Simplified Differential!

[MikeDL]

I pulled the engine and transmission yesterday and as per Grady's suggestion, took a look at the stretch bolt from the axle flange. What you see in the picture is the kind of bolt that indicates that the transmission still has the "simplified" differential which can apparently fail in a pretty violent way.
So, I now have another significant rebuild operation in front of me. In searching through the PP archives, I learned that this was fixed in a later Porsche update. Where would I go to track down the instructions and necessary parts for the update operation? Are there any other things I should look to do when I tackle this job?



Thanks!

[Grady Clay]

Mike,

Not to worry, you are 90% there just finding this. I used to have a collection of these bolts from OE new to tits necked down to only 10% diameter and finally broken off. The results of both tits braking are catastrophic transmission failure and possible loss of vehicle control.

The parts are readily available in a donor 914 transmission. I’m sure I listed them and diagrams in prior posts. Do a search on my name and “simplified differential.”

Good for you looking – you just saved a 901/911 transmission, and your ars.

A quick look shows the following transmission types with the “simplified” differential:
901/03
901/06
901/07
901/10
901/12
901/13
901/75-84
902/01
902/02
902/50
905/00
905/01
905/13
905/20
911/00
911/01
911/80-84

This certainly isn’t an all inclusive list and many survivors have had the update. There exists a Porsche+Audi recall list. Who has a copy?

The alternative to the upgrade is having the differential pin slide out and saw the transmission in half. Not good.

If anyone has images, please post them.

Best,
Grady

[MikeDL]

Thanks so much for the help, Grady!

I did a few searches and came across a thread about the simplified differential where you discussed the issue with pictures and a great explanation. Is this the one you mentioned, or am I missing one?

broken transmission, any ideas?

In particular this picture is terrific because it tells me what I should be looking to find:



It looks like I need the two bolts, block #3, roll pin #1, and pieces #5 & #6. Is the best place to look for these parts a junk transmission with a compatible differential? If so, what differentials could the pieces come out of? I saw in a different thread about transmission where you listed some of the changes between 1968 and 1971 - some of which sound incompatible (e.g. axle flange bolt pattern and shaft diameter). Can I ignore those issues for the purpose of finding the needed parts and just look for a "fixed differential" 901?

Also, the reason I was wondering if there were published update instructions is that both of my 911 shop manuals ignore the subject of differential work, recommending that I "entrust it to my Porsche specialist".

[Grady Clay]

Mike,

What year, type and S/N is your transmission?
Is it aluminum or magnesium?

You are correct, you need the two long “elastic bolts”
901.332.902.12 It won’t have the tit (blue circle.)
Make sure the ones you get have this washer centering
step (green arrow).



The #3 block “armature” 911.332.208.00 and the #5 roll
pin (6x28) 900.308.005.00 are needed.

The threaded piece 911.332.206.01 will have the extension
(red circle) that fits in the armature block (brown arrow.) There
are some threaded pieces that don’t have this extension and
consequently don’t keep the big shaft in place.



Depending on your transmission, some #6 differential gears
weren’t cut to fit this threaded piece. In that case you need
to replace the set of four pinion gears. Inexpensive good used
will do just fine.

If you use a donor 914 transmission, remember that the gear
cluster must be removed before you can get the differential out.

This set of parts was first production installed in about ’71 and is
used through the ’77 915. In ’78 they figured a better (simpler)
way that manages to occasionally crack the differential housing.

This is a very easy DIY update on the ’69-’71 Mag case
transmissions. I recall it being somewhat more (still DIY)
on the ’68-’69 aluminum casting transmissions. Perhaps
that is where I remember the threaded piece issue.

I don’t know what manuals you are referring to but I suspect
they are talking about resetting the ring & pinion gears and
the pre-load on the differential bearings – something you
don’t need to do with this upgrade.

Best,
Grady

[MikeDL]

Here is the information from the bottom of the transmission:

901/03 2281674

You identified it (in another thread) as "delivered in 911T ;68, non USA. 5-speed, geared AFMSX, 7/31"

Which fits because the car is a euro 1968 911T.

I think the case is magnesium but perhaps I just need to scrape off more caked on filth. The finish looks pretty rough and a dull grey color. There's a picture of the tranny on my web site, but I don't think it'll be much help until I get it cleaned up.

So I need to find a donor tranny from 71 to 77, preferably from a 914?

I have the Clymer and Hayes manuals. Neither has anything on differentials beyond how to remove and replace them as a unit.

[randywebb]

"very easy DIY update on the ’69-’71 Mag case transmissions."

_ It is very easy once the trans. is removed and opened up - is that what you mean? Or is there some clever way to do the #3 and #5 parts w/o removing the unit - taking off the side cover maybe?

[Grady Clay]

Randy,

Either way. I’m a proponent of simply taking an engine and transmission out of the 911 to do any extensive work. Things get done better on a nice bright, clean workbench, properly cleaned, tools and spare parts at hand, and not having road dirt in your face.

I think this should be priority #1 for DIY Pelicans.

Yes, you can undo the two inboard CVs, remove the “elastic bolts”, un-do the clutch cable, remove the throttle link, remove the differential side cover and you have the differential assembly in your hands. Aside from the “simplified update” parts, all that is absolutely necessary are a differential cover O-ring (or gasket), two CV gaskets, a set of CV bolts & Schnorr washers and some transmission lubricant. Depending on the 911 and person, there may be some sand & gravel installed with the differential.

Either way it is a relatively simple update in a mag case.


I will be interested to see Mike’s ’68 911T. The 901/03 is the aluminum casting. It will have the smaller 901 differential gears. It will probably take an unusual set of parts (differential gears & threaded piece possibly from a ‘69) in order to properly fit the update armature block.

Best,
Grady

[Scott Clarke]

Mike-
Did you find the parts you needed to get this done? I need to do the same to mine.
-Scott

[MikeDL]

No, I haven't. I need to take mine apart and figure out exactly what I need - I'll be posting here once I do.

[randywebb]

I thought I'd revive this thread rather than create a new one....

Grady - in one thread you referred to the simplified differential as being in the 1968-70 cars.
Would a '71 also have that?
My 911 trans is 911/01 #7114447 supposed to be from a '71. Does it have a simplified differential? I think I recall the little points on the ends of the bolts... tho maybe a bit shorter than in the pic above.

[YTNUKLR]

I have a spare '67 complete diff (open) if you need it for parts

Thanks, Scott

[ErVikingo]

I have a complete diff if you are interested (replaced it with a ZF ).

Cheers, Juan

[Grady Clay]

Randy,

To the best of my lame memory, some of the ‘71s had the update originally installed. I have never seen a 915 without it. Interestingly, I have also never seen a 914 without the update; they preceded the 911 by 1-2 years. In ’69-’70 we were replacing the stretch bolts with tits at every opportunity until I discovered the 914 parts.

I would like to again see the P+A sheet on this subject, mine is long lost. Think the date was in ’72.

The prior non-“simplified” differential is the ’67 and earlier with the aluminum caps in the middle of the axle flange. Any ’68 through ’71 is suspect of having a “simplified” differential unless you have actually confirmed the update pieces are installed. It just isn’t worth the risk that something like ” 901 [911] let go with a bang!” happens. There are precious few of these transmissions remaining.

Best,
Grady

[ErVikingo]

Grady,

Can I tell if the one I took out had the update without taking it apart????

Thanks much !

[Clay Perrine]

For those of you who are interested...

I have two complete 914 differentials (no bearings or ring gears) that I would sell. To install them all you have to do is install your spacers and bearings, and move the ring gears over. I do not have the bolts for the drive flanges, though. IIRC they are available as replacement parts.

PM me if interested.

[Scott Clarke]

I took a look at mine last night, and thought these pictures would be of interest. These components are from a 911/01, '70 model year, made late in '69. First is the pair of #5's listed above. Grady-
Are these compatible with the armature? Note that these are splined on the outside. These splines fit into the same female splines of the differential gears that engage the drive flanges.

Here is where they fit. We are looking inside the diff, with the splines that receive one of the threaded inserts (#5) at the bottom of the picture.

Last, this is the differential gear axle (#2) pushed outside of the diff. The hole is where the axle bolts engage. I assume that the roll pin (#1) fits in the same hole?

[Scott Clarke]

Clay-
PM sent.
-Scott

[Jim_Brown]

One more time around on this thread...

My ‘69T is equipped with a mag case 901/13 with the simplified differential (SN 7193323).

Here are the worn ends of the 901 stretch bolts. A big thank you to this board and to Grady Clay. It looks to me like I was very close to a full blown 901 destructo episode.



I’ve located a donor 914 tranny for the differential parts. Here are the two differentials (901 is on the left):



I disassembled both differentials hoping I could put my 901 back together with the armature, roll pin, stretch bolts, and threaded end pieces from the 914 (#5 in the diagram Grady posted). No such luck. My 901 differential is smaller, with smaller spider gears and longer axle shafts than the 914. The pin/shaft that holds the spider gears didn’t fit (see pic below). The threaded pieces fit well in the spider gear, but are too long for the smaller 901 differential.



On the left in the picture below is the #5 threaded piece from the 901 differential with no extension to retain the armature. It looks like I'll need to have the 914 piece (on the right in the picture below) machined to remove about 4mm such that it matches dimension A on the 901 piece.



Before I destroy something here, what other options do I have? I couldn’t get the spider gears from the 914 to fit in the 901 differential - too big. The output flanges on the 914 axle stubs are too small for my CV joints so I can’t do a full swap...

Thanks,
Jim

[Scott Clarke]

Jim-
Did you try flipping the 901 threaded piece upside down? I was able to get my 911 pieces to fit both ways, so perhaps it is possible.
-Scott

[Jim_Brown]

Scott-

I gave it a try - no such luck. The diameter of the extension on the other side of the 901 piece is too large to fit in the armature.

Thanks,
Jim