996/997 Turbo, GT2 and GT3 Clutch and Gearbox

High-horsepower cars or dedicated track cars like this 997 GT3RS inflict a great deal of stress on their gearboxes, specifically their ring and pinion. This RS is in the shop for a clutch failure that wiped out the bell housing. We're going to take a look at a few things associated with these gearboxes and what we can do to help preserve them as we beat the tar out of them.

The gearbox is out and, as suspected, the clutch pressure plate is coming apart. The rivet that secures one of the springs has broken, allowing the spring to move out of position and cause damage. Use of the Sachs Competition clutch products in this application will prevent this failure.

Here's a closeup of the damage to the gearbox housing. The inside of the bell housing has been cut away by the runaway spring. We've seen this before and this one isn't damaged beyond use.

The vehicle's owner elected to replace this section of the gearbox housing as well as remedy some other known issues caused by wear and tear. Here is an image of the new housing.

This is glimpse of the gearbox shafts and how they are arranged inside the gearbox. Under heavy load, the pinion shaft will want to shift itself away from the ring gear(in the direction of the arrow), thus, exerting force on it's support bearing.

This is typically what the drain plug magnet looks like when a gearbox  has this issue. Monitor the noises your gearbox makes and check with your mechanic if you suspect you hear something out of the ordinary.

When these gearboxes see hard on and off throttling, the pinion support bearing, AKA 4-point bearing, will become over-stressed and will degrade, causing a grinding noise that will resonate through the chassis in every gear at every speed. This bearing is retained by a plate and shimmed to a specific setting. Often, bearing wear will cause the shims to spin, too.

Here is a closeup of the hard stressed bearing race.

This particular gearbox has seen some hard use. The pinion adjustment shims have been spinning on one side and have been moving around on the other side of the tensioning plate as a result of the force on the plate and the failure of the bearing. Note the grooves in the shim to the left and the debris stuck to the shim on the right.

Here's a close up of the tensioning plate on the side where the shim has been spinning, effectively machining out the surface. This tensioning plate will be replaced.

Our course of action is to replace all of the shaft support bearings and tensioning plate. Here, the new plate and bearings are being installed.

Here, we are measuring clearances to set the preload on the pinion shaft bearings.

Here we measure the pinion shaft position relative to the center axis of the differential. This step is critical to the smooth operation and lifespan of the ring and pinion. If this setting is not carried out correctly, the ring and pinion will be noisy and will wear prematurely. This step requires expensive tooling and training. 

The differential in this gearbox also had excessive slip caused by worn clutch plates. Factory diffs in the street cars tend to wear quickly. We were going to replace the plates and ramps but then we discovered that the bearing boss was worn out. This diff repair suddenly became a replacement. We decide to install a Guard Transmissions limited slip differential: a durable, high performing upgrade.

Here is a pic of the old differential during disassembly. This special fixture is required to service the differentials.

After we finish installing the differential, we assemble the gear stacks, forks and housing. The final step in setting up the gearbox is the backlash(play between the ring and pinion). Special tooling required here, too.

Gearbox oils of choice: Mobil Delvac 75W-90 and Mobilube PTX 75W-90, the latter of which can only be purchased through Porsche.

Routine maintenance, regular inspections and keen eyes and ears can help prevent costly damage. If you are tracking your car or running high horsepower applications it is important to have your equipment checked more frequently. Wear and tear can never be eliminated in these situations but costly repairs can be avoided if an issue is rectified before it becomes severe.