Rebuilding a Porsche 915 Transmission is not for everyone. It takes special tools that the average Porsche driver does not own. Some of the tools we use in this re-build are no longer available. So honestly, maybe a very small percentage of you are ever going to try to rebuild your Porsche 915 Transmission.
That being said, you can still learn about what is involved so that you are educated when it comes to looking for a transmission rebuilder. Also, you may also get a better understanding of where the noises are coming from in your transmission and how the whole thing works.
Porsche 915 Transmission Tear Down
We start with taking the transmission apart. Draining the oil is the first part. The Porsche engine stand yolk works on the transmission to hold it firmly in the same setup you use for the engine. We do not bother with cleaning off the transmission beforehand, everything will get clean soon but just get at it first.
Looking for Damage during Diss-assembly
As we are taking apart the 915 transmission, we are looking for signs of wear and or damage. Starting with the rear housing, undo the bolts that hold it on. The rear cover has lugs for tapping, do not pry on the gasket surfaces.
As you take the cover off, be aware of the reverse pin, don’t let it drop out and get lost. You need a big area to start laying out your 915 transmission. Just like the cars, the parts seem like way more than when it is together, and for a compact transmission, it sure can expand. We use a metal table specifically for transmissions, it has a draining lip at the front. Any excess oil can drain off into a bucket.
You will expose gears when the cover comes off. These are 5th gear and reverse gear. Some 915 transmissions do not have a fifth gear. You will need to remove these gears to get to the intermediate part. The gear shift pivot has to be uncoupled to remove the gear shift fork. Again do not pry on the gasket surfaces of the pivot cover.
Drive out the roll pin from the castellated nut. We use an impact gun and a 27mm socket to remove this nut. You may have to hold the shaft at the end of the transmission to undo these nuts. Next up is the pinion shaft nut, this nut is pinned in place and is a one-time use. This is a 36mm nut, and again the impact gun works well for us.
915 Transmission Intermediate Case
When the intermediate cover comes off the gear shift will come out with it as well. Place it all on your bench. Now you have exposed the main part of the transmission. The main shaft and pinion shaft. There are two bearings that support the shafts in the intermediate case. we will address these bearing races in the case as we progress.
Now you can look over the wear and tear of the gears and synchros. On this 915 transmission, the wear was very obvious on the third gear synchro ring. The synchro ring should feel like sandpaper, and this one has polished spots and some material has worn off.
We replace all synchro rings almost every time we open a gearbox. Any other parts that need to be replaced are inspected to check for wear. Cost-wise, we do add all of the bearings to the quote but advise that gear damage is expensive and done on a need-to basis. The other wearing parts are the “dog teeth”, sliders, and shift forks.
If you ever need to change a gear, you need to change it’s corresponding gear. They are numbered with matching numbers on all of the gears.
Next undo the 8mm nuts on the intermediate holding plate. The nuts are nothing special , but the lock washers are a smaller size than normal, and come only from Porsche. We try to keep these to re-use, though we like to replace the nuts. The nuts on this 915 transmission were quite rounded off from previous builders.
The gears should come out as one big assembly, put this on the bench as is. We will have lesson on each of these shaft separately. You will notice some shims on the case. These shims control your pinion depth, you will need to hold on to these. We will cover pinion depth in another lesson.
Check the bearings in the case, they can have a tendency to come loose. If you have a worn out housing because these bearings let go, you have a couple of options. First you could get another front case, or you could have a steel insert machined into your case. Lucky for us the bearing housings were fine!
Pulling the Differential from a 915 Transmission
We have already removed the half shafts from this transmission, by undoing the center bolt with an impact gun. We move onto the cover next. Use the lug points that Porsche has cast into it for you. Once the cover is off the whole differential comes out as one unit. We will have a course on measuring the diff as well! This is a not a limited slip differential, or LSD. You can call it an open housing diff. We have a rebuild video on an LSD. Check that out here.
Lots of Cleaning
You will now have the gearbox mostly all apart. Cleaning, cleaning, and more cleaning is in your future. Not only do we use a hot degreasing parts washer, after rinsing in hot water we clean all parts with an evaporative cleaner.
The Porsche 915 transmission case can be made of magnesium or aluminum. Many times all the covers are aluminum, but not all times. You can recognize magnesium from its dark grey color. We do not want to remove the protective finish on magnesium. The finish is a chemical process where the surface of the magnesium reacts with chemicals to change it.
Porsche chose magnesium for the weight savings. Magnesium has a specific high strength to weight ratio, and it is 35% lighter than aluminum and 75% lighter than iron. Typical magnesium alloys weigh ~25% less than their aluminum counterparts at equal stiffness.
Long story short. Do not use abrasives when cleaning magnesium. No blasting, wet or dry! The aluminum parts are fine to go into the wet blaster and the ceramic polisher.
Removing the bearings and last parts from the 915
We have a few more parts to take out of the cases. Best to do after cleaning for some of them.
We remove the bearings and seals with a bearing punch, a bearing punch has more surface area to help with removal. We knock out the bearing races as well, careful not to hurt the sealing surfaces.
In the front housing, we need to remove the clutch fork and shaft, as well as the removing the guide sleeve. There is a pin that holds the clutch shaft in place. Remove this pin with a punch, but do not punch it all the way, pull the last part with a pair of side-cutters. You will need a new pin and seals for this shaft.
The guide sleeve on this transmission was glued in place and on closer inspection, kind of worn out. It took an impact screwdriver to start undoing the screws that hold this guide sleeve in place. Remember in 915 transmissions you do not need silicone sealants anywhere!
In the Next 915 Transmission Lesson
We look at the input shaft. Taking the input shaft apart, replacing the worn parts, and reassembling the shaft. After that, we cover the pinion shaft in a similar fashion.
It may not look like you need any special tools to take a 915 transmission apart, but believe me, putting back it together is another story. Watch all of the lessons before you do a tear down.