Olds Posi Traction

[marcar1993]

The ratio is a the division of the ring gear into the pinion. 41 tooth ring gear, 10 tooth pinion: 41/10=4.1 or 4.10 gears.
There are set ratios available for each differential set up, 7.5" 10 bolt, 9" ford, dana 60, because the ring gear size and pinion size sets the number of teeth each gear needs to have to make them mesh properly.
In a dana 60, you can get 3.55 gears, but in a 10 bolt, its 3.42 or 3.73. All comes down to how the teeth lay out over the span of the ring gear.

 

[olds307 and 403]

I can get a 9" housing done on a jig for a G body within driving distance for $600 Canadian. Seems like a no brainer vs what people ask for 8.5" G body rear ends around here. Kind of like building a TH350 vs a TH400. What I like about the Type O in my 70 S is decently strong and has bolt in axles. Just gears are limited to 3.42 and 3.90 gears plus just one modified 8.8 Ford clutch style posi available. I am looking at $1000 in parts to get the 3.90 posi I want with new bearings.

 

[Pharozen]

The 9 inch is just as strong in stock form as the 8.5. I mean in stock form, they both would have 28 spline axles. The Ford does have bolt in axles, that is the bearings are pressed on and the axles is held into the rearend by a flange that secures the bearings. Where as the 8.5 has C clips. In the event of an axle failure, the axle and wheel could come free from the 8.5. If you notice, the 8.8 Ford is in the GM or Salisbury/Saginaw style rearend with C clips and the more efficient gear design.

Ford has its 'own' numbering or marketing....3.70 versus the GM 3.73. Or its 4.10 versus 4.11

Ford 9 inch has the ease of swapping gears by having an additional center section. The weak part is the front pinion support and actual design of the gears, the GM style pulls the gears together and the Ford tries to push the pinion forward or apart. Technically the GM style is more efficient, or takes less power to turn or drive. The hot rod or aftermarket industry adopted the 9 inch, and everything for them is still in production. Now, that being said, new 12 bolts, 8.5 10 bolts and even the Dana 60 are now in production for cars like ours, but, they are pricey.

This has been very informative and helpful.. Thanks a lot.

 

[Mike buttafuoco]

This has been very informative and helpful.. Thanks a lot.

Also wanted to mention that the ford 9" housing can be "beefed up" with a brace fairly easily and is a steel housing whereas GM has steel tubes with an iron mid section that is plug welded to the tubes. Any diff can be built the hill with your budget being the deciding factor but for my money, ford 9" is the way to go. You can always perimeter weld the tunes to the center section on a GM (I've done it) but it takes a little knowledge of metallurgy and you need heat! Lots of it. I have built both and my weakest links have always been the trannys.