Can't figure out the backlash/pre-load check on my differential

[Hindsight]

I'm following the FSM and am a little confused.

With the transmission on the table with the left side down (per the manual):

Why does tightening the right-side adjuster change the back-lash? The height of the ring gear in relation to the pinion is set with the left side adjuster and changing the left side adjuster should change the backlash correct?. Tightening the right side adjuster should only increase the pre-load on the bearing no?

When it comes to checking and adjusting the tooth contact pattern, the FSM says that all problems are solved by increasing or reducing the thickness of the pinion height adjustment washer. Is this really true? Does the height adjustment of the ring gear make no difference in contact pattern?! If so, then there is really no reason to even bother checking the contact pattern if you aren't replacing or changing the pinion gear no?

Thanks

[Hindsight]

Many more hours of research later and I have an answer to my first question. The FSM is terribly written. I checked the procedure for the 6 speed and while it follows the same general idea, it is more complete and accurate. You dont change the preload by just tightening the right side adjuster; you have to move both left and right opposite but equal amounts. This allows the ring gear to move up and down which changes the backlash. Once that is sorted, you turn the right side adjuster one tooth tighter which just sets the bearing preload.

[Jaime]

Yup. Side-to-side determines backlash, so you can't move the ring if the contact pattern is wrong. You need to lift or drop the pinion, then reset the backlash to change the pattern.

[Hindsight]

Thanks Jamie, and according to further reading, if the backlash is correct and the contact pattern between heel and toe is not correct, then there is nothing you can do to change it all including adjusting the pinion depth because all the pinion depth changes is the contact between the face and flank.

[Jaime]

Rings and pinions are sold as matched pairs. If you get the pinion height and backlash correct, the tooth engagement will be correct. If it's not, then the pinion isn't at the proper angle to the ring - which would mean your transmission case is toast.

Pinion height affects several aspects of the pattern because of the wedge shape of the teeth. If the pinion is too high, then there will be too much space between the teeth, which will cause you to move the ring too close to take up the slack when setting backlash.

[STiPWRD]

Hindsight, did you have to buy a pack of shims for installing your LSD? I'm trying to get a list of things needed like the dial indicator. Also, did you reuse your old bearings? I couldn't find the part number for those. Thanks

[Hindsight]

No, no shims. The shims are for the pinion shaft only and if you aren't changing it or the case, there is no need to change the shims.

The diff height (location) is adjusted not by shims, but by turning adjustable carriers (which carry the differential bearings on each side).

You need a dial indicator, but not the push-in type; you need the lever type. McMaster has a budget version accurate to .0001. Also a magnetic base for it. Also a piece of steel stock you can drill a hole in so you can bolt it to the case and then attach your dial indicator mount to.

I did not re-use the old bearings. Part number can be found on parts.subaru.com under the manual transmission section - front differential. When I get home tonight, I'll get my receipt and post the part number.

I'm going to post a full DIY on this sometime later in the week that should explain it much better than the dumb FSM which caused me to waste an entire day and feel defeated lol.

You also need gear marking compound. Torque wrench. Vice grips. A 10lb weight. Shop press (can maybe get away without this). Anyway, more to come in the writeup.

[STiPWRD]

Cool, thanks!

[Hindsight]

806340080 is the part number for the taper roller bearings on the diff. Need two of them, of course. This is for an 07 WRX 5MT. Not sure about other years or models. They aren't listed in the transmission group, they are listed in the Driveline and Differential group.

I should have the DIY guide done by the end of the week. Need to take some more pics and wait for some new gear marking compound to arrive because the cheap crap I got off Amazon is nearly impossible to read. I really shouldn't care since I didn't change the pinion and I know how to check the backlash with a gauge but I guess my OCD is getting the better of me on this.