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 > Technical Articles: / Porsche 911 (1965-1989) >
Full-On 901 Synchro Change!

Pelican Technical Article:

Full-On 901 Synchro Change!

James Tomlinson


Socket set, punch, Allen wrench, crescent wrench, hammers,

Applicable Models:

Porsche 911 (1965-71)
Porsche 912 (1965-69)
Porsche 914 (1970-71)

Parts Required:

Synchro, gaskets, sliders

Hot Tip:

Work in a clean environment.

Performance Gain:

Better transmission performance

Complementary Modification:

Check CV joints and boots.

Full-On 901 Synchro Change!

So your gears are a grinding--I feel your pain. You too can rebuild the 901. It's not that hard. Really! If all of the oil seals are in decent shape (no leaks!!!), then you can change the synchros while leaving the tranny in the car and attached!

Tools - You'll need a few tools. Most of them ya got, but there 2 special ones which really help

Synchro hub removal tool (#PEL-SIR-P225) - This puppy helps you remove the dog teeth from the actual gear. There are ways to get around buying this one. I'll address those later.

Shift fork alignment tool (#TR-SFJ-001) - There is no substitute for this. If you want to rebuild the tranny, ya need it!

30 mm deep socket (I found one at Autozone for $6.99)

2 mm pin punch

13 mm socket wrench with extension

17 mm socket

17 mm allen for the drain plugs

19 mm deep socket (you can substitute a 3/4 inch)

"Big Ass" Crescent wrench

Plastic and brass hammers

A thin file

Gasket scraper (or a single edged razor blade)

Some sort of cleaning fluid (I like to use diesel fuel)

Lint free rags

A couple of brass brushes

3 jack stands

2 drive up ramps

A couple of big plastic boxes.

A plastic bin sink wash tub.


4 gaskets (all-inclusive set: #914-300-901-00) - Intermediate plate to transmission housing, End cover to intermediate plate, the fork gasket and rear cover plate gasket. The factory manuals don't show the 3rd or 4th gaskets for side shifters, which use an o-ring with the side shifter and no rear cover plate on the rear cover.

Up to 5 synchro bands (#911-302-301-06)

Up to 5 sets of dog teeth (1st: #10-1550-310, rest: #10-1550-315)

Up to 3 sliders. 2nd/3rd and 4th/5th are one type (#911-302-411-10) and Rev/1st is different (#911-302-226-00).

What will you need? It depends on what is actually wrong and how many gears grind. If it only grinds in 1st or 2nd, you may only need parts for those areas. I'll show you good and bad parts. If the grinding just started, you may be able to get away with just a few synchro-bands. As a bare minimum, you need to get 2 brand new synchro-bands, for 1st and 2nd. These get the most wear and you might as well replace them while you're in here!

Muffler gaskets for 914

Clean - Clean - Clean

You're about to get into the inner workings of the transmission. You do not want to get dirt inside. We all get oil leaks and some of this oil ends up on the transmission. Clean the dirt and grime off the transmission. You might try one of those pressure washer engine degreasers at the u-spray it car wash places before starting this job. Besides, no one likes working on a greasy mess. Clean at least the end cover and the intermediate plate on the outside.

While you're under there, take a look at the CV joints and check the boots. Squeeze them while holding a strong light on them. Cracks? Dry rot? Replace them! And don't forget to repack them with grease!

While you're cleaning, pay special attention to the nuts which hold the transmission cover on. Clean the threads as best as you can. Then spray them liberally with penetrating oil. While they're soaking in it, remove both drain plugs and drain the tranny oil. Remove the shift rod cover. Clean the shift rod area and soak its nuts with penetrating oil.

Back the car up onto the ramps. For safety sake, disconnect the battery. You don't want someone accidentally starting the car while this is all going on. Support the heat exchangers with wire or jack stands. Remove the muffler and the muffler hanger from the transmission. Disconnect the transmission ground strap from the end cover.

Remove the shift rod
Tail shift: You disconnect the two 13mm nuts which hold the shift rod support bracket. Then push it back on the shift rod. You may want to go ahead and completely remove the shift rod from the car and renew all of the shift bushing (See the tech article on shifting improvements).

Side Shifter. Using an allen wrench, remove the shift rod set screw and separate the shift rod from the shifter arm. Remove the 2 nuts which hold the shifter arm to the transmission. Pull the shifter assembly out of the transmission. Disconnect from the shift rod. Soak it in your favorite cleaning fluid.

Remove the speedo cable
Place a jack stand underneath the transmission and support it. Remove the bolts transmission support bolts. These are through bolts and you'll need to place a wrench on top as well as the ratchet on the nut.

Remove the Tail cover
All of the tail cover nuts should be 13 mm. Don't worry if the nut is rusted on and the stud backs out. We'll take care of that later. Remove the nuts and the washers. Put them into a jar with some cleaning solution. On a tail shifter, remove the rear cover nuts washers and cover. Put something underneath the transmission, because you are about to have reverse drop out on the ground!!! Using a plastic and/or brass hammer, tap the cover off. Tap lightly on the "ears", the rear transmission supports.   Collect all of the pieces that fell on the ground and remove any loose pieces from the inside of the end cover. Put them in a clean plastic box labeled reverse. Make sure all loose pieces are there.

Tail shifter: remove the fork. It is in the same spot the side shifter uses for the shift selector. Use a big screw driver to pry it out, but only at the "pointy" ends, not in the middle. Note which way the "toes" are pointing.

Before we remove the intermediate plate, we need to loosen/remove a couple of things.

There is a rod on the left side which is connected to a brass  1/2 moon thing. This is you 1st/rev gear selector. The  1/2 moon thing is a shift fork. It is attached to the 1st/rev slider. Push the rod and the slider into the transmission (Towards the front of the car) and engage it to 1st gear. Now look at the same level as the first shift rod on the intermediate plate. You'll see the protrusions of the other 2 shift rods. Using a ratchet extension and a hammer, tap both of these rods into the intermediate plate. This engages all gears, locking up the entire transmission so it can't turn. Use the 30mm socket and a breaker bar and loosen the rear bolt with the speedo worm drive (Yeah! THE BIG BOLT). Just make sure you can turn it by hand, but don't remove it yet.

Using a 13mm ratchet, remove the little bolt from the shift fork. Slide the shift fork and the slider off the transmission. You may need to use a large screw drive to pry the slider out of engagement with first gear, depending on the condition of first gear. Label this 1st gear shift fork.

Take the pin punch and remove the roll pin from the castle nut on the right side of the transmission. Use a 19mm Socket wrench (I like a  3/4 deep socket) and a breaker bar to loosen (but not remove) the castle nut.

Remove the intermediate plate and the gears: Using a brass hammer, lightly tap the intermediate plate towards yourself. Once it's loose, hold tightly (It's heavy!) and pull the intermediate plate and gear stacks out of the transmission.

Now the fun begins!

Part 2 - Rebuilding the gear stack

Gear Stacks with the Shift forks removed

I hope you cleared a good work space because this is going to be fun. Get yourself 2 nice clean plastic boxes. Label them (If you're into that) Pinion and Drive shaft. If you're only replacing the 1st and 2nd gear synchros, you only need one box, and you don't need to remove the shift fork from the pinion shaft

Remove the shift fork from the shorter shaft (drive shaft) and place in the drive shaft box. Don't remove the shift rods from the intermediate plate. Do the same for the pinion shaft.

If all you want to do is fix 1st & 2nd gear, just remove the drive shaft by removing the large 30 mm bolt and pulling 1st gear off. Place all of the 1st gear stuff into the drive shaft box. You should be able to get down to the big washer, slightly angled washer. Put a label on this washer, as 1st gear.

Tap. And I mean TAP  the drive shaft with a hammer. The fit of the bearing runner is very tight. You are not driving a nail! You are tapping. Tap tap tap. Tap tap tap. Once there is a little room, you could use a gear pulled, but lightly taping will work. Once the collar is off, put it in "the box"

Now, you have a couple of choices. The gear puller is the best solution. But all you need is another tool. I hear you cry. My gear puller also helps on pulling the CV joints apart, among other things. And it's a cool looking tool. If ya got it, use it to remove 2nd gear which will pull the other collar and the other washer off. If you don't have one, you can use the BRASS  hammer to whack the gear stack down. It does work, just be gentle, but not too gentle. The brass hammer takes the abuse, the gears do not. Just turn the stack.

If you are repairing/inspecting 4th gear, do the same with the pinion shaft. You might want to do this anyway, since the pinion bearing is a weak point and 1/2 of them have been bad that I've seen. If the ball bearing falls out of the pinion bearing, you have a bad one. If the brass cage is shattered, you have a bad one!!!!!!!

Once 2nd gear is off, everything should be easy to access and should slide right off. Now is the time to look at the parts.

The gears themselves are always in mesh and spin on those needle bearing collars. So, they are NOT connected to the shaft, except when the slider engages them.

Take the slider, and place it over the synch band and press down. That is what happens when you engage a gear. The band compresses, and the slider slides over it and engages the dog teeth.

First let's look at the slider. It is a ring, with lots of little internal slots, in 3 groupings, and 3 large slots. The slider slides over the triangle member on these 3 large slots.

Now let's look at the slider itself. The small slots should have a point at each end (Except 1st gear which should have a point only on one side). Once the synchro band wears out, which is when you start to hear grinding, the slider is no longer engaging the synch band. It then engages the dog teeth at a different speed and uses the pounding of the dog teeth to get them to the same speed. Not good!

On my 2nd gear, At higher RPMs, I could get 2nd to synch. This is because the rapidly spinning shaft forces the synchband outward and allows it to engage the slider a bit, less or no grinding.

The slider can get some wear from the contact with the synch band, but the majority will come from moving contact with the dogteeth. 3 things happen.

#1. The Slider loses some (or all) of its pointiness

#2. The Dogteeth lose some (or all) of its pointiness

#3. As the slider wears, the metal is pushed back and mushrooms a little bit, making the slot uneven. This makes it hard to shift into a gear and may prevent the slider from fully engaging the dogteeth, allowing it to "pop" out of gear.

This is a slider with normal wear is could be put right back in (It's a 4th/5th gear slider, but it is exactly the same as 2nd/3rd). I'd make sure the small slots are straight with a file. Also, use a brass brush and some solvent to remove any gunk in the slots.

The points will force the gear to rotate slightly as these points encounter the dog teeth points. Worn teeth require more force to get the slider into gear and may lead to the slider "popping" out of gear. 1st gear on my 914/6 comes to mind.

The little circumferential scratches come from synchro contact and are normal.

If the points are a little blunted, OK. Clean up the ring as best you can. If the points are gone and flat. It's done. Time for a new one. I am investigating the possibility of "repointing" the sliders; taking a blunted slider and grinding back on a point. Not sure of the success of this idea. Hmmm. I will report back on this.

The 1st gear slider is different from the other 2. 1st it has points only in one direction. 2nd, it has gear teeth (reverse!) extending radially out from the edge, which makes its OD bigger. It costs about twice what a 2nd-3rd/4th-5th slider costs. This is the slider I am considering repointing. My concern is that after repointing, the engagement may be less and it may pop out of gear. If the dog teeth are good, you don't need much of a point.

Remove the synch bands from the gear by removing the circlip with a pair of snap ring pliers.

Synch bands:

Take a look at these synch bands:

The bad one here is really bad. If you look at any synch band. The side towards the slider will wear more than the side towards the dog teeth. Porsche factory manual suggests that anytime you have one out, even if it is still good, turn it around so the better surface will be towards the slider. And I thought this was just a mechanics trick. Nope. Porsche approved! Looking edge on, the side with more metal (thicker) is the "good" side, if you lost track.

If the bad side is so bad it will fit into the dog teeth loosely, it is absolutely done. Pay the $20 and get a new one. If it falls out, it's too loose. It should compress slightly when going in.

Dog Teeth
The dog teeth are press fit into the gear. The Porsche/VW removal tool is a 2 piece plate which locks onto the dog teeth. You then use either a press or a hammer to separate the dog teeth from the gear with the cylinder. (The tool pushes the gear). If you are replacing with new ones, it makes little difference how you pull the dog teeth off. If you are using a donor tranny, get this tool. It is cheaper than 1 set of dog teeth.

The "teeth" biting into the synchband here are the dog teeth. These are the part that engages the slider.

All Dog teeth are the same for all gears. All Synchro bands are the same for all gears. So, why do 1st gear dog teeth cost less in the catalog? The dog teeth come with brake bands, the little guys which sit under the circlip. 2nd-5th, need to be synchronized in both directions (Spinning the gear faster and spinning the gear slower--i.e. upshifting and down shifting). 1st gear only needs to be synchronized in one direction, down shifting. So only the down shift brake band is installed. The only time you would you would need to synch the other way would be if you were moving backwards and tried to synch 1st, which is a no-no. Don't do it.

The Little bands between synchroband and the inside lip of the dogteeth, are the brake bands.

Part 2B "Re-assemble the stack"

Pinion shaft

Reassemble up to and including #6. 1-5 are on the other side of the intermediate plate and done afterwards.

Drive Shaft

Reassemble up to #11. 1-10 are on the other side of the intermediate plate and are done afterwards.

Slide both shafts into the intermediate plate and then assemble the rest onto each shaft. Don't torque anything down yet. You may need to tap bearing collars back into place.

Using your fingers, snap the sliders for 2nd/3rd into 2nd and the 4th/5th into 4th. The gear shafts are locked together. Torque the castle nut on the pinion shaft and put its retainer roll pin back in. Torque the main shaft bolt with the deep 30mm socket. Snap the sliders back to the central position. You may need a screwdriver to snap them off of new synchrobands.

Slide the shift forks back onto the shift rods for and put the nuts and washers on loosely. Slide the shift fork alignment tool onto the gear stack. The shift rods can rotate slightly. Make sure that all three shift rod slots match up position wise.

Center each slider into the middle of between each gear set. Tighten the shift fork nut. 1st gear, adjust the shift fork so the slider is just off of the synchband for 1st. Tighten its nut.

The gear stack is back together!

Part 3 - putting it all back together
Tranny is back in the car. I made a "trial run" on the in place tranny refurb. Not too bad. I installed my 6's tranny back into the car with only the differential installed. It's pretty weird, being the "jack" for the tranny. I went ahead and reconnected the CV joints and reinstalled the starter.

Well, this is what the "gut less" tranny looks like.

This is a tail shifting 6 (a 901/10!), and an early one. I'll take a shot of the differential cover. It still has the support built in for the clutch cable guide, a left over from the 911. Some Early 914/4's have this too!

The gear stack slid in with little or no problem. One "trick" is to engage 4th gear, which helps position the stack to slide in. I did have to turn the gears a little bit (kind of like putting in a distributor). Uh, a hammer helps. Seriously! You lightly tap the intermediate plate to move the gear stack in, the last inch or so. I found that the 2 dowel points to be good positions to tap.

Once the gear stack is in place, for a tail shifter, you need to make sure the shift selector rod is inserted in its hole properly. Then tap the intermediate plate home.

The hard part: Putting the cover on. Make sure you've removed the speedo drive. Side shifter, it's a breeze. Reassemble reverse onto the reverse shaft. Keeping the cover tilter back a little bit (having the engine and tranny tilted down helps a little bit here. Slide the cover home and get the reserve shaft into its home. The final "washer" on the reserve stack has to be aligned. Make sure it is in the correct position. Adjust it into the correct position with your fingers while the cover is about 1 inch from closure. Once everything is all set, tap it into place with the hammer (Damn thing sure comes in handy, but, don't get wild with it!)

For a tail shifter, it's a bit tougher. You have to do everything on your back! The shift selector fits into a recess in the tail of the cover. I expect this is where the 911 shift shaft used to go. You have to position it with your fingers from inside the cover, until it's in the correct place, all the while not letting reverse fall off.

When reverse falls over, yell a bit, curse and find the bits as they roll away. Clean 'em off and try again. Follow the rest of the directions as a side shifter.

Once the cover is on, I put a couple of nuts into place.

On the tail shifter, I put the support fork in. For a side shifter, install the gear selector console. Test out the gear selection with the external selector. Don't worry if it's a little stiff. If everything seems ok (being able to select each gear), then go ahead and close up the cover and torque the nuts. Be careful, they only need about 18-20 foot-lbs.

Reconnected the main shifting shaft. I checked the gear shift and made sure I could shift into every gear. All set, went ahead and reconnected the tranny ground strap and torque the bolts on the cover and the side access port.

Reinstall the gear selector cover. Re-install the muffler.

Reinstall the battery!

AND take it out for a spin!

Comments and Suggestions:
Bob Comments: P S do not use water soluble solvents for cleaning you gear stacks. It will destroy the needle bearings and races. Thank you.
August 17, 2017
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it.
- Nick at Pelican Parts
Bob Comments: Hello gear grinders. I bought 2 901 trannys for parts. And after a trial by error I found that if the syncros and sliders are not easy to engage the gears on every input your fighting a battle you can't win. Also if the shift fork detent sit in the rods happy the shift rod will bind up and won't go into gear.
August 17, 2017
  Followup from the Pelican Staff: Thanks for sharing your repair process and experience. These type of comments add so much to the Pelican tech community.
- Nick at Pelican Parts
Chuck68 Comments: I will replace the pinion shaft bearing, the one in the housing#27 on above diagram.... do you have any tips/instructions on removing the race from the housing?
901 transmission
March 17, 2017
  Followup from the Pelican Staff: I've not done this before but at the shop I have seen the guys use a slide hammer attachment to pop it out, I think there is an access notch that you can hook with the slide hammer attachment and pop it out. Will post to the forums and see if someone who has performed the task can give a tip. - Casey at Pelican Parts  
yondeman Comments: I am having a problem removing the gear stack and intermediate plate. It comes out about 4-5 inches but binds up. Now i have balls falling out and the shiter shafts detent balls came out this is a practice tranny but still what am I doing wrong.
July 24, 2016
  Followup from the Pelican Staff: See the steps mentioning rebuilding the gear stack. You may need a puller or a soft-faced hammer to get it apart. - Nick at Pelican Parts  
Paul Comments: I am working on rebuilding my 901 transittion for my 914. It was stuck in gear. After dissassemby the shift rod for 2nd and 3rd was stuck. ON closer inspection the shift rod is very very loose in the intermediate plate. Looks like it needs new bushings but that doesn't appear to be a part that is available. Maybe it was just a bored hole in the intermediate plate. From the looks of new intermediate plate on your web site it appears that there is a bronze bushing in there. Do you think the intermediate plate can be modified and a new bushing installed there? Any other thoughts?
May 17, 2016
  Followup from the Pelican Staff: I don't think so. You may be able to have anew part machined. Or you never know, a repair kit maybe available. Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
Stathis Comments: Hello Wayne,
The problem with my 901 transmission is the difficulty to shift and lock the 1st gear grinding noises and sometimes the 2nd. Also there is a "differential" noise only when I shift the 4th speed. Don't have any problem to up neither down shifting - but the 1st. What should I replace? Can you please pass me a list with all suggesting parts and P/N. Also oil leak problems are visible. Thank you in advance
January 30, 2016
  Followup from the Pelican Staff: If you detach the linkage from the shifter and manually shift at the trans, does the problem go away?

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
gmcgowan99 Comments: Is there a kit or parts list for this rebuild for a 74' 914?
March 14, 2015
  Followup from the Pelican Staff: I am sure we can put one together. 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
j914 Comments: I am having trouble with the final step. The shift fork on the top of the transmission is coming in contact with the top of the case and it wont let me remove the indeterminate plate
May 29, 2014
  Followup from the Pelican Staff: Are you installing it backward? If not, check that it is aligned properly and not hung up on the shaft. - Nick at Pelican Parts  
Steve Aki Comments: I can't seem to 'tap' in the two shift rods; I can get one in or the other but not both. I have followed the write up and pushed the 1st/Rev gear all the way forward and that allows me to only slide one rod forward. What am I doing wrong?
April 17, 2011
  Followup from the Pelican Staff: The only thing I can suggest is to make sure that the detent lockouts are not blocking the insertion of your rods. You might have to manually hold the lockout in place to get the rods in. - Wayne at Pelican Parts  
cwitt Comments: Anybody have a similar write-up for the 915? I've seen Zimmerman's blog, but am interested in a if-you-don't-have-all-the-factory-tools take on 915 synchro replacement.
October 29, 2010
  Followup from the Pelican Staff: 915s are tougher - you really do need a lot of the specialized tools to get all of the gears and synchros apart. Less so on the 901. - Wayne at Pelican Parts  
ruben Comments: no, in the main casting between the gear stack and the diff housing, its the smaller non conical baring shell. the plate and the rest is tip top
April 29, 2010
  Followup from the Pelican Staff: Should be tight - if not, you need to have a machinist put in a steel retaining ring to sure it up. - Wayne at Pelican Parts  
ruben Comments: i can spin the incomming shaft baring shell in the main casing round with the retaner on, is there an oversise baring shell avalible or is this normall?
it has no side ways play yet
April 29, 2010
  Followup from the Pelican Staff: I think you mean you can spin the bearing in the intermediate plate that connects the front and rear of the transmission. Yes, this is typical, but it's also worn out. The intermediate plate needs to be replaced, or you need to machine in retaining collars to hold the bearings in place. - Wayne at Pelican Parts  
todd Comments: Hello,

I just replaced the dog teeth and synchronizing ring on 1st gear. I have the gear sets attached to the intermediate plate on the bench. I have noticed that fifth gear on the pinion shaft is 1/16 of an inch farther out then fifth gear on the input shaft. Is this something abnormal I tightened down the stretch bearing bolt and no difference Any suggestions?

I have attached a photo

October 24, 2009
  Followup from the Pelican Staff: That doesn't look correct to me, looks like the bearings are off too, which means that the shaft alignments are off. Make sure that the bearings are not installed incorrectly in the intermediate plate, I think that this can cause that. - Wayne at Pelican Parts  
Vic Comments: While I am inside my gear box, any recomendations for strenghening my gear box for street use behind a 76mm x 104mm 8.6: 1 compresion single spring head combination?
September 28, 2009
  Followup from the Pelican Staff: The only thing I can think of is the aluminum intermediate plate upgrade that replaces the existing magnesium one. - Wayne at Pelican Parts  
Hylar Comments: Thanks for your help. The new bushings have made the car shift like new.
August 21, 2009
  Followup from the Pelican Staff: - Sorry for the long delay in replying. We have recently added staff and are trying to get caught up. Thanks for the kudos!  
none Comments: Hi
I wonder if you can help me. I have a 1967 912 with a 5 speed transmission. Recently I have noticed I cannot shift from 2nd or 3rd into 5th. I used to be able to. Now I have to shift into 4th first. Once in 4th the shift into 5th is fine. Do you have any idea what may be happening?
Hylar Friedman
August 6, 2009
  Followup from the Pelican Staff: First thing I would do is to replace all of your shift bushings in the linkage - those are the first to go. Also, check the shifter itself - it's an odd design that's press fit together and the metal often fails / fatigues after 40+ years. - Wayne at Pelican Parts  

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Page last updated: Wed 2/21/2018 02:19:44 AM