Still doing lots of research on exactly what I'll have the budget to do, but have a few q's to ask in the meantime.

Acquired axle is from a '72 F100, 61.25" WMS-WMS, big bearing, 31-spline (I'm guessing, but don't fully care), 3.25 ratio stock (care even less about that).

Current plan is to go with a 35 or 40 spline spool (at least until I can even consider affording an ARB) and axles to match, with 4.86 gears.

Q's:
I've seen hints in certain places that I'll have to get a different 3rd member in order to run the larger axles (something in regards to "case bore"). Other places say I can use the stock one. What's the real deal here?

Am I correct in believing the stock 3rd member will be adequate for any gear ratio I choose?


Thanks in advance. Any other hints and tips you can add will be helpful, this being my first 9" and all.

To do it right and to have the strongest possible I would get a 3rd member with the 3.25 bearings. That why you can go either way you want. I've heard you can use the stock 3rd for 35 spline but nothing i would ever think about.

Read some of the tech at the Mark William's web site.
You can use 35 spline axles in a MW spool and a factory case. 35 and 40 spline axles use a 45* pressure angle, thus a Dana 35 spline and a MW 35 spline are not compatable.

I understand wanting bullet proof, but wouldnt 31 spline be plenty strong.... just asking

Originally posted by 5150xj
I understand wanting bullet proof, but wouldnt 31 spline be plenty strong.... just asking

Might have to be for the time being, considering the price of the 3.250 bore case or MW's spools and axles.

Was considering trying my hand at competition with the new buildup, but I suppose it'll have to come piecemeal right now... 31sp for the time being, probably just weld the spiders or minispool it, then upgrade to a new case and 35+sp Detroit or ARB as the money allows. At least that's easier to do with the 9".

In the rear,I've been running 35 spline axles with a spool in a 3.06 stock case for two years with zero probs.I switched from 31 spline after I broke a shaft in the DRIVEWAY fuckin around.

I used to run a 31 spline detroit in a 3.06 case in the front end and blew the carrier bearing cap in two doing some full throttle mud antics.Have since switched to a Strange case with 3.25 bearing,35 spline Detroit and shafts no more Diff. probs.