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Rebuilding First Gear on the 901 Transmission

Pelican Technical Article:

Rebuilding First Gear on the 901 Transmission


5 hr


$50 to $300




Impact wrench, 30mm deep socket, synchro removal tool, snap-ring pliers

Applicable Models:

Porsche 911 (1965-71)
Porsche 912 (1965-69)

Parts Required:

Synchro ring, 1st gear dogteeth, 1st/Reverse slider (if worn), transmission end cover gasket

Hot Tip:

Having the right tools is a requirement for this job

Performance Gain:

No more grinding into first gear

Complementary Modification:

Add Swepco fluid to the transmission, Replace shift bushings
101 Projects for Your Porsche 911

This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.

One of the most frustrating elements of owning a classic Porsche is the existence of a worn out transmission. Porsche transmissions in particular have not been known for their excellence in shifting ability. Combine this with the fact that there really isn't too much information available on repairing them, and you have a recipe for poorly maintained transmissions. Adding to the problem is the fact that internal transmission parts and repair tools can be some of the most expensive parts to purchase.

Well, there is a little bit of relief available. For early 911/912 owners (1965-71), there is a relatively easy fix to repair first gear on the five-speed 901 transmission. The 901 is unique in that the gears for first and reverse are located at the rear of the transmission (the part that faces the front of the car) and are easily accessed without tearing apart the entire transmission. That's the good news. The bad news is that you need to remove the engine from the car in order to get the transmission out. But once you have access to it, repairing that nasty grind into first gear is a relatively simple matter.

Before you decide that your first gear is trashed, you should check a few other factors that might be contributing to grinding. Make sure that your motor and transmission mounts are firm and not cracked (Pelican Technical Article: Replacing Motor and Transmission Mounts). Also make sure that your clutch is adjusted properly (Pelican Technical Article: Clutch Adjustment). Check that your transmission shift linkage bushings are in good condition, and that your shift linkage is properly adjusted (Pelican Technical Article: Shifting Improvements). If all of these check out in good condition, then it's probably wise to take a closer look at the transmission itself.

The first step obviously, is to remove the engine and transmission from the car. For more information on this particular task, see Pelican Technical Article: Engine Removal. Once you have the engine and transmission separated, it's a wise idea to place the transmission on an engine stand. It's easier to keep it steady, and you won't find yourself bending over too much.

An important step to take before your start working on the transmission is to empty all of the transmission fluid out of it. If you forget this all important step, you will be reminded of it when you start to take it apart, and transmission fluid empties out all over your garage floor.

Removing the rear cover is as simple as unbolting the nuts that hold it on. Be careful when you remove the rear cover, because the reverse gear and bearings are attached to a shaft that is pressed into the cover, and they might fall out. After the rear cover is off, take one of the nuts, and reattach it to one of the studs with a spacer in-between the nut and the intermediate plate. The reason for this is to prevent the intermediate plate (the approximately 1.5" thick plate that is wedged in-between the case and the end cover) from separating from the case. When playing with the gears on the transmission, this plate will try to separate from the case.

Once you have the rear exposed, remove the brass shift fork and the 1st/Reverse slider. This fork is simply bolted onto the shift rod for 1st/Reverse gear. Now exposed, the removal of the 1st gear synchro hub assembly requires the use of an impact wrench. There really is no other way to remove the expansion bolt, because there is no practical means of keeping the transmission pinion shaft from turning. Use a 30mm deep socket to remove the bolt using the impact wrench.

The disassembly of the synchro hub starts with the removal of the large snap ring that holds on the synchro ring. Use a pair of snap-ring pliers to remove the ring from the assembly. It's a wise idea to wear safety glasses during this step. Carefully remove the synchro ring, the anchor block, the thrust block, and the brake band, keeping in mind their original positions so that you can reassemble them easily later on.

The dog teeth, or synchro-hub, should be inspected at this point. If the teeth are badly worn, the hub needs to be replaced. Chances are if the transmission was grinding, then the synchro hub is probably worn beyond recognition. When the synchro ring wears out, it allows the internal teeth inside of the transmission to contact each other when they are not turning at the same speed. This results in that awful grinding noise when changing gears. The grinding of the gears is not really what you hear, but it is the grinding of the synchro hub and the slider that create this horrible noise.

To remove the synchro hub from 1st gear, you will need a synchro hub removal tool. I don't recommend attempting the removal of the synchro hub without this tool, as it will almost certainly have to be cut off of the gear with a small rotary tool. First gear can be very expensive to replace if you accidentally damage it while cutting off the synchro hub. Buy or rent the tool: it's a good investment.

Installation of the new synchro hub is straightforward: simply tap it on with a soft hammer. Reassemble the unit with the brake band, thrust block, anchor block, and the new synchro ring constrained by the large snap ring. Your new assembly can be installed back into the transmission using the impact wrench again. Make sure that the expansion nut is zapped on tightly as you don't want this assembly to come loose in your transmission. The factory torque specification for this bolt is about 114 Nm (84 ft-lbs).

The slider deserves careful inspection before it's installed back into the transmission. Carefully inspect the points that mate with the synchro hub and assess their wear. If you have doubts about whether you need to replace the slider, take it to your mechanic or a transmission specialist and ask them to assess its condition. If you were performing this repair on the Porsche 914, then it would be less of a concern because the slider can be replaced in about a half hour on that car without removing the transmission. On the 911 however, you will need to drop the engine again, so extra consideration is worthy here. It's not an easy decision, because new sliders cost around $325.

After you've reassembled the slider and shift fork, carefully check to make sure that the 1st gear slider locks onto the synchro hub and stays in place. It's common for these parts to wear to the point where they will have a tendency to slip out of gear. Make sure the fit is tight, and that they are firmly locked together.

Now, replace the end cover on the transmission. Make sure that you properly reassemble reverse gear, and align and replace all the bearings and spacers that fit on the rear cover shaft. Also use a fresh transmission cover gasket, and make sure that you scrape off the remains of the old one with a razor blade prior to installation. Tighten the nuts up on the case to about 23 Nm (16.9 ft-lbs). I also recommend that you replace the shift rod selector seal in the end cover. The old seal simply pries out and the new one can easily be pressed in.

When you finally get back into your car and start driving it, you may experience some difficulty getting the car into gear, as the break-in period makes shifting pretty tough. Don't worry: the new synchro ring needs to be worn in just a bit before you will feel a significant improvement. Adding Swepco 201 transmission fluid may help the break-in process as well.

This is what is visible when the rear cover is removed.
Figure 1

This is what is visible when the rear cover is removed. The first gear shift fork (manufactured out of brass) is wrapped around the 1st/Reverse slider. Before you play around with the shifter (it's good to familiarize yourself with how it works), make sure that you place a nut and spacer on one of the end cover studs (shown by arrow). This will keep the intermediate plate from separating from the transmission housing.

After removing the shift fork and slider, you will need an impact wrench to get the center stretch bolt off of the assembly.
Figure 2

After removing the shift fork and slider, you will need an impact wrench to get the center stretch bolt off of the assembly. Removing the bolt without the help of an impact wrench is nearly impossible, since the shaft turns freely.

Removal of the snap ring reveals the thrust block, the anchor block, and the brake band.
Figure 3

Removal of the snap ring reveals the thrust block, the anchor block, and the brake band. It is important to remember that 1st gear uses only one brake band, whereas all the others use two. Surrounding the whole assembly is the synchro ring (shown by green arrow). It is important to reassemble the unit exactly as shown here. The thrust block (yellow arrow), brake band (tan arrow), and anchor block (red arrow) must be assembled in their proper order and positions in order for first gear to function properly.

The all-important synchro hub removal tool is shown here.
Figure 4

The all-important synchro hub removal tool is shown here. I don't recommend attempting this job without access to this tool. Buy it or rent it: the amount of time you will save will be tremendous. Without it, you will have to cut off the dogteeth synchro hub, which is a job that is tedious and could damage your first gear. The tool wraps itself around a groove in the synchro hub (dog teeth), which then allows you to knock out the gear from the tool using the large center punch. A couple of good smacks with a hammer will affirm that this really can't be done without the use of this tool. If you have a press available, then the procedure is made a little bit easier.

Old and new synchro hubs side by side offer up a good reason why 1st gear on this transmission didn't work too well.
Figure 5

Old and new synchro hubs side by side offer up a good reason why 1st gear on this transmission didn't work too well. The normally pointy gear teeth are completely worn down and rounded off at the edges. Not only does this cause additional grinding, but it also makes it more difficult to shift into 1st gear when the teeth finally synchronize. Having a transmission that is difficult to place into first is likely to have worn dog teeth like these on the synchro hub.

The other half of the equation is the slider.
Figure 6

The other half of the equation is the slider. This is what the dog teeth (synchro hub) mate to when they are engaged in gear. While usually showing less wear than the synchro hub, the slider has a tendency to wear out quicker when the synchros have been worn beyond their useful life. Shown here is a slider with semi-rounded points similar to the worn dogteeth. Also shown is a worn groove in the middle (indicated by the arrow) that occurs from the synchro ring wearing on the slider. New sliders have a slight groove that locks in the synchro ring when the transmission is in gear.

The final piece of the puzzle is the synchro ring.
Figure 7

The final piece of the puzzle is the synchro ring. After this begins to wear, everything else goes downhill as well. The old and the new rings side by side show how significantly the synchro ring can wear. The arrow points to the subtle, yet important contoured surface on the new ring that is completely worn away on the old one. This surface is an important factor in preventing the gears from grinding. When it wears away, the teeth on the synchro hub and slider begin to grind and gradually become destroyed. It is important to replace the synchro ring before this starts happening. The worn synchro ring also contributes to the transmission popping out of gear when driving. The synchro ring snaps into the slider and holds the transmission into gear.

Comments and Suggestions:
PETE Comments: I am putting a 914 gearbox in a vw karmann ghia with vw engine . will my input shaft go into the bearing of a 356 gland nut , DUE TO THE DIFFERENT DIMENSIONS BETWEEN THE PORSCHEVW , VW GEARBOX ?
February 24, 2017
  Followup from the Pelican Staff: I have NO idea. I will create a post in the forums and see if another member can offer guidance! - Casey at Pelican Parts  
Porsche novo Comments: only when driving for a longer time, 1st and reverse gears become a problem to engage. Does the metal expand due to heat and changes the linkage by minimal amount?
July 2, 2015
  Followup from the Pelican Staff: Could be the signs of a worn clutch. When the trouble persists, if you shut the engine off, does it shift easily?- Nick at Pelican Parts  
Max Comments: I came, I saw, and I conquered. This was an incredibly helpful article!
I took my first to a local transmission shop and for a beer the guys pressed off my dog-teeth and pressed on the new ones.
November 19, 2012
  Followup from the Pelican Staff: It's amazing the power of a free beer has. - Nick at Pelican Parts  

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Page last updated: Sat 2/24/2018 02:00:59 AM