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Replacing 1st Gear on the 901 Transmission
 
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Pelican Technical Article:

Replacing 1st Gear on the 901 Transmission

Tim Polzin

Time:

1 weekend

Tab:

$500

Talent:

*****

Tools:

Floor jack (or two drive on ramps), jack stands, wheel chocks, safety glasses, torque wrench, metric wrench and metric socket set, flathead screwdriver, Phillips screwdriver, 30mm deep socket, 17mm hex key, rubber hammer, 13mm open-end wrench, impact wrench, snap ring pliers, Dremmel tool with grinding wheel, wedge, press and a clam shell bearing splitter, vise, lint free shop rags,

Applicable Models:

Porsche 911 (1965-68)
Porsche 911E (1969-73)
Porsche 911L (1968)
Porsche 911S (1967-73)
Porsche 911T (1969-73)
Porsche 912 (1965-69)
Porsche 914 4-cyl (1970-76)

Parts Required:

Slider, shift fork, synchro ring and brake band, dog teeth, 1st gear, 2 muffler gaskets, transmission cover gasket, replacement gear oil

Performance Gain:

Much improved shifting thanks to a replaced 1st gear set

Complementary Modification:

Make the shifting improvements to your side shifter or tail shifter equipped trans before replacing the 1st gear set
The 901 transmission came standard on the 914 and the early 911. Although a very sturdy gearbox capable of easily handling 200HP, the 901 often suffers from sloppy shifting caused by years of neglect. There are several variants of the 901 transmission, but for the bulk of this article, the variations are insignificant. This repair can be performed if you own a 911, 914 tail-shifter (70-72), or a 914 side-shifter (73-76). If you own a 914, there are many ways to improve shifting by replacing small yet important parts, but these will be documented in our other article, Improving Shifting on Your 914. The main focus of this article is the diagnosis and repair of 1st gear on the 901 transmission.

The 901 transmission was designed with 1st gear and reverse located at the rear of the transmission housing. This enables the repair of these two gears without the significant disassembly of the entire transmission. Figure 1 shows the transmission with the rear cover removed. Located on the rear, 1st and reverse can be repaired without removing the transmission from the car - a benefit that makes this job a whole lot easier if you own a 914. Unfortunately, early 911 owners are not as lucky because the engine and transmission needs to be removed from the car in order to perform this repair. In the photos that accompany this article, I had the transmission removed from my 914 because I was in the middle of performing an engine swap.

The first step in repairing 1st gear is to determine the cause of your shifting problems. It is wise to eliminate all of the other relatively minor problems prior to disassembling the transmission. One common symptom of 1st gear neglect is the generation of grinding noises when shifting into 1st gear while the car is in motion. Another symptom is the inability to shift into 1st gear when the car is at rest. These problems, although commonly routed inside the transmission can be exacerbated by other problems with car. Prior to disassembling the transmission, all of the tips and tricks listed in the 914 Shifting Improvements article should be performed. If none of these succeed in improving your shifting problems, then maybe its time to replace some worn out components in your transmission.

The 901 transmission has a synchro-mesh mechanism which allows the gears to be shifted when the car and the transmission are spinning at different speeds. There are 6 main components used in 1st gear. These are the slider, shift fork, synchro ring and brake band, dog teeth, and 1st gear. The 1st gear slider is shown in Figure 2. The slider is a large gear that is wrapped around the drive shaft. This slider gear is controlled by the shift fork which is attached to the shift linkage. The shift fork is displayed in Figure 3 . When shifting into 1st gear while the car is rolling, the slider slides along the drive shaft and makes contact with the synchro ring. The synchro ring is shown in Figure 4. The friction between the slider and the synchro ring spins up the transmission to the same RPM as the drive shaft. The brake band prevents the meshing of gears until both the drive shaft and transmission (temporarily disconnected from the engine by the clutch) are spinning at the same speed. At this point, the brake band relaxes, and allows the mating teeth of the slider to mesh with the dog teeth that are attached to 1st gear. The slider teeth are shown in Figure 5. As the slider fits over the synchro ring, it snaps into place. Unfortunately, words and pictures cannot fully explain this process. When you have the transmission apart, take some time to play with the shifting of the gears, especially in and out of 1st gear. This should give you a good idea of how the whole mechanism works.

Since the synchro ring uses a contact friction interface to spin the transmission up to the same speed as the drive axles, it is understandable that this part would wear out over the life of the car. Fortunately, the synchro ring for 1st gear, which usually gets the most wear, is relatively easy to replace. Unfortunately , as soon as the synchro ring begins to wear out, the slider and the dog teeth begin to grind and wear down. If this happens, the slider, dog teeth, and synchro ring most likely need to be replaced at an approximate cost of $400 for the parts alone. Therefore, as soon as your transmission begins crunching, you need to replace the synchro rings. Otherwise you will do irreparable harm to the more expensive elements of the transmission. Beware of people selling 'rebuilt' transmissions for $400/exchange. In most cases, to properly rebuild the 901 transmission you need a minimum of $400 in parts at wholesale costs. These 'rebuilds' often only include the replacement of the synchro rings - and sometimes only for 1st and 2nd gear. When buying a rebuilt transmission, make sure you know the condition of the sliders and the dog teeth before you spend your hard-earned cash. A good rebuilder would most likely be happy to supply you with pictures of the sliders and dog teeth if you asked.

Ok, enough warnings. It's time to inspect and repair your 1st gear assembly. Here's a list of what you will definitely need:

  • 30mm deep socket
  • 1st gear synchro
  • 2 muffler gaskets
  • 1st gear slider (if needed)
  • 1st gear dog teeth (if needed)
  • 17mm hex key

Begin by raising up the rear end of the of car. I recommend using two drive-up lift ramps for this task. The reason is that you will need to prevent both wheels from turning if you don't have an impact wrench. The parking brake alone is often not strong enough to prevent the wheels from turning when loosening up the expansion nut that holds 1st gear to the drive shaft. You may be able to improvise a solution by having an assistant press on the brake pedal when you need to untorque the nut. I have never tried this though, so I'm not sure if it would work.

The next step is to support the engine and transmission. You will need to disconnect the rear transmission mounts from the car, so the engine-transmission assembly must be independently supported. Place a jack stand underneath the transmission case. The rubber engine mounts will flex slightly, so it doesn't have to be pressed up against the bottom of the case. Just make sure that the jack stand is close enough to the case to support the engine-transmission assembly when disconnecting the rear transmission mounts. I don't recommend jacking up the car and placing the jack stand underneath the transmission because the weight of the entire car will be resting on the jack stand.

Once you have the engine and transmission assembly supported, or the jack stand in place, then empty the gear oil out of the transmission. You will need a 17mm hex wrench to remove the oil drain plug. The oil filler and drain plug are both located on the right-hand side of the case (for this article, we will assume that the transmission is oriented in the car as it is in a 914). Remove both the filler and drain plug and clean them thoroughly. A common upgrade performed is to replace the filler plug with an old drain plug, which is magnetized. Hopefully, this would help reduce the metal particles floating in the gear oil. After the transmission is empty, then disconnect the speedometer cable that connects to the rear transmission cover. It also may be wise to disconnect the shift linkage, since the transmission may be dropping down a few inches. If you decide to keep the linkage attached, then just be careful not to place any force on the linkage when disconnecting the transmission mounts. At this point you need to remove the muffler to gain access to the rear of the transmission. Once the muffler is removed, be careful not to place any force or pressure on the exhaust heat exchangers. They are connected directly to the cylinder heads, and any adverse forces placed on them may crack the exhaust studs on the heads. At this time, it may be wise to check your muffler bracket. This bracket prevents cylinder head damage as described in the tech article, 914 Muffler Bracket Repair.

After you remove the muffler, disconnect the ground strap from the transmission to the car. Once the ground strap is disconnected, you can loosen and remove the 2 rear transmission nuts that hold the transmission to the car. Make sure that the transmission/engine assembly is securely supported. Once the entire assembly is stable and supported, remove the nuts and washers that fasten the transmission rear cover to the housing. Depending upon what repairs have been performed on the transmission before, the cover may or may not slide off easily. A few taps with a rubber hammer will often loosen it up. Make sure that the intermediate plate does not slide off, because the rear transmission cover normally holds the intermediate plate to the transmission housing. When you remove the rear cover, be careful because the reverse gears and bearings may fall out. Once the rear cover is removed, replace at least two of the nuts with spacers on the rear studs. This will prevent the intermediate plate from becoming separated from the transmission case when shifting the transmission. A picture of this nut and spacer is shown in Figure 6. Once the intermediate place is secured, you can shift the transmission through its gears. Now would be an opportune time to observe how 1st gear works.

Once you've picked up the idea of how the slider and synchro work, loosen the 13mm bolt on the shift fork and remove it from the shaft. Remember its approximate position along the shaft. The slider needs to slide off of the drive shaft with the removal of the shift fork. The transmission should now look like Figure 7. Check the slider for significant wear on the inner teeth. Reasonably sharp slider teeth are shown in Figure 5. These gear teeth should be pointed and sharp. Also check the amount of free play that the slider has on the drive shaft. There should be a little movement perpendicular to the axis, but not too much. Finally, inspect the channel where the synchro sits when the slider is engaged. Worn out sliders will have a very deep groove. Brand new ones have only a slight groove machined long the inside. If any of these areas show any significant amount of wear, the slider should be replaced.

Now, it's time to remove the expansion bolt which holds on 1st gear. This is done by placing a 30mm deep socket over the brass speedometer drive. Be careful of the speedometer drive that is attached to the expansion nut, as shown in Figure 8. Note:If the car is on ramps, make sure that the parking brake is engaged (working on both wheels), the front wheels are chocked, and no one is underneath the car. The nut is directly connected to the differential and hence the wheels. If the bolt is on too tight, you may be able to push the car off the ramps by turning the socket wrench or breaker bar. The best approach is, of course, to use an impact wrench. Once the nut is removed, the entire 1st gear assembly can be easily slid off of the drive shaft. The entire 1st gear assembly is shown in Figure 9. The transmission without the first gear assembly is shown in Figure 10.

At this point, remove the large circlip that holds the synchro ring and brake band assembly together. Pop off the synchro ring, being careful to remember the orientation of the brake band within the synchro ring. The 1st gear assembly without the circlip and synchro ring is shown in Figure 11. The synchro ring should be replaced with a new one - they are inexpensive enough to be worth it. Take a close look at 1st gear and its dog teeth. If they are flat and worn, they should be replaced. Sometimes, the wear pattern is not too obvious to spot. Figure 12 shows a set of dog teeth that are sharp around the outside, but very dull around the inside. The dog teeth are pressed onto 1st gear, and removing them is not easy. There is a special Porsche tool designed to remove the dog teeth from 1st gear. However, with enough patience, the dog teeth can be removed by prying and tapping with a wedge placed in-between the dog teeth and 1st gear. This is not an easy task. The use of a dremmel tool combined with a small grinding wheel may also be used to literally cut the dog teeth off of 1st gear. Make sure to note the location and orientation of the dog teeth with respect to 1st gear. Tim Polzin had this to add to the process of removing the dog teeth. "The dog teeth will drop right out using a clam shell bearing splitter and a press. Just put the edge of the splitter in the notch used by the circlip and press the gear out."

It is important to note that first gear only has one brake band, whereas the rest of the gears have two. Figure 11 clearly shows only one brake band (half-moon circular steel band) around the inside of the synchro hub. Make sure that you only place a single brake band into first gear. Otherwise, it will be very difficult to shift into first gear when the car is stopped.

Assembly is the reverse of disassembly. Press the new dog teeth onto 1st gear using a vise. You may need to place some spacer blocks on both sides of the dog teeth to get equal pressing force on both sides of the dog teeth. Any friendly Porsche shop should be able to do the removal and replacement of the dog teeth for a nominal fee, or even for free. The reassembled 1st gear assembly is displayed in Figure 13. Once the 1st gear assembly is complete, then replace it on the drive shaft. Figure 14 and Figure 15 shows the reassembled 1st gear assembly on the transmission. Replace the (slider groove piece) and retorque the expansion nut to 75 ft-lbs. Next, replace the slider and shift fork, tightening the shift fork at its approximate original location along the shaft. The completed reassembled 1st gear with slider assembly is shown in Figure 16. At this point check to make sure that the entire assembly shifts smoothly into 1st gear. Once the car is back together it will take a few miles to wear in the synchro ring. Therefore, the shifting should naturally feel a little stiff in the beginning. Now reattach the end cover making sure that the reverse gear assembly is installed correctly. Use a new gasket in-between the rear cover and the transmission housing. Also, make sure that both the case and the rear cover surfaces are clean. Retorque the rear cover bolts to 6-7 ft-lbs. Raise the engine-transmission assembly back up and reattach it to the transmission mounts. Make sure that you reattach the ground strap, or you might have problems starting the car later on. Reconnect the shift linkage, and the speedometer cable. Lower the car off the ramps, and you're ready to try out your newly rebuilt 1st gear!

Don't forget to refill your transmission full of fluid. The plugs are located on the side of the transmission, and you will need a transmission oil hand pump to get the transmission fluid into the tranny. We recommend Swepco 201 as it will help your old synchros shift like new!

This task can be completed over a weekend, however, you may want to assume that the car may be inoperable for a couple of weeks. The task of changing out 1st gear involves the inspection of several components within the transmission. If any of these components need replacing, it may be difficult to get them in a hurry (within a day or so, especially on a weekend). Pelican Parts can supply you with everything you need to complete this task - including the technical support. We'll even help you out on the task by taking a look at your parts. If you are unsure as to whether or not your slider or 1st gear is any good, then just drop it in the mail to us. We'll take a look at it and give you advice on what needs to be replaced, and what seems good enough to use. As with most of the components within transmissions, all will exhibit some type of wear. Depending upon the wear pattern, and the amount of wear compared to a new part, the used part that you have in your transmission may simply be fine. Or, we may have a good quality used part at an affordable cost that you may want to use. Give us a call, or drop us a line if you have any questions.


PLEASE NOTE THAT AS OF 8/24/2016 PORSCHE NO LONGER STOCKS FIRST GEAR REPLACEMENT PARTS. 

1st gear and reverse assembled
Figure 1

1st gear and reverse assembled

1st gear slider
Figure 2

1st gear slider

Shifting fork for first gear
Figure 3

Shifting fork for first gear

Brand new synchro ring
Figure 4

Brand new synchro ring

Close up of teeth on 1st gear slider
Figure 5

Close up of teeth on 1st gear slider

Nut and spacer to hold intermediate plate
Figure 6

Nut and spacer to hold intermediate plate

Transmission with shift fork and reverse removed
Figure 7

Transmission with shift fork and reverse removed

Speedometer drive and guide sleeve
Figure 8

Speedometer drive and guide sleeve

Top view of 1st gear assembly
Figure 9

Top view of 1st gear assembly

Transmission with 1st gear assembly removed
Figure 10

Transmission with 1st gear assembly removed

1st gear assembly without synchro ring
Figure 11

1st gear assembly without synchro ring

Dulled dog teeth
Figure 12

Dulled dog teeth

Complete 1st gear assembly
Figure 13

Complete 1st gear assembly

Synchro ring, dog teeth & 1st gear
Figure 14

Synchro ring, dog teeth & 1st gear

1st gear assembly
Figure 15

1st gear assembly

Reassembled 1st gear assembly with slider
Figure 16

Reassembled 1st gear assembly with slider

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Comments and Suggestions:
mjr730 Comments: Hello,
I just followed this write up and was able to do this synchro change in an afternoon. Great write up, thanks. This didn't solve my 1st gear grind issue completely though. I should have asked before putting it back together because I did notice in figure 11 that one of the blocks in the brake band assembly looked different. Looking at the picture it is the one on the right. Mine looked similar but what I would call the legs that point toward the brake band were much shorter almost to the point of not being much longer than the tab on the block that fits in the opening. I still don't understand exactly what the brake band does. Could too short of a block not reaching the end of the brake band cause a grinding issue. The synchro did help some. At a slow roll or stopped it goes into gear pretty good, but it grinds badly on a downshift when going very fast at all. Do you have these little blocks I am talking about available? Thanks for any help you can give me. Monte
September 22, 2015
  Followup from the Pelican Staff: If the block is took short the synchro braking (speed matching) may not occur. Let's find out if you have the right part.


Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Dick Shift Comments: This articul recommends Swepco 201.
I was taught and seen it reinforced many times Porsche transmissions with servo SYNCHRO's like 1965-1986 911's were designed for and work [shift]better and last longer with NON-synthetic 85-W140 weight gear oil.Swepco 201 is 80-W90 to thin for servo SYNCHRO's. Swepco 212 80-W140 would be a better choice.
July 22, 2014
  Followup from the Pelican Staff: Personally I prefer factory fluids or fluids approved by the factory. Thanks for the input. - Nick at Pelican Parts  
Dwain Comments: This is very informative and I certainly appreciate it. I only wished it laid everything out like Svenson does for the 914 engine. I might actually be brave enough to give it a go!!!
THANKS AGAIN
June 20, 2011
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 

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