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Axle conversion and Posi diff

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Waylon109

Axle conversion and Posi diff

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?

1981 Ford F-100 Explorer 2WD in Dark Caramel Metallic, Single Cab Longbed, Clark 4spd OD, 4.9L Straight Six, 2.47 gears

85pig

Re: Axle conversion and Posi diff

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.

1985 F150 4X4 300 I6 4-Speed
1970 Torino Cobra "Twister Special" 429CJ 4-Speed
1965 Mercury Comet Caliente Convertible 331 5-Speed

Gary Lewis

Re: Axle conversion and Posi diff

Administrator

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.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Waylon109

Re: Axle conversion and Posi diff

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?

1981 Ford F-100 Explorer 2WD in Dark Caramel Metallic, Single Cab Longbed, Clark 4spd OD, 4.9L Straight Six, 2.47 gears

Gary Lewis

Re: Axle conversion and Posi diff

Administrator

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.

Gary, AKA "Gary fellow": Profile

85lebaront2

Re: Axle conversion and Posi diff

Administrator

In reply to this post by Waylon109

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.

Bill AKA "LOBO" Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 2009 Flex Limited with factory tow package Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

85pig

Re: Axle conversion and Posi diff

In reply to this post by Gary Lewis

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.

1985 F150 4X4 300 I6 4-Speed
1970 Torino Cobra "Twister Special" 429CJ 4-Speed
1965 Mercury Comet Caliente Convertible 331 5-Speed

mat in tn

Re: Axle conversion and Posi diff

my 82 f100 with the small pattern has the same 31 spline axles. the wheel end is a small pattern.

ArdWrknTrk

Re: Axle conversion and Posi diff

Administrator

In reply to this post by Waylon109

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?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.