915 - R&P Axle Spline Differences - What years will fit my application?

[RarlyL8]

Ran into a problem before we even got started.

A little background info.
I purchased an '84 Euro 915 from a friend last year who removed it from his car due to an R&P whine. The tranny had been rebuilt 6 months previous and had made the noise from day 1. Upon examination it was determined that the genius who rebuilt the tranny did not set the pinion gear properly causing the noise and subsequently wear. The R&P must be replaced.

SO, after some searching I found a good R&P for the late 915. Not cheap.
Fast forward a year and I'm finally ready to get this thing fixed and in my car only to find that the R&P I bought is not correct for the tranny.

Here is a picture of the current ring gear. The splines are about 1/2 the size and number as the ring gear I bought and the housing is a LOT more beefy than the one I bought. I'll get a picture of that later.

So what years 915 have a ring gear that looks like this?
Anybody have a set they want to swap me for the other style?

SERIOUSLY bummed at this point. Time is a premium for me and this is a major setback. Any help appreciated.

[Matt Monson]

That's not an issue with the ring gear. You just need to remove the ring gear from the differential housing and swap it over. That's a differential issue. Use the diff you took out of the tranny and you should be fine. You are going to have to reset the preload and backlash anyways, so there is not point in leaving the ring gear attached to the differential it was delivered with.

You will want to buy new locktabs, but I generally consider it ok to reuse the bolts once. If you have any doubts to their history (look for tool marks from another mechanic being in there) then pitch them and buy new bolts as well...

[Grady Clay]

Brian,

What you have pictured above is a differential assembly with the aluminum gear to drive the RoW transmission oil pump.

There is the issue with the splines on the transmission axle flanges that mate with the two side spider gears in the differential. Porsche enlarged the spider gears, added needle bearings on two spider gears and changed the splines on the axle flanges (18 to 43). I’m pretty sure this is at the same time as the differential housing change. I haven’t (yet) researched exactly when that was and with what 915/xx transmission types but it is basically ’84 to ’85 model change. I’m sure there are other details that need to be documented.

I think this is simultaneous with the stub axle change from castle nut and cotter pin to self-locking nut.

Have you considered using an LSD in your application?

Best,
Grady

[Grady Clay]

Matt,

Will the 43 spline spider gears fit in the earlier differential housing?

I’m pretty sure the later differential housing will fit all earlier 915s. What do you think?

Best,
Grady

[Bill Verburg]

As Grady said, thats the drive for the oil pump
here's mine


It mates to this

[RarlyL8]

Thanks for the responses.

Ignore the oil pump gear ring. That is a good thing.

My problem is the axle flange spline size and the general wimpiness of the unit I bought.
The '84 ring gear housing is much more robust.

So you are telling me that this design is unique to the Carerra?

What are my options?
Do I need to sell the older style R&P and buy a Carerra R&P or can I swap out the ring gears and buy a set of the old style flanges?
My current tranny is a 915/61 in my '78 SC. Does that tranny have the old style R&P and flanges?

Yes, an LSD would be SWEET! Now if I only had an extra $1500 laying around ...

[Bill Verburg]

3 different joint flanges and matching diffhousing used on Carrera which are different from SC

these are the joint flange #s
915/67/68 use 915.332.209.12 MY84
915//69/70 use 930.332.209.03 MY84 t/l also used on 930/34/35
915/72/72 use 930.332.209.05 MY 86+86 also used on 930/36/37

[Geary]

Originally Posted by RarlyL8

I purchased an '84 Euro 915 .... The R&P must be replaced.

I found a good R&P for the late 915 ... (but) find that the R&P I bought is not correct for the tranny.

Here is a picture of the current ring gear. The splines are about 1/2 the size and number as the ring gear I bought and the housing is a LOT more beefy than the one I bought. I'll get a picture of that later.

You may have to explain a bit more clearly .... are you sure you're talking about the ring gear? The ring gear has 31 teeth on it, but no splines, and you're not really showing it to us... it's on the other side of the diff housing.

The splines that I see in the picture are those of the internal side gear, which mate to those on your output flange ... but you either have 43 smaller splines, or 18 larger splines ... if you say the picture shown has splines that are "1/2 the size and number" as what you purchased, I'm totally confused. Early flanges have 18 large splines, and late ('78-'86) flanges have 43 small splines.

Originally Posted by RarlyL8

So what years 915 have a ring gear that looks like this?

The diff housing shown can be found in '78-'86 915s, and the late 8:31 R&P can be found in '76-'86 915s.

[Matt Monson]

I guess I am just a little slow. I still don't understand how this is an issue with splines and axles. Here's the issue as I see it with the solution included:

An complete '84 915 transmission was purchased. It had a bad ring and pinion.

A replacment ring and pinion was purchased. It sounds as if the ring and pinion showed up attached to the differential carrier, which is likely an early 915 course spline differential.

The solution is to rebuild the '84 915 transmission using everything from that transmission and adding the replacement ring and pinion. It shouldn't be rebuilt using the carrier that came with the replacement ring and pinion, just the ring and pinion.

There was no change in spec or bolt pattern throughout the evolution of the 915 ring and pinion. If the only part changed is the ring and pinion, there should be no issue unless the part purchased was not a 915 ring and pinion...

To clarify it for my little pee wee brain, could I trouble you to post a picture of the replacement ring and pinion that was purchased? Also, a reference to the production date that is stamped into the ring and pinion would be nice as well...

[RarlyL8]

It sounds as if the ring and pinion showed up attached to the differential carrier, which is likely an early 915 course spline differential.

The solution is to rebuild the '84 915 transmission using everything from that transmission and adding the replacement ring and pinion. It shouldn't be rebuilt using the carrier that came with the replacement ring and pinion, just the ring and pinion.

BINGO!

I did not mean to be vague but I have never taken one of those assemblies apart before and did not know if the ring gear alone was transferable to another carrier of a different year/design.

If it is I am golden. Any caveates on doing that?

[Matt Monson]

See my original post in this thread about lock plates and bolts...

[RarlyL8]

Better safe than sorry, where do I buy new lock plates and bolts?

[Matt Monson]

I bet Wayne sells them. Since it's his site, I would send you there first. If he doesn't have them, I can get them for you.

[RarlyL8]

Thanks! I'll check.

[Geary]

I imagine you're going to need more than just lock tabs, as replacing the R&P is essentially rebuilding the transmission ... NOT a home job, if you're not mechanically inclined.

[RarlyL8]

I accomplished my first oil change last week. Changing a tranny R&P can't be that bad.

All kidding aside, I've never done a complete tear down on a 915 so this will be a good experience. The challenge will be setting the R&P depth and relation. I'll start a thread on the rebuild and we'll see how it goes.