified Eaton for the front, so you have this rear that looks like it came out of a car, then a massive HO52 up front with 6 lug wheels. The 3/4 ton conversions look a lot more normal as the front and rears are both matching Eatons (with the usual 4.56 gears)
So that really makes sense why the 3.90 exists, and is so rare. I don't think anybody has ever found evidence of Chevy offering it as a RPO (I of course could be wrong on that point). NAPCO must have had them custom made for their conversions, and then probably they or the manufacturer had some others they sold as parts for the aftermarket.
This is the best theory on the subject I have come across, and it makes sense to me. What do you all think? Love to hear other opinions on this.
Tom
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