De-Simplifying the Differential

MaquinaVerde · 04-22-2007, 04:20 PM

Today I had a chance to investigate and upgrade the simplified differential in my 70 911. I found the threads created by people who had done this before extremely helpful so at the risk of repetition I'm going to post some pictures I took of the process. Grady's information and Randy/Scott's account were particularly helpful, as was the article in Upfixin vol 4 (co-written by Grady). Project911's experience provided the motivation.

I wasn't smart enough to do this during the several months that my transmission was out while the car was being painted, but I was smart enough to eventually buy the parts and wait for an opportunity to dig in.

This weekend I was able to get my project 911 back on the road for the first time since July of '05. I drove it to and from the Pomona swap meet this morning, and have put a total of about 75 miles on the car since saturday. When I got back and started poking around underneath I noticed that 3 of the 4 CV boots (which I decided to reuse since they looked good) were weeping grease and one of them was spraying the wheel well. Needless to say, lesson learned. After cleaning up the mess I took off the axle shafts to replace the boots and repack the joints(again). I couldn't help noticing that the differential cover was quite exposed and some wheels in my head started turning.

The general opinion seems to be that removing the trans from the car is preferable to keep the inside of the transmission from being contaminated. My car is still pretty clean underneath so I decided to take the chance and go for it.

After draining I used an old steel stake to help me remove the old expansion screws. Once the bolts came out it was clear that my diff had never been retrofitted.

The differential cover came right off, and weighs practically nothing. Magnesium is neat.

Diff pulled, with the axles out of the way it comes right out with plenty of room. Really Really Easy.

The bolts exhibit a similar amount of wear as those seen in randywebb's thread. This is not a comforting wear pattern, and I'm sure glad that they're coming out before rather than after a failure. The main thing that caught my eye was the color - These bolts are gold and unlike any other color pictures I've seen. Perhaps they were replaced at some point? The car only has ~75k miles so it isn't super high mileage.

New and old hardware side by side

What the still-simplified diff looks like without the old bolts

The center spider gear shaft simply pushed out with firm finger pressure...I'd hate to see what would have happened if the bolts had let go.

Armature Added

A roll pin completes the upgrade

Finally it all goes back in with a new O-Ring. Putting it back in was as easy as taking it apart.

The parts I used were:
999-701-286-40 Rubber O-Ring
2x 901-332-275-12 Hex Screw 10x75
911-332-208-00 Diff Armature
900-308-005-00 Cotter Pin

Obviously, some gear lube was required.

I highly recommend that anyone who suspects they have this kind of differential check it out and perform the upgrade if necessary, it is child's play compared to most other mechanical operations we perform on a regular basis.

DUK · 04-22-2007, 04:25 PM

Nice write up Matt, Definately one for the archives. BTW nice clean car. Now get out there and get it dirty.

Grady Clay · 04-22-2007, 05:16 PM

Matt,

Great job and excellent photos.

You did this in the nick of time. I have seen the tits broken off with just this degree of wear.

Yes, the originals were cad plated as you see.

Good researching! Can you post links for the on-line sources you found?

I had forgotten about the Pano piece. For reference it is Up-Fixin der Porsche, Volume IV, page 47. Frank Barrett was RMR-PCA Newsletter Editor at the time (’74) and co-authored much due to my lame writing, pre-word processor. We still see each other regularly. He is now on this Forum.

I encourage everyone with these years (late ’67 to early ’71) 911 and 912 with the simplified differential to do this update. The carnage that happens when these tits wear and break off is unbelievable. The big differential pin slides out; the inertia of the car breaks the pinion, input shaft and case. The transmission is not repairable. Very little is salvageable.

Best,
Grady

MaquinaVerde · 04-22-2007, 05:32 PM

Thanks Duke, I'll be posting a thread on my car within the next week or so, my task list now is:

1) Put the axles back on
2) Readjust the clutch (and I had it adjusted perfectly before)
3) Raise the car 1/2-5/8"
4) Full alignment/corner balance

Grady,

When I pulled the first bolt out I was shocked at just how much wear there was and how identically it matched all the other cases out there. This really is a big deal, and I appreciate all the information you have put out there. I am very very glad I didn't put it off any longer.

Also based on the recommendation in your posts I upgraded my pinch-type struts to wedge pin type, I'm hoping for the same benefit as the differential upgrade: No loud expensive noises (or accidents). An added side benefit is the ability to bolt on S calipers.

The threads that were the most useful to me were:

1) Project 911's simplified disaster.
901 let go with a bang!

2) Randy Webb and Scott Clarke's upgrade
How To: Simplified Differential Upgrade

3) MikeDL's "Simplified Differential" thread
Simplified Differential!

I'm sure there are others. I still find the Upfixin article to be the most concise.

randywebb · 04-22-2007, 05:36 PM

I'm glad the threads helped you.

EVERYONE with a 901 or type 911 transmission should check theirs. You can check by just pulling off the side cover & drive flange next time you do a trans. oil change and CV joint inspection. Pulling the trans. out is not required.

Grady Clay · 04-22-2007, 06:02 PM

Matt,

When reinstalling the axles, here is THE thread:
Reconstructing Constant Velocity (CV) Joints

While long, it is worth studying it in full.

“I still find the Upfixin article to be the most concise.”
That is the benefit (or bane) of print medium.
On the internet there isn’t an editor to reduce the wordcount.

Best,
Grady

randywebb · 04-22-2007, 06:23 PM

"there isn’t an editor to reduce the wordcount."

- Actually, each person who posted on that thread could go back and edit down or delete extraneous material (such as Attaboys or whatever).

Grady Clay · 04-22-2007, 07:24 PM

HeHe, Randy I agree.

I tend to use complete sentences and other so our non-English
speaking members can understand or the language translator
programs work. Since this is archived, I want it to be precise
and not subject to misinterpitation.

With the late ’67 through ’69 there was a “threaded piece” that
was smaller diameter (red arrows) by about 1/3. As I recall,
there were several versions ’68-‘69 some without any provision
for the armeture.

It is critically important that the “threaded piece” properly fit
the “armature” as Matt shows. My recollection is that we
replaced the “threaded piece” to fit the “armature” with the
‘67-’69 901 transmissions. As I recall it was a special update
part, not OE.

Now comes the issue when some PO installed the “armature”
without replacing the threaded piece.” This allows the
differential shaft to move out of center by about 6 mm. I
don’t think the shaft can contact anything. The shaft is going
to stay in a displaced position and develop wear patterns that
are detrimental.

Whenever you are in one of these transmissions, check this.

The good news is Porsche introduced the appropriate “threaded
piece” earlier than the non-simplified update. This allowed
Matt’s update with his ’70 not requiring the later “threaded
piece.”

The update parts can be salvaged from 914 transmissions also.

Best,
Grady

Jandrews · 07:30 PM

Very nice documentary.

I have a '71 911/01 Trans and am wondering about this. First, a couple questions...

Are there any symptoms that this kind of wear is occurring?

I am having trouble picture what is wearing those bolts like that. Where do they go and what is causing that wear?

Why do the replacement bolts appear to be shorter than the originals?

Is there any reason not to replace that roller bearing while you are in there?

What function does the armiture perform? Does it exhibit any difference in driving after you install that piece, or is it simply a "bracing" apparatus?

Thanks,

JA

Grady Clay · 04-22-2007, 07:46 PM

“Very nice documentary.

I have a '71 911/01 Trans and am wondering about this. First, a couple questions...

Are there any symptoms that this kind of wear is occurring?
None

I am having trouble picture what is wearing those bolts like that. Where do they go and what is causing that wear?
The wear is when the differential is “differentiating,” ie, one axle is at different speed from the other.
JA,

Why do the replacement bolts appear to be shorter than the originals?
They don’t have tits.

Is there any reason not to replace that roller bearing while you are in there?
Never do that without resetting the major transmission specifications – pinion depth, R&P back lash and differential bearing pre-load.

What function does the armiture perform? Does it exhibit any difference in driving after you install that piece, or is it simply a "bracing" apparatus?
It simply helps to prevent destroying your transmission.

Best,
Grady

Jefpl8 · 05-10-2007, 04:41 AM

Hi Grady - Jeff Plate here with that '68 911 that is about 10 VIN increments from yours.

In your article "How's Your "Expansion Screw for Joint Flange" you make mention that the '68~'69 models the screw alone can be replaced. Can you explain that a bit more? Does that mean I do not have to buy a cotter pin and armature?

Also, I have a receipt from a PO that says the tranny had first gear rebuilt back in '95. Is it possible that the PO also had this problem addressed (IE was this problem widely known)? Can a 901 simplified diff make it to 140k without this fix?

By the way thanks for your advice from about a year ago in getting my car to pass emissions. Since that time, I've done a good deal of wrench turning and the car is really starting to come together.

Jefpl8 · 05-21-2007, 09:27 AM

Well, I dropped the drivers side inner CV joint and pulled the expansion bolt. There was some wear, but it did not look to be too bad (see pics). Does anyone know if there is a wear limit spec?

randywebb · 05-21-2007, 09:42 AM

I would just replace it ASAP. You never know when it is going to get a larger load on it from a bump or braking in gear or acceleration.

The results can be catastrophic.

And the parts only cost about $70.

MaquinaVerde · 05-21-2007, 01:30 PM

Jeff,

My worn bolt measures 4.65mm along the shaft (where you measured) and 4.17mm along the along the narrowest (most worn) part.

I purchased my parts from the local dealership, and they cost about $160 including tax and took a couple of weeks to come from Germany.

randywebb · 05-21-2007, 02:27 PM

wow!

Telephone (not internet) Pelican or use Stoddards. Maybe the parts have shot up in price since I did it.

Jeff Hail · 05-21-2007, 02:51 PM

I am going to frame this. Its a modern art masterpiece

randywebb · 05-21-2007, 03:09 PM

vntgspd · 06-13-2007, 07:12 AM

Okay...Help!

I have a '69 901 and I sourced the "armature", shaft, pin, "threaded pieces", and shorter stub axles that appeared to go with it. When I went to install, there was not enough clearance between the "threaded pieces" to fit the armature.

Grady, is this what you are referring to when you say that "we replaced the “threaded piece” to fit the “armature” with the ‘67-’69 901 transmissions. As I recall it was a special update part, not OE."

Are there different spider gears I can use? Is this "special" piece available somewhere? Am I SOL?

Thanks!

David

Jim_Brown · 06-13-2007, 08:05 AM

David-

I had to remove 4mm from the ends of the threaded pieces sourced from a 914 donor differential to get everything to fit in my '69. There are a couple of photos in the MikeDL Simplified Differential thread Matt has linked above.

Good Luck,
Jim

Grady Clay · 06-13-2007, 10:45 AM

David,

Which ’69 901 5-speed do you have? The 901/07 is the typical aluminum case transmission and the 901/13 is the typical mag case version. What is your transmission number?

Porsche had all this stuff in flux in this period and didn’t document it very well (meaning hardly at all). Part of my current mission is to get all this info posted on Pelican.

Yes, there were quite a number of threaded pieces – some fit the armature, some can be machined to fit and some won’t work.

I’m interested about your question about different transmission axle flange lengths. For a given transmission Type, I think the only difference is between an open differential and a ZF LSD.

Yes, I recall some situations where we had to change the spider gears. I think this was the situation where the later threaded piece didn’t fit the spider AND the armature. This may be the situation where the (never delivered in a car) update part comes in.

Best,
Grady