How to set up the Differential in a Porsche 915 Transmission

porsche 915 transmission open differential

Now that you have finished rebuilding both the input and pinion shaft, it is time to tackle the differential. The differential gets the power from the engine to the ring gear and drive shafts to the rear wheels. There are a lot of different measurements to take to set up the differential.

Setting up the 915 Differential

In setting up your 915 transmission differential we will cover the following measurements

  • Bearing Preload
  • Bearing Drag
  • Pinion Depth
  • Pinion Backlash

Taking the differential apart

Start by pulling the side bearings off the differential. Locate your puller correctly, there are cutouts for the puller arms. We use a spacer and an impact gun to pop off the bearing. The speedometer plate will slide off and the shims on this side will be underneath. Overall the shims will add up to 6.25mm. But where the shims are located is important to set up the differential, so keep things in order as you take the diff apart.

De-grease and clean the S-2 spacers, the speedometer side is S-2, the ring gear side is S-1. Measure the S-2 shims. Take apart the other side of the differential in the same way. Clean and measure the shims to get your total shim pack measurement.

Bearing Pre-load

Clean all of the parts and return your shims to their original positions. Look at the shims and place them in the correct position, using the cutout and relief to set them. We are putting on new bearings and we just tap them in place with a punch. The bearing race will have to be added to the case to match the new side bearings. Tap in the new races into the transmission case, making sure to seat the race fully and evenly.

Now you can put the matching race into the side cover. Because we changed the side bearing we need to confirm the bearing preload. Place the differential into the case and put the side cover on with no seal and only two nuts. Then using a feeler gauge you can take a reading on the side cover.

The optimum number we are looking for is 0.3 to 0.4 of an mm. This case comes in around 0.325mm which is good to do for pre-load.

Feeler gauge in a differential cover
Using a feeler Gauge for the differential cover to measure bearing pre-load

Bearing Drag or Running Torque

Next, we want to measure running drag on the side bearing. To do this we need to add more nuts and washers to the side cover and nip them down and torque them to 25-newton meters. Remember, no O-ring is required at this point.

Work the bearing around, back and forth a few times to seat it. Depending on the brand of bearing you are using, we are looking for different drag numbers.

  • SKS – 300 to 420-newton centimeters
  • FAG – 350 to 650-newton centimeters

We use a running torque meter to get a reading on the drag.

Measuring differential bearing running torque
Measuring differential bearing running torque

Measuring the Pinion Depth

This is the part of the differential setup that takes the special tool you may have only heard about but never held, the VW385 tool.

The ring and pinion have numbers stamped on them. These numbers represent the depth and the optimum setting from the factory for the ring gear

Set up the VW385 tool by sliding on the mandrils to the shaft. The mandrils locate on the side bearings. Next, we need to set up the depth gauge to the design dimension to measure the pinion depth. The dimension we are looking for is 66.30 plus the number from the drive gear, which is the deviation number .19mm. together they give us a depth setting of 66.49mm.

The numbers on the ring gear are the matching gear number and the optimum backlash number. Yours may be different, but in this case, the backlash is .16mm.

Drive gear showing the deviation number for this transaxle.
Drive gear showing the deviation number for this transaxle.

To set the pointer we use the forked tool which has magnets on the end. You set the depth mike to 66.40mm. This has to be the one that comes with the tool cause it is making up for centers of the tool, transfer, magnet disc, etc. When you set the forked tool in place, you then zero the dial gauge on the end of the tool. Now you are ready to put the VW385 tool into the transmission case.

Start with the special spacer onto the end of the pinion shaft. Then loosen the diff holding parts and install them into the transmission case. Put the cover on and hold it down with a couple of bolts. Tighten up the diff tool till there is no movement. Only get it tight but not too tight that you cant turn the tool.

What to do if you need to change the pinion depth.

We came in .01 of an mm from zero on our pinion depth. The reason this is acceptable is the shims to change the depth only come in set sizes. The shims come in 0.1, 0.15, and 0.2mm sizes. If you need to change the depth you will have to do the math and order the shim you need from Porche. The shims we are talking bout are the steel ones at the center case line.

Shims to adjust pinion shaft depth
Shims to adjust pinion shaft depth

Pionion Shaft Backlash

The last measurement we want to take is pinion shaft backlash. The backlash was scribed on the ring gear. In our case, it is 0.16 of an mm. You can remove the VW385 tool and pack it away for next time! We have already set up the bearing drag, but this is the backlash of the ring gear. Drop in the crown wheel and put on the side cover again, still with no o-ring. Make sure you keep turning the gear easily while tightening the cover. Torque down and use at least 4 nuts with washers.

Install the VW521 tool that we used earlier for drag. Tighten it into the housing. Put the pointer and shaft over where the ring and pinion meet and at a depth of 83mm. Put on your dial gauge to measure in the direction of travel, like at 90 degrees to the shaft. Clamp up the rear gear to stop the transmission from moving.

Zero the pointer remembering to put pressure on the dial gauge pin. Move the VW521 tool to measure the play. We came in at 0.15, with the optimum at 0.16. So again with shim sizes, this is an acceptable outcome.

If you need to make a change to backlash you would adjust the S1 and S2 shims. You would rearrange but not change the overall total, just where they are positioned.

Measuring pinion backlash
Measuring pinion backlash

Up next, Final assembly

On the final assembly, we will show you how to set up the shift forks. And yes, you can finally put on the large o0-ring on the side cover. Thanks for following along with this long rebuild process. Don’t forget to ask your questions in the comments.

If you missed the first part of this series check out How to Rebuild your 915 Transmission

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