Porsche 915 Transmission Repair Tutorial Part 2: Disassembly, Inspection, and Reassembly

Welcome to Part 2 of the Porsche 915 transmission repair tutorial. If you have already removed the transmission from the vehicle and completed the initial tear-down from Part 1, you are ready to dive deeper. This guide focuses on disassembling the internal components, inspecting critical parts for wear, and reassembling the transmission correctly. The 915 is a robust unit found in many classic 911s, but it requires patience and precision to rebuild successfully. Let's get started.

Before You Begin: Safety and Tools

Working on a transmission involves heavy parts and small bearings. Always wear safety glasses and gloves. Ensure your workbench is clean and well-lit. You will need a bearing separator, snap ring pliers (internal and external), a brass drift punch, a torque wrench, and a transmission jack. Also have a clean parts tray and degreaser on hand. Do not reuse any gaskets or seals; replace them with new ones from a quality kit.

Required Tools Recap

• Bearing separator
• Snap ring pliers (internal/external)
• Brass drift punch and hammer
• Torque wrench (ft-lb and in-lb ranges)
• Transmission jack or sturdy stand
• Clean rags and solvent
• New seal and gasket kit

Disassembly of the 915 Transmission (Continued)

From Part 1, you should have the transmission removed and the front cover and shift housing taken off. Now we will separate the main case halves and remove the internal gear cluster.

Separating the Case Halves

Place the transmission on its side with the differential side down. Remove all bolts securing the main case half (the one with the side cover). There are typically 12-14 bolts. Pry gently with a flathead screwdriver at the seam, but be careful not to damage the mating surfaces. Once loose, lift the case half straight up. You will see the gear cluster and differential.

Removing the Gear Cluster

With the case half off, the gear cluster is exposed. Remove the snap rings holding the main shaft bearings in the case. Use a bearing separator to press the bearings off if they are tight. Note the orientation of the synchro hubs and sliders. Take photos for reference. After removing the snap rings, you can lift the main shaft assembly (with gears and synchros) out of the case. The pinion shaft and differential may come out together or separately. Mark their positions.

Disassembling the Main Shaft

Clamp the main shaft in a soft-jawed vise. Remove the snap ring at the front of the shaft (near the input spline). Slide off the first gear, synchro assembly, and spacer. Work your way back: second gear, synchro, third gear, etc. Each gear has a needle roller bearing; catch these as they fall. Lay out parts in order. Be careful with the synchro keys and springs; they can fly off.

Inspecting Key Components: Synchros, Bearings, and Gears

This is the most critical step. Worn parts will cause grinding, missed shifts, or noise. Inspect each component thoroughly.

Synchro Rings (Brass Rings)

Check the internal threads and friction surface. If they are smooth or have lost the sharp edges, replace them. Compare to new ones. Also check the synchro hub for worn engagement teeth. The hub should slide smoothly on the shaft splines without excessive play.

Bearings

Spin each bearing by hand. They should feel smooth with no roughness or noise. Check the needle bearings in the gears; they should not be discolored or pitted. Replace any bearing that feels gritty. Also inspect the main shaft and pinion bearings in the case. If they show wear, remove and replace them.

Gears and Shafts

Look for broken teeth, chipped edges, or discoloration from heat. The engagement dogs on the gears (where the synchro slides) should be square with no rounded corners. Slight rounding can cause pop-out. If any gear shows significant wear, replace it in pairs (e.g., both third gears). Check the pinion gear teeth for scoring.

Differential

If your transmission has an open differential, inspect the side gears and spider gears. They should spin freely. For limited-slip units, check the clutch pack thickness. Measure the backlash of the ring and pinion – this is advanced but essential. Write down the backlash and preload settings for reassembly.

Reassembly Tips for Smooth Shifting

Reassembly is the reverse of disassembly, but with careful attention to tolerances.

Lubrication and Preload

Use a high-quality GL-4 transmission oil (not GL-5) for synchro life. Pre-lubricate all bearings and synchro hubs. When installing the main shaft, ensure all snap rings are fully seated. Torque the main shaft nut to spec (typically around 180 ft-lb). Check end play on each gear; it should be minimal.

Installing Synchro Springs and Keys

Place the synchro spring in the hub, then insert the three keys (balls) into the slots. The spring engages the keys. It’s tricky; use a small screwdriver to compress the spring while sliding the slider over the hub. Make sure the keys fit into the sliders’ notches.

Setting Bearing Preload

For the pinion bearings, shims are used to set preload. Measure the original shim thickness and add or remove to achieve the same preload (usually 5-10 in-lb of rotational torque). The main shaft bearing preload is set by the snap ring depth; use selective snap rings if needed.

Reassembling the Case Halves

Apply a thin layer of sealant to the case half mating surfaces (use the specified sealant, e.g., Loctite 518). Lower the top case half onto the bottom. Install all bolts and torque to spec in a cross pattern. Do not overtighten.

Common Pitfalls and How to Avoid Them

• Mixing up synchro hubs – Take photos and label each synchro assembly. They are not interchangeable between gears.
• Forgetting the shift rail detent balls and springs – These live under the shift housing. If you skip them, the transmission will feel vague.
• Not replacing the plastic shift rod bushing – This often cracks and causes sloppy shifting. Replace it.
• Using the wrong oil – GL-5 is too aggressive and damages synchros. Stick with GL-4.
• Incorrect torque on the ring gear bolts – Use thread locker and torque to spec (often 60 ft-lb). Loose bolts destroy the differential.

Final Recommendation: When to Seek Professional Help

Rebuilding a 915 transmission is not for the faint of heart. If you encounter excessive wear on the pinion bearings or ring and pinion set, especially if they require resetting backlash and pattern, consider sending the transmission to a specialist. Also, if you do not have a press and bearing separator, the cost of tools may outweigh the savings. However, for a DIY mechanic with patience, this tutorial series provides the steps to refresh synchros, seals, and bearings. Always double-check your work with the factory service manual. A well-rebuilt 915 will reward you with crisp shifts for many miles.