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Street Race Truck More... The Toolbox

8 Inch to 9 Inch Upgrade


In this Tech Tip, John responds to a customer looking to upgrade the 8” stock differential in his Cougar to a 9” open diff.

8 Inch to 9 Inch Upgrade

“Here’s my dilemma..... I have a 1968 Mercury Cougar with an 8 inch stock differential. I found a 9 inch from a 1968 Cougar GT 390 car with open diff, and 28 spline axles (which should fit perfectly under car with minor  mods if any). I have a limited slip 31 spline carrier of which I’m going to replace 3.00 gears with hopefully your “RICHMOND” 3.55’S that you have for $165 new....but dilemma is finding exact fit 31 spline axles. 1967-1970 Mustang axles should be correct application for me. I’m not going to exceed 350 horses on motor, so I don’t need Currie, Strange or Moser unless I can get them at a great deal from you guys??? I’ll need to get info and pricing in advance (hopefully FREE SHIPPING on all) so I can buy a Visa card to load with “EXACT” amount so the REAL BOSS of the house (alias the wife) doesn’t see it on our credit card (hint, hint…). So, if you can help a guy building a car with champagne taste on a beer budget, I’d appreciate it immensely”

  • Stan

Hi Stan, You inquired about axles for your 9” housing and there are a couple of things to keep in mind. All housings do not take the same outer bearing and seal configuration.  If it is the new style with the big bearing ends then our “Cut-to-Fit” axles such as P/N 91045127 in short and long should solve your needs.  If it is an older style with the small bearings then we apologize may not be able to help.  You can check this out and then if correct order the axles and the gear set along with a new gasket and lube.

  • John W.

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