The Stovebolt Site Forums The Shop Area Driveline Limited slip rear - 55T/ HO33

Did GM make a posi-carrier version of the 55T/HO33…..55-63 truck read end?

I know there were posi version of the CAR rears that were very similar….

Nathan - According to what I could find through my resources, 1959 was the first year where a 1/2 ton Positraction option was listed in dealer documents. This one originally dated September 1958 for 1959 Chevrolet Pickups on the last page (screenshot below) shows that listing. No idea if it would fit earlier 1955.2-58 rear axle housings???

I picked up a posi 3rd member years ago, ended up trading it away after I swapped out a dana 44 power loc axle from a '60 GMC.

Are there any externally-visible differences with the Posi third member?

If it's out of the housing, yes
Still in the housing should be able to turn 1 axle & opposite side should turn the same direction & pinion will rotate.

Sorry….i meant visually with it installed in the axle housing. Or perhaps a casting number or something. I think the car Posi units have some letter that denotes it? Been a while since I’ve seen one.

Nathan - I was able to do a little more quick research on your question. The below screenshots are taken from pages 56 and 57 of this 1959 Chevrolet Truck Engineering Features PDF that provides a fairly detailed description of how you can externally/internally ID the Positraction rear axles beginning in 1959.

I did further searches through the PDF's in the GM Heritage site and found that for C10 Model 1/2 tons from 1960-62, the 2 letter prefix on the rear axle serial number sequences were either MC or MF. Both of those would be followed by a 2 numeric digit number indicating the day of the month built followed by 1 or 2 more numeric characters for the month of the year built (see 3rd pic showing 1960 info). The 1959 data file is missing the serial number listing info so unable to confirm those details. Good luck in your search.