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Jul 21st, 2025 | | | | Joined: Aug 2011 Posts: 51 Wrench Fetcher | | Wrench Fetcher Joined: Aug 2011 Posts: 51 | On the rear axel, stamped into the housing of the drivers rear siade is the number
WA 415 and it has the 12 bolts holding the cover in place.
The guy I got the truck from said it had 3 73 posi trac gears.
How can I verify he is correct?
Thanks in advance
oh ans IF we decide to swap the engine and trans to a 350 small block with a 700R4 trans, do I need to change out the rear end? | | | | | Joined: May 2006 Posts: 8,351 'Bolter | | 'Bolter Joined: May 2006 Posts: 8,351 | The WA code does come up as a 3.73 ratio. Only way to know if it's really a Posi is going to be to take the cover off and look. usually, a factory Posi will have a tag on one of the cover bolts stating "Use Limited Slip Fluid Only", though it's not unusual for those to be missing. Your diff is the early 12 bolt, so it will have the 17 spline axle shafts. Thsi could have a bearing on rebuild parts for the Posi, if it has one. It will also effect things if you choose to install an aftermarket limited slip/locking differential, as thay are only made in the later 30 spline. This sin't too big of a problem, you'll simply need to go to a 30 spline axle shaft if you go that direction.
That rear will work just fine with a 350/TH700R4.
Bill Burmeister
| | | | | Joined: Jul 2008 Posts: 1,262 Shop Shark | | Shop Shark Joined: Jul 2008 Posts: 1,262 | Counting the teeth is the most accurate but another way to check the rear end ratio is to jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading. As for checking if it has a posi.... a big smokey burnout will tell you if one or two wheels are spinnin'  Actually, if you jack both tires of the ground and spin one tire by hand the other tire will spin backwards on a non-posi, the same direction for a posi.
Last edited by Hot Rod John; 09/18/2011 12:49 PM.
| | | | | Joined: Aug 2011 Posts: 51 Wrench Fetcher | | Wrench Fetcher Joined: Aug 2011 Posts: 51 | Well it is not a posi trac rear end.
Thanks again for the information. | | |
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