901 speedo drive gear

[DByers]

I have a small leak (dime size after about a week) coming from the speedo drive gear on my 901 trans. Car is a 71 911T. It looks like I will have to remove the trans support bracket to get at it, my question is I am sure there are some O rings that need to be replaced and what part # might those be? Also any special tool need to remove the drive? This is the only leak I have and I will be replacing the tranny fluid and thought I would tackle this at the same time.
TIA

[john walker's workshop]

o-ring 999 701 084 50
radial seal 999 113036 50
support trans, remove lower mount. the drive is two sections held together with the big nut on the outer angle drive piece. take them out as a unit after removing the setscrew/bolt that you can see on the end of the trans. the o-ring os obvious, the seal is in between the two sections. there is a metal ring over the seal that pops off easy enough, them pry out the seal and squeeze the new one in. i'vee seen the housing bore pretty worn out to the point where the o ring would still leak, so it may need some sealer if that's the case. the drive wobbles around, and after 30 years, wears the bore oversize.

[DByers]

Thanks JW.

[john walker's workshop]

let me add that upon reinstalling the unit up into the housing bore, take care and be sure that the dimple in the unit that accepts the tip of the set screw/bolt is centered at the bottom of the bolt hole. you can stick a rod or something in there to feel it in place, or check it with a mirror and flashlight.

[Jandrews]

Excellent description as usual from JWW.

I'll add one experience I had when making this change. I found that radial seal to be very difficult to remove. In the two examples that I worked on, it was lodged or "sealed" in the barrel extremely tight. On the first one, I ended up scratching up the barrel bore significantly by picking at it with a dental pick. That aluminum is fairly soft and will damage easily if care is not taken when trying to pry/lift that seal out. The seal is actually made of a metal ring covered in a rubber/plastic material. In my case, it was this seal that was allowing fluid past the shaft, not a problem with the o-ring.

All I am saying is be very careful when working on removing that seal. It can be tight, and before you know it you can destroy the barrel housing. By the way, I got a good replacement from PartsHeaven after I messed the first one up.

Hope this helps.


JA

[Grady Clay]

Grady


Dan,

Your thread was looking sooo lonely ….

I’m going to try and be very detailed for any others who want to attempt this successfully. Have your digital camera in use for every step to add to the thread.

Yes, you can do this without removing the transmission. You are correct; you do have to remove the transmission mount and the rear sway bar.

I would first remove the shift linkage access plate in the cockpit behind the seats and confirm the shift rod seal isn’t leaking.

Your safety is of primary importance so support the 911 with substantial jack stands under the torsion bars at the spring plate. I would additionally chock the front tires, put a couple of there and wheels under the tunnel toward the rear (leaving enough room to access the parts), and leave the rear tires on.
You are going to need a hydraulic jack and a third jack stand to support the transmission.
Make sure everything is very stable and your back-up protection is in place. You may be wrestling with stuck fasteners and you don’t want to have the 911 fall on you.

Some steps require patients and care, not to be breezed through. This is particularly true of the diagnostics.



1) Loosen the fill plug first. Drain the transmission oil while the 911 is level. Let it drain thoroughly for a couple of hours (warm is best) or overnight. Otherwise you will have a large puddle in the front of the transmission to drain on you when you remove the speedometer drive.

2) Remove the two sway bar support brackets and just let the bar hang from the links. You can wire it up to the rear and out of your way. You may have to coax these bolts with penetration oil or heat if they are reluctant to come out. This is the time to inspect the sway bar mounting on the chassis for any cracks or damage. Inspect all the sway bar clamps, bushes, and links for needed attention or replacement. This is not necessarily a “while you are there” issue since this stuff is very accessible.

3) Loosen (don’t remove yet) the four nuts holding the transmission mount to the transmission. You will need to use penetrating oil. I would not be surprised if the studs come out of the transmission.

4) Loosen (don’t remove yet) the two transmission mount to chassis bolts. This will probably require a LONG wrench and significant force. Be careful to not upset the 911 on the jack stands.

5) With the jack, lift the engine and transmission slightly and place the 3rd jack stand under the transmission, on the center rib, in the general area of the differential to drain plug. Make sure it is in a secure location where it can’t slide. This should leave you room to work. I would leave the jack up under the engine mount as an added safety measure.

6) Remove the two mount-to-chassis bolts and then the four mount-to-transmission nuts (with studs if necessary.) Inspect the two large bolts for any sign of damage to the threads (like cross-threading.) There are two aluminum spacers between the mount and transmission. Note their orientation for reinstall.

7) Inspect for the location of the oil leak, you don’t want to fix the wrong part. Generally an oil leak source can be identified by the cleanest oily spot. That is because the oil washes away the dirt at the source and below.

8) Liberally apply penetrating oil to the speedometer drive where it enters the nose piece of the transmission, to the #7 M8 “hex bolt with pin” that secures the drive (see illustrations below), and to the speedometer cable nut.

9) Remove the speedometer cable nut. Be careful to not apply so much force that the speedometer angle drive (the lower part of the speedometer drive assembly) is damaged. Once loose, you can push the cable back into the tunnel or wire it to the side.

10) You should see if there is rotational play of the drive housing. Using a thin 22 mm wrench on the angle drive nut, see if you can rotate the speedometer drive a few degrees. It is no big deal if you can’t but this is a test on reinstallation.

11) Remove the angle drive. This takes two open end thin wrenches (22 mm and a very customized 27 mm). Hold the 27 mm stationary and apply the force on the 22 mm against the 27 mm wrench. Once the angle drive is removed you can get a socket on the 27 mm adaptor if necessary. Many mechanics seem to use a hammer and chisel on the 22 mm hex – poor form unless the part is a throw-away.
You don’t want to rely on the strength of the pin in the hex bolt when applying a lot of force.
If this area has gear oil in it, this confirms the lip seal is leaking. If dry, that suggests the O-ring seal was the culprit.
If the rubber boot around the shifter nose piece is sufficiently pliable, you can push it back into the tunnel to gain more access. This boot is replaceable by disconnecting the shift linkage in the tunnel (grub screw, not pinch bolt) and feeding the new boot in from there.

12) Mark the position of the speedometer drive housing relative to the nose piece casting. Use a scratch awl or a very small chisel. This will facilitate the reinstall in the proper orientation.

13) Rotational and axial free-play of the speedometer drive shaft. Take some small pliers and gently test the rotational and axial free play of the speedometer drive shaft for future reference. Be sure to not scratch the tang surfaces of the drive shaft.

14) Remove the retaining bolt. It will take some mild contortions with a wrench. You should have good purchase on the hex and be able to “snap” it loose. Use a hammer on the wrench if necessary. Note that the bolt has a locating pin on it. This is what keeps the drive in place yet allows it to freely center on the O-ring seal. I think your ‘71 has a copper sealing washer under the bolt head. If you need to replace this bolt or washer, make sure the replacement is the same length and you use the same thickness sealing washer.

15) If this drive assembly has been in place for 32 years or reinstalled dry, it may not want to just slide out. Go by your local shops and see if you can find a defective angle drive. You can use it as a tool to extract the speedometer drive without damaging your angle drive. More on that below.

16) Once the speedometer drive is out you can see the external O-ring seal and the internal lip seal after you remove the #14 “centering disc”. Inspect the O-ring for any slight damage. Remember, it will have been scratched when coming out of the transmission nose piece by any corrosion.
Take some oil and lube the external part of the speedometer drive shaft. While rotating, carefully remove the shaft by the gear end. Inspect the shaft for damage (grove from lip seal.) Inspect the lip seal for damage. Note the seal orientation – the open side is out according to the diagram.

17) Inspect the bore where the speedometer drive is in the transmission nose piece. Probably the part below the O-ring seal is corroded. Clean it the best you can with solvents and rags. Is there any damage where the O-ring seals? There is going to be a subjective judgment call here. The issue is weather the new O-ring has a proper surface to seal against. Some cleaver use of a camera will allow Pelicans to offer opinions.

18) Test fit the hex bolt with pin back in the transmission nose piece. The threads (internal and external) should be clean and the bolt should be able to go in and out with just finger pressure. If not, clean everything again and chase the threads.
Inspect the sealing surface on the nose piece where the sealing washer seals. There should be sufficient un-corroded surface for a proper seal.

19) Test fit the large transmission mount to chassis bolts. If there is the slightest damage, replace the bolts and chase the threads in the chassis. Apply anti-seize to the chassis threads. It is possible that some ham-fist PO screwed up these threads. That possibly isn’t a DIY repair and can be done later but shouldn’t slow your mission.

20) Remove the O-ring from the speedometer drive taking care to not scratch the groove it sits in. Those are sealing surfaces also. Again inspect the O-ring for damage. Inspect the grove for any scratches or corrosion damage. A competent machinist can repair most damage and you can purchase larger cross-section and different diameter O-rings (non-Porsche.)

21) Re-finish the corroded surface in the nose piece with some fine paper abrasive. Be absolutely certain no abrasive stays in place or gets into the transmission. Don’t touch the area where the O-ring seals. It is possible for a machinist to move the grove in the speedometer drive 1-2 mm inboard and add an outboard spacer. This would give the O-ring a new surface to seal on.
If the bore of the transmission nose piece it too damaged, replacement is the best option.

(end page 1)
Continued

[Grady Clay]

(begin page 2)

22) Replacing the lip seal is relatively straight forward. You need a “hook tool” that will go past the seal from the outside and snag the seal. A slap hammer works great to get the seal out. Be very careful to not damage the bore where the seal sits.
I don’t think there is a P-Tool to install this seal. There are a lot of commercial tools available, find one that fits properly.
I would recommend some sealant around the outside of the lip seal, absolutely necessary if it is a metal seal.
If you don’t replace the shaft, try and push the new seal in as far as possible to give it a fresh surface on the shaft. If you clean/polish the shaft, always do it in the circumferential direction, never axially. Make sure there are no burs or sharp edges in the tang end of the shaft that could damage the new lip seal.
Be sure to lube the seal and shaft prior to reassembly.
Remember, the direction of the seal is open side out unless the seal is designed otherwise.

23) Install the new O-ring carefully; don’t stretch it more than necessary. Roll it into position. Inspect and remove any burs or sharp edges on the speedometer drive housing. Grease everything.

24) OK, now for reassembly. The critical issue is to get the new O-ring past the corroded part of the nose piece without damage when installing the speedometer drive. Grease everything. It is useful to mark the bottom of the speedometer drive with a punch or file in-line with the locating hole. This allows you to rotationally index the drive to have the hex bolt pin find the locating hole. You don’t want to have to remove the drive and possibly damage the O-ring.
Make sure the drive is pushed in as far as possible and is free to rotate. You may need to rotate the shaft so the gears properly mesh.

25) Reinstall the Hex bolt with pin and a new sealing washer. You should be able to turn the bolt in by fingers until it seats on the washer. Continually check that the drive housing can rotate slightly. It should not bind from the hex bolt with pin. When you are certain everything is in place, torque the hex bolt with pin to 18 ft-lb. Again check the drive housing is not in a bind.

26) Now check to see that when the transmission pinion shaft turns, the speedometer drive shaft rotates. With the transmission in neutral and one rear wheel blocked, turn the other wheel and verify the speedometer drive shaft rotates and has some rotational and axial free-play everywhere. This should be the same as your test prior to disassembly.

27) Get the other boots and the “while you are there” things in place.

28) Reinstall the angle drive with anti-seize on the threads and grease between the nut and the drive. A last minute squirt of oil at the speedometer drive shaft and lip seal wouldn’t hurt.

29) Reinstall the speedometer cable on the angle drive with anti-seize on the threads. Make sure the boot is in place. If it is cracked, put the crack downward and use a zip-tie.

30) Reinstall the transmission mount with the four nuts and washers. Remember the two aluminum spacers between the mount and the nose piece. Leave the nuts backed off about three turns so the mount is very floppy.

31) Reinstall the transmission mount to chassis bolts with anti-seize on the threads. Note the proper orientation of the large washers. At least use new lock washers. Torque to specification (which I don’t seem to have handy at the moment) or slightly less due to the anti-seize.

32) With the hydraulic jack, remove the third jack stand and allow the transmission to sit on the mount. Tighten the nuts and torque to 18 ft-lb.

33) Lube the sway bar bushings and ball cups. Reinstall the sway bar with new hardware and anti-seize on the threads. Torque to 18 ft-lb.

34) Refill transmission, double check everything.

35) Check everything for a third time (don’t need to cross fingers) and test drive.

36) 911 lives happily ever after and you drive with satisfaction.




Speedometer drive removal tool is locally manufactured from a defective speedometer angle drive. You cut the angle drive off as close to the angle as possible. Square the cut end in a lathe and thread the inner piece to fit a slap hammer. Simple, cheap and effective. You have to remember to remove the hex bolt – ask me why.




Planned spare parts to have available:
Transmission oil.
New hardware.
Copper sealing washer for hex bolt with pin.
External O-ring seal.
Internal lip seal.

Possible:
Speedometer cable assembly.
Speedometer angle drive.
Speedometer drive shaft.
Hex bolt with pin.
Clutch cable and associated pieces.
Rubber boots.
Helicoil thread repair.

While you are there:
Throttle cushion.
Throttle bell-crank plastic bushings.
Clutch cable plastic piece upgrade to metal.


Expected problems include fasteners not easily coming off, the angle drive failing (or had failed but was still operating), the speedometer cable nut is cracked needing a new cable, rubber boots need to be new,

It is not unlikely that if the studs come out of the transmission, the magnesium threads will come also. The proper and easy fix is stainless steel Helicoil thread repair. Single-size M8 kits are available at most auto parts stores.

These magnesium transmissions were originally coated with a product trade name “Tectyl” from the Factory. That stuff is currently available. Do a Pelican search. This is very necessary for the long-term survival of the magnesium parts.

Be prepared (financially and emotionally) to remove the engine and transmission. That is not defeat, just sometimes necessary and appropriate.

The last resort is to have a flatbed tow to the shop. Not a big deal if necessary.

The reason good shops are just that is they anticipate potential problems and have accumulated the means of fixing them properly. Few DIY home shops have that ability. This is good CYA backup plan B.

I am not advocating anyone NOT attempt this DIY repair. If everything goes reasonably well, it’s easy and a fun weekend project.


JW and all others please add to this.


Best,
Grady

[Grady Clay]

Image excerpts relative to above.
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(C) 1969 Dr. Ing. h.c. F. Porsche A.G.

Best,
Grady

[scottb]

Grady: I've been reading your knowledgeable posts for a long time, but this one takes the cake. People like you and JW are what makes this board an invaluable tool!



Scott

[DByers]

Grady,
Thank you very much for those detailed instructions. Between you and JW there is not much I wouldnt tackle on my 911. I will be sure to document my RR on this and post back when done.

Scott, you ready for another SB Run? Look for one soon!

[john walker's workshop]

i don't type real fast, so i try to keep it short, but nice elaboration grady!

[Grady Clay]

Dan,

As you do this project, post if there are any additions or corrections to the list. Particularly anything that is not clear. I’ll go edit the list (of course I found a few typos after I posted.) Photos of each step will help the next Pelican.


JW, I don’t type real fast, either. I’m just persistent and not running a shop at the same time.
I heartily agree with something you posted recently. It is no skin off any knowledgeable Porsche person’s nose to post all the details of any given problem, diagnostic and repair technique. Everyone learns, particularly the poster when it turns out there is another option or technique. This Forum is similar to publishing a professional paper for peer review.

I have been talking to some of my former mechanics, several who own Porsche shops, about ways to use our accumulated knowledge. These guys are mostly in the 60-75 age, the “kid” just turned 50. There are over 300 man-years of Porsche experience in this small local group and there are more. Most would like to semi-retire but still continue doing something Porsche and have some income.
I want to see all that knowledge put to use and passed to the next generation.
What do you think?

Best,
Grady
gradyclay@hotmail.com

[scottb]

Dan: Name the time a place. A run this time of year would be fun!

[Grady Clay]

Dan,

How did it go

Best,
Grady

[DByers]

Grady,
Have not had time to get to that project yet. Family obligations (3 kids) and honey doos are first in line. I am hoping by the end of this month to attack this. I will document everything though and post here. I like to prepare well in advance of any project, car or other wise. Keeps the busted knuckles to a minimum.

[Randy Webb]

Which one of you techies will send JWW a free speech translator for his computer?

[Grady Clay]

Dan,

I understand completely. My youngest is a college freshman. Now I am looking for things to do. It is difficult to exaggerate how much fun we all have had with Scouts, OM, Science Fairs, Horses, sports, Go-Kart & SCCA racing, back packing, skiing, and much more. Enjoy while it’s available, they grow up fast.

Sunday afternoon Pelican KobaltBlau dragged my sorry bod to several computer stores and I am now set with a digital camera and all the trivia. No longer am I confined to just placing something on the flatbed scanner. I found a transmission with a complete speedometer drive in place so I will practice my digital photographic skills on this thread. It is a virgin aluminum case 901 but every thing in this area is almost exactly the same as your ‘71.

We will rely on you to photo-document for posterity everything you do. Difficulties, mistakes, and misunderstandings are probably most important. Please don’t be shy.

Best,
Grady