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The Ford 9-inch rear end and the Ford 9-3/8" rear ends are very similar but only a few parts between 3rd members will interchange. Ring & pinion gears isn't one of those things that will interchange though. If the ring or pinion in that 9-3/8" rear end ever wears out or, a tooth gets chipped or broken off, there are no replacements for them --unless you could find some NOS gears or, a decent set of used gears. Aftermarket support for the 9-inch rear end is vast but, there's no support for the 9-3/8".
A 9-inch 3rd member will fit in a 9-3/8" rear end housing but a 9-3/8" 3rd member won't fit in a 9-inch rear end housing --at least not without notching the housing flange where the 3rd member bolts up.
Clearance notches (for the ring gear) shown on a 9-3/8" housing.
No clearance notches on the 3rd member-to-housing flange on this 9-inch rear end housing under my truck.
....I removed the 28-spline axles from the 9-inch rear end in my '69 F100 and replaced them with a pair of 3300# 31-spline axles from a '71 F100 that had a 9-3/8" rear end. I did this so I could remove my 3.50 geared, 28-spline, open differential, single vertical rib, 9-inch 3rd member and replaced it with a 31-spline, Traction-Lok, 3.50 geared, 9-inch, Daytona, N-case 3rd member.
My original '69 C7AW-E 9-inch 3rd member.
....replaced by this '75 D0OZ-B 9-inch Daytona N-case 31-spline Traction-Lok 3rd member.
The Ford 9-inch rear end and the Ford 9-3/8" rear ends are very similar but only a few parts between 3rd members will interchange. Ring & pinion gears isn't one of those things that will interchange though. If the ring or pinion in that 9-3/8" rear end ever wears out or, a tooth gets chipped or broken off, there are no replacements for them --unless you could find some NOS gears or, a decent set of used gears. Aftermarket support for the 9-inch rear end is vast but, there's no support for the 9-3/8".
A 9-inch 3rd member will fit in a 9-3/8" rear end housing but a 9-3/8" 3rd member won't fit in a 9-inch rear end housing --at least not without notching the housing flange where the 3rd member bolts up.
Clearance notches (for the ring gear) shown on a 9-3/8" housing.
No clearance notches on the 3rd member-to-housing flange on this 9-inch rear end housing under my truck.
....I removed the 28-spline axles from the 9-inch rear end in my '69 F100 and replaced them with a pair of 3300# 31-spline axles from a '71 F100 that had a 9-3/8" rear end. I did this so I could remove my 3.50 geared, 28-spline, open differential, single vertical rib, 9-inch 3rd member and replaced it with a 31-spline, Traction-Lok, 3.50 geared, 9-inch, Daytona, N-case 3rd member.
My original '69 C7AW-E 9-inch 3rd member.
....replaced by this '75 D0OZ-B 9-inch Daytona N-case 31-spline Traction-Lok 3rd member.
Ok, so I am guessing you swapped it out because you wanted better highway cruising speeds? That's the thought I have tossing around. The RPM's are way too high around 70-75 mph
Ok, so I am guessing you swapped it out because you wanted better highway cruising speeds? That's the thought I have tossing around. The RPM's are way too high around 70-75 mph
The ratio (3.50:1) is the same between my original 28-spline 9-inch 3rd member and the 31-spline N-case 9-inch 3rd member I replaced it with so, no change there.
I did it because the N-case had 31-splines and a Traction-Lok differential and it's an N-case --the ultimate in strength for a 9-inch case. I only gave $75.00 for the N-case 3rd member and, I've had it sitting around for a number of years. It was cheaper for me to find some 31-spline axles from a '68-'72 F100 with a 9-3/8" rear end than it would have been to buy a 28-spline Traction-Lok differential, swap my ring gear over onto the differential and then have to set the gears back up.
I don't want to lose the rear end gearing. To remedy the problem of higher RPMs on the highway, I'll eventually remove the inline 240 and C-4 automatic and install an overdrive transmission --behind a fuel injected Ford small-block V-8.
The ratio (3.50:1) is the same between my original 28-spline 9-inch 3rd member and the 31-spline N-case 9-inch 3rd member I replaced it with so, no change there.
I did it because the N-case had 31-splines and a Traction-Lok differential and it's an N-case --the ultimate in strength for a 9-inch case. I only gave $75.00 for the N-case 3rd member and, I've had it sitting around for a number of years. It was cheaper for me to find some 31-spline axles from a '68-'72 F100 with a 9-3/8" rear end than it would have been to buy a 28-spline Traction-Lok differential, swap my ring gear over onto the differential and then have to set the gears back up.
I don't want to lose the rear end gearing. To remedy the problem of higher RPMs on the highway, I'll eventually remove the inline 240 and C-4 automatic and install an overdrive transmission --behind a fuel injected Ford small-block V-8.
I've been tossing around the idea of taller tires, less expensive.
