dimented24x7

iTrader: (2)

So I've been messing with another 9-bolt, and trying to get a good contact pattern with a new used gear set. Seen lots of pictures of what "good" should be, but no real explanations of why one is "good", but another is "bad".

What I currently have with the existing pinion shim is a pattern that is centered, but very sharply defined about 1/3 up from the root of the ring gear tooth, like whats shown below. This matches how the grenaded gears looked when I got this housing. Top 2/3 of the pinion was completely ground off and the tips of the teeth were badly chipped. Looks like the PO thought centered was better, but obviously here the sharp pattern edge is the top edge of the pinion teeth riding on the flanks of the ring gear.

Decreasing the pinion shim reduces the sharp edge and gets a more rounded pattern, but it diverges away away towards the heel and toe in opposite directions once it starts to center up. I assume a rounded pattern centered on the flank is what is preferred, regardless of position relative to heel/tow?

It seems to be when I think of the teeth as hyperbolic surfaces. Moving the pinion in and out with shims controls how high up or low down on the tooth the contact is. Once the proper backlash is then set, the actual location relative to heel/toe is a function of housing tolerances and wear, which I have no control over. So I assume I should try to center the pattern, and not be too concerned unless its WAY off to the sides towards the heel or tow, correct?

big gear head

Correct. It is more important to get the pattern centered between the root and face than it is to center it between the heel and toe.


The picture that you posted shows the pattern making heavy contact at the face of the tooth and very little at the root. This means that the pinion needs to move farther in toward the ring gear center line.


Just remember that a used gear will not give a good pattern because of wear. This is where you have to make good guesses and hope for the best. The drive side on a used gear almost never gives a good pattern and is usually heave at the root. The coast side will usually give you a pattern that is good enough to go by, but it's still taking a chance.

dimented24x7

iTrader: (2)

I played with it and this seemed to be about the best I could get out of it. Less or more and it diverges towards the extremes. I started with no shim and worked my way in. If I start adding more shim to move the pinion in, the pattern becomes more shallow and towards the tip of the teeth. Comments?

Since this is a 9-bolt, new gear sets are harder to come by. Doesn't seem too worn, but they do have some miles.

big gear head

Coast side looks pretty good for used. Drive side isn't what I usually see in used gears, but is probably acceptable. Most of the time I see a sharp line at the root of the tooth from heel to toe on the drive side.

dimented24x7

iTrader: (2)

Thx for the feedback. If I add a little more shim I see just that, a sharp line across the drive side. Im about 2 mils below the stock shim the housing came with.

big gear head

Let's see what that looks like.

dimented24x7

iTrader: (2)

Ok, heres what it looks like with about .003" more shim. Has a much more distinct pattern on the drive side, like you said. I assume this is probably about as good as it will get?

big gear head

Yea, probably as good as it gets. Reading used gears is usually a guessing game. It's always best if you do a pattern check before disassembling the rear end so that you know what to look for when you put it back together. Most of the time you don't get the chance to do that.