There are specific tools needed to set the depth / backlash on the ring / pinion gears. Spending a little extra money will pay dividends in the long run. Just ask any reputable Porsche shop if they simply "eyeball " these measurements.

 

Their is a huge difference between eyeballing and not using the Porsche tools. Not everyone who does not have the Porsche tools is eyeballing. I have been shopping for the specific tools to buy for the differential for 3 years without luck. That being said, knowing what you have to check and measure allows a talented transmission mech to work around not having the differential tools.

 

After measuring the stock differential flange to bearing race and comparing taking into account the new bearing thickness versus the old worn we determined that the inner bearing needed 3.8mm(2mm & 1.8mm) for shims. So, this week I got some of the shims I ordered. A little bit about the Porsche shims. The shims range from 1.5mm-3.8mm in .05mm increments. Once we got the inner bearing shimmed, i measured the height from the flange to the bearing race. So then I pressed in the inner bearing race. Then installed the outer test bearing with 1.8mm shim. The installed the outer bearing race into the side cover. The new stub shaft seals will be left out until the final bearings are installed. Then i installed the stub axle and a magnetic dial indicator. Then index the dial indicator on a flat surface and measure the outer bearing gap by lifting carefully on the stub axle. I checked this measurement in several different locations and a bunch of times to and duplicate a measurement of .025" which is .66mm. So i need 2.5mm of shims plus what will be needed for preload. Most people will take that measurement and add .4mm. So by my measurements i will need 2.9mm which does not exist in any combination of the Porsche shims.

The bearing preload is not nearly as critical as gear lash. And with the pinion depth not being messed with we are not changing gear lash. We are checking gear lash as an indicator of ring gear depth. This is to verify that the measurements we took from the original diff is setting the correct ring depth instead of blindly trusting how the factory shimmed the original diff. Because our measurements do not account for wear. And we did see a .002" difference in depth between the new bearing and the old worn ones. We can assume this is due to wear but can not be certain. So we will double check with die grease and measuring gear lash. As mentioned before, the measurement point for gear lash is over the ring gear bolts. So once I have the outer shims sorted I will bolt an arm to the flange that will allow me to measure over the bolts with the side cover installed. I will take measurements with the same magnetic base dial indicator. Once shim measurements and ring depth are determined the final bearing will be installed.

A note about test bearings. We bought two of each bearing and took one and honed the inside to slip fit. If you do this measure both bearings and note the difference if any. Ours we exactly the same. And when using the test bearing, do not press in the outer race off the test bearing. Press in the actual bearing race. As long as both bearings are the same dimensions the test bearing will not change measurements when used with the actual bearing once that is pressed on with the final shim stack. But you can measure the test bearing with the actual outer race to be sure they measure out the same.

 

So yesterday I received the final shims needed for preload on the LSD. So once I shimmed the LSD for the ring gear depth the same as the stock diff I installed the outer test bearing with a 1.8mm shim. I bolted the side cover on and with a dial indicator i was able to determine that i actually needed a 2.4mm shim to take up the space between the outer bearing and the cover. So with the shim for the space of the case you add .40mm for preload so 2.8mm total.

Once the differential is installed with the correct preload you can check your gearlash.

Warning******
Do not try and check gear lash without bearing preload. Your readings will be off.

My ring gear called for a lash of .14mm. You will find this etched on the ring gear. Per the manuals, the tolerance is .05mm under . So this equates to a range of .00354... -.0055(thousands of inches). My gear lash measured at .004. You never want to be over the gear lash. Only under per the manuals. And make sure you are measuring the lash without a shaft seal on the transmission.

So now I will press on the actual bearings and shims for final install. Also make sure to replace shaft seals and side cover o-ring. I will post more pictures later.

 

So here is a picture of the measuring fixture i made for measuring gear lash. The line corresponds to the measurement location Porsche calls for which is also the outer circumference of the ring gear. The porsche/VW tool is a collet that locks inside the inner part of the bearing flange. For a helical or open diff this is the only way to measure because using the flange will also measure the helical gear lash as well. The tool shown below will only work for a static locking diff because the differential gears are locked with that design. When you use this axle flange type setup you must install and tighten stub axle bolt so that the axle does not move in the diff. If not you will also measure the lash of the stub axle splines which you don't want.

After installing the final shims and bearings I installed the shaft seals, side cover o-ring and stub axles. Transmission is ready for mounting to the engine.