In this lesson, we take a closer look at the Input Shaft. In the last lesson, we took apart the Porsche 915 transmission but did not tackle the gear sets. The input and pinion shaft are the main parts of the transmission. We cover the pinion shaft in the next installment.
Taking Apart the Input Shaft
The input shaft gears are held on with a large nut. This nut is a one-time use and must be replaced. To remove the nut we do not care about the peened in part we just have to support the shaft safely to undo it. We use Porsche Special Tool part number 000-721-256-00 or P-256 but this part is no longer available new.
The gear sets are also held on by the two bearings and we need to set the input shaft in the press to release them. The shaft is pressed down through the supported gears to let go. Now all the gear set is free and can be removed.
Using a large clean area (we have a transmission table made of steel) lay out the gears as they come off. Degrease all of the parts bit by bit so that you can keep them in order. Dry off all the parts and they are ready for inspection.
If the gear or splines on the input shaft are damaged, you will have to replace the whole shaft and gear. Remember you will have to replace the matching gear on the pinion shaft. You can measure the wear on the shaft. We will also check the run-out before putting the shaft back into service.
Disassembling the Gear Sets
The gear, synchro ring, dog teeth, energy bands, and slider comprise the gear pack. You can see the wear in the parts, the synchro ring gets shiny and the dog teeth get knocked down. Porsche does not sell you just the dog teeth. Porsche wants to sell you the whole gear. Again, if you change one gear you need to replace its running mate. This would be super expensive. The after-market has filled the need. You can buy the dog teeth rings but will need another special tool to remove and replace.
Start to disassemble the gear pack by removing the top shim with a set of circlip pliers. After removing the synchro band, inside you will find the anchor blocks and the energizer band. In the fourth and third gear, the energizer band comes in two parts with an anchor block between each band. If you are not changing the dog teeth, you can replace the synchro ring and reassemble the gear set the same way it came apart. The gear set should sound rattly, and the ring will be firm to move.
Replacing the Dog Teeth Ring
On third gear, we have excessive wear on the synchro and the dog teeth. So, on this gear, we will remove and replace them. This is the time that you need another special tool. You can cut the dog teeth ring off with a grinder. This method is dangerous, messy, and can cause damage to your very expensive gear. The right way is to use the tool. Use a press to remove and use the tool again to replace it. The dog teeth are set on very tight.
Synchro engagement tool for removing dog teeth
Most of the work needed on this input shaft is to replace the synchro rings. We need to replace dog teeth on a few gears and when we do that it is good to also replace the slider. We are also replacing the bearings. When you replace a bearing on the input shaft, you must also replace the bearing races in the transmission case. The bearings are not all the way through their life, but we are bringing the gearbox back to the start of its service life.
Reassembling the Input Shaft
Now that we have rebuilt the individual gear sets, we are ready to get all the parts back onto the input shaft. Clean everything again and dry. During assembly we will use no oil or lubricants, everything will be clean and dry.
Use the splined shaft holding tool, part number p-355a, using a vise to hold the tool. If you cannot find this tool you can get an old 915 5th gear as it has splines.
We press or heat the bearings and reassemble the gear sets onto the input shaft. Best to watch the video for this process. The gears and parts will show you how they go back together from the wear marks.
Up Next in Your 915 Transmission build
In the next lesson, we cover the pinion shaft. The pinion shaft is treated much like the input shaft. Changing out the synchro’s, checking the dog teeth wear and replacing the bearings.