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1low37 06-23-2013 01:55 PM

Gear Tooth Pattern on Ford 9"
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Hi Guys, I have been trying to get a "good used" Ford Posi 9" diff set up correctly and cannot seem to get the drive and coast patterns in the middle of the ring gear. Specs, Reusing the bearings as they appear ok. Ring gear run-out is about .002-.003" Changed the yoke and used a Ratec supersleeve. Pinion bearing torque is 7 in-lbs, 3.25 gear Ratio, factory (or at least what came out of it was a .018 pinion carrier shim.
I have been up and down the shim stack spectrum-from . 000 to over .025 and nothing seems to get the pattern off the toe of the coast side; without really screwing up the drive side. I have posted pictures of the (factory .018 shim) pattern first, with .012" backlash. Then with a .015 pinion shim with .013 backlash, which seems to center the drive side up nicely but still does not get the coast side up the middle much. Its driving me crazy. I have tried so many shim and backlash settings I am cross-eyed :pain:. I just wanna go for a drive in the hotrod. I have seen a pattern in the Yukon Gear set-up article and it shows a pattern much like this one with the drive centered and coast biased towards the toe and they say its OK. Any suggestions?
Thanks 1low

big gear head 06-23-2013 04:54 PM

First of all you are trying to do the impossible. You can not get a text book pattern on used gears no matter how hard you try. These gears have wear on them and there is no way that the pattern is going to look like what you think it should. Most of the time with used gears I try to get the coast side to look good and don't worry about the drive side. This is because the drive side has much more wear on it than the coast side and you are more likely to get a good looking pattern on the coast side. In your case it seems that both the drive and coast have enough wear on them that you can't get a good pattern on either side. This is why I try to do a backlash and pattern check BEFORE I take anything apart so that I know what I need to be looking for when I put it back together. I think in this case I would use the original pinion shim and hope for the best.

cranky1 07-05-2013 07:11 AM

I like to paint the pinion and not the ring. It gives a more pronounced read than painting the ring with the checking grease does.

1low37 07-05-2013 07:40 AM

Thanks for the advise. I reassembled the unit with the shim that came with it and the .014 BL that I measured before starting this journey. I took a bright flashlight and peered around inside the posi unit and found some small chips on the spider gears. SO my fears that I got hosed in this transaction are most likely true. I put it in the truck and drove it around a bit; aside from what I think is an e-brake shoe on backing plate squeak it seems ok. But no freeway miles yet.
I am now hoping it stays together long enough to save up for a proper diff build. All I can hope for is it doesn't come apart catastrophically. Hopefully there will not be an eulogy on this one.

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