Waylon109 Posted March 21 Share Posted March 21 Hello everyone, I have an ‘81 F100 with the 300 I6, Clark 4sp trans, and a 2.47 open diff with the 5x4.5 bolt pattern and 28 spline axles. I want to upgrade to 3.00 or 3.25 gears and a posi third member, but that takes 31 spline axles and I can’t seem to find any that fit that bolt pattern for the truck. Can anyone point me in the right direction? Link to comment Share on other sites More sharing options...
85pig Posted March 21 Share Posted March 21 Sounds like the 81 is still a 9" diff, there should be some "car" center sections floating around that ran limited slip, 28 spline diffs/axles (the car bolt pattern was also 5x4.5). I've got a 2.70 geared, 28 spline LS in my shop slated for 3.50 gears for my son's 71 Torino. Not 100% sure, but I think you can have a 31 spline third member swapped to 28 by changing the side gears. Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 21 Share Posted March 21 Sounds like the 81 is still a 9" diff, there should be some "car" center sections floating around that ran limited slip, 28 spline diffs/axles (the car bolt pattern was also 5x4.5). I've got a 2.70 geared, 28 spline LS in my shop slated for 3.50 gears for my son's 71 Torino. Not 100% sure, but I think you can have a 31 spline third member swapped to 28 by changing the side gears. Right. That's pretty much what I was going to say. But I'll add that I would seriously consider staying with the bolt pattern you have. Otherwise you'll either need two spares or have to change the front to the larger pattern as well. Link to comment Share on other sites More sharing options...
Waylon109 Posted March 21 Author Share Posted March 21 Right. That's pretty much what I was going to say. But I'll add that I would seriously consider staying with the bolt pattern you have. Otherwise you'll either need two spares or have to change the front to the larger pattern as well. Do they not make 31 spline axles for the 5x4.5 bolt pattern? Also, I’m not sure about what length of axle to look for. I assume that axles from a car version of the 9” would be a direct replacement? Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 21 Share Posted March 21 Do they not make 31 spline axles for the 5x4.5 bolt pattern? Also, I’m not sure about what length of axle to look for. I assume that axles from a car version of the 9” would be a direct replacement? I don't know about 31-spline axles with the smaller pattern. But the differential won't care what length axle is inserted. As long as the side gears where the axles go in have the right spline count for the axle that differential should go in any 9" case. I think. Link to comment Share on other sites More sharing options...
85lebaront2 Posted March 21 Share Posted March 21 Do they not make 31 spline axles for the 5x4.5 bolt pattern? Also, I’m not sure about what length of axle to look for. I assume that axles from a car version of the 9” would be a direct replacement? If that is the car bolt circle, yes they did, but finding a pair for a truck might be a problem. My 1966 Shelby had a 9" with 3.89 gears and 31 spline axles. Link to comment Share on other sites More sharing options...
85pig Posted March 21 Share Posted March 21 I don't know about 31-spline axles with the smaller pattern. But the differential won't care what length axle is inserted. As long as the side gears where the axles go in have the right spline count for the axle that differential should go in any 9" case. I think. 31 spline axles with the smaller bolt circle would be somewhat common in the performance Ford cars - Mustangs, Torinos, Cougars, etc. One could probably measure the length of your current axles, and see if you can find a comparable length axle from a car to keep the 5x4.5 bolt circle. Link to comment Share on other sites More sharing options...
mat in tn Posted March 22 Share Posted March 22 31 spline axles with the smaller bolt circle would be somewhat common in the performance Ford cars - Mustangs, Torinos, Cougars, etc. One could probably measure the length of your current axles, and see if you can find a comparable length axle from a car to keep the 5x4.5 bolt circle. my 82 f100 with the small pattern has the same 31 spline axles. the wheel end is a small pattern. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted March 23 Share Posted March 23 Are you looking to swap the whole 9" axle, or just the pig's head? There are dozens of places that build these things, from so cheap I wouldn't trust them to Strange or Currie axles that hold 1,000 HP. Your title says "conversion" so I have to assume you're looking for a center section that will take your 31 spline axles? Link to comment Share on other sites More sharing options...
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