Beefing up my early 915 - opinions please

[ischmitz]

I keep hearing horror stories of blown differential side covers and broken ring and pinion's on early 7/31 915's exposed to track usage. So I am wondering if I should do some preventive work on mine. I figure the original 150HP engine did not really cause much wear during its first 120.000 miles. I had it apart for a blown 3rd/4th sleeve due to a bothered shift and put new synchros in at the time. The innards had little to no wear and all races were firm in their seats.

Since it's been holding up fine to the 3.6 for a couple of years now and some 20.000 miles with some track events and spirited driving but respectful shifting. However, an upgrade now might be a lot cheaper than full rebuilt sometimes down the road.

Some suggested to use the later style 915 covers with the second webbing ring and I remember seeing some aftermarket billet style covers, too.

So where do I source one of those side covers and what do they cost? Which one should I consider? I searched the Wevo website with no luck.

Cheers and Happy New Year,
Ingo

[cowtown]

Powerhaus sells one, I know. I picked up a late-model reinforced cover from Peter at Rennspeed (Rennsport? The guy who sells the fiberglass pieces cheaply) for $50.

[RarlyL8]

I'm looking for billet side cover myself. Please elaborate on where you purchased the unit.
Ingo, don't know if you followed my thread on this subject a couple of months back. I hurt my 915 during a hard launch. Damage was found to be in the pinion bearing area.
Wevo has a one-peice pinion bearing retainer for ~$250 that I may purchase along with the more robust side plate. From what I have seen the damage starts with the pinion gear moving laterally pushing the ring gear towards the side plate with great force. In my situation the car got into horrific wheel hop causing the gears to rappidly and forcefully beat on the side plate and bearing retainers. The side plate may have "stretched", I don't know. The plan is to re-inforce these areas and add a strong LSD that will distribute the stress to both axle bearings.
My tranny has an 8:31 which is supposedly more robust than the 7:31. I do believe that these transaxles can live with large engines if you don't abuse them. The parts I mensioned above just may give you an edge during that "oh *****" occasion that happens every now and then.

[ischmitz]

Yeah, I remember that thread and the carnage. I also saw the Wevo bearing retainer plate but no mentioning of the billet side cover. It makes sense that under heavy load you load up the pinion pushing on the retainer plate and the ring gear gets pushed sideways against the cover plate at the same time. Both result in the ring and pinion to change its contact pattern and wear excessively over time.

Then there is the catastrophic failure of the side cover. It simply disintegrates under the extreme sideways pressure of the ring gear and lets go of the differential all together. It doesn't really matter whether it is a limited slip, or and open or closed one. I would even think wheel spin due to an open diff is a plus here since it drains some of the torque.

Apparently, the side cover is the weakest link here and needs to be adressed first. So I hope someone can point me to some sources. There should be a web site that offers them somewhere, right.

Cheers,
Ingo

[MuffinMan]

I just installed the WEVO side plate. The nice thing about their unit is that is uses the larger 930 carrier bearing, giving more peace of mind. I started a 915 rebuild thread a couple of months ago, but have been very lazy about finishing it with the final pics & what not. I'll try to do that this weekend & you can see what some of the WEVO goodies look like, as I also have the WEVO bearing retainer plate and some WEVO transmission cooler items.

[ischmitz]

Rob,
How much is the WEVO sideplate and can you buy it as a separate item? I did not see it on their website or on Summit's website
Ingo

[Joe Bob]

Ingo and I were having this same discussion a couple of days ago....my 83 3.6 powered SC has a low rumble in second and third when really getting it on.....80,000 miles on the trans, 20K on the 3.6 install.

When Ingo was driving he felt that it had a "solid feel" while shifting and was not as loose as his earlier model 915.

Is this something to worry about or am I just being a nervous Nellie?

[ischmitz]

Wait until Tyson tells his story. He apparently blew out a side cover not too long ago on a 2nd to 3rd upshift with a 3.2. I think it IS something to worry about

[MuffinMan]

I believe the model number for the WEVO side plate is XT-100 and it's ~$368, and the 930 bearing is ~$47. You can get it either through WEVO directly or by asking Pelican to order that specific part number.

[MuffinMan]

Here is a picture of the WEVO sideplate. To the right are the 930 carrier bearing on the bottom, and the stock/original 911 carrier bearing on the top.

[Joe Bob]

Is this a trans on the stand fix or can it be done with it still in the car after dropping the axle(s)?

I have a factory LSD and I assume this is done on both sides?

[ischmitz]

Can be done with the tanny in the car and there is only one side cover

[Grady Clay]

I don’t know who makes the billet side cover but aside
from apparently being much stronger, it uses the larger
930 differential carrier bearing – very desirable. It’s too
bad Porsche didn’t do that for the 915 when they
reinforced the cover.

The big issues seem to also be the wear of the 4-point
pinion bearing where it can allow the pinion to move
axially even when fully torqued. I measured two this
past week that allow the pinion to move axially 0.120 mm
and 0.105 mm respectively measured using P285 with a
0.001 mm dial indicator. I applied up to 5 kg force axially
in either direction. It is clear from the deflection per unit
force where the end play stops and you are just slightly
deforming the casting elastically. Note that the smallest
pinion shim adjustment is 0.050 mm (they come from
Porsche 0.100, 0.150, and 0.200 mm.)

The aluminum main differential casting where the pinion
and input shaft 4-point and roller bearings press into
clearly become deformed where the outer races are no
longer a press interference fit. I looked at several this
past week where the outer race cold be easily pulled out
with my finger tips. I’ll measure those bores on several
this coming week.

During the ’84 Model Year production (starting Dec. 16, 1983)
Porsche changed the machining of the main differential
casting. They changed the roller pinion bearing from the
previous 915 62 mm OD (999.110.032.00) to the smaller
930 55 mm OD (930.302.391.00) in order to leave more
material between the pinion bearing and the input shaft
bearing. I don’t know (yet) if that made a significant
difference.

I think the biggest issue is wear from failure of the lubricant.
I think this is from several causes. First is too high
temperature. How many have a temperature gauge on
their transmission? Next is the creeping gain in torque with
the installation of larger/higher HP engines. These
transmissions were design with a constraint on the torque
and power expected. Additionally are the choice of lubricant
and the frequency of gear oil changes. How many change
transmission lubricant every few thousand miles or after
every track event? Of course we are using these
915s in their geriatric years – 20-34 years old. How many
have an all new 915?

It is instructive to read this from WEVO.
The late magnesium case 915s have a steel bearing
carrier cast in place.

I wonder how many “rebuilds” even measure the bores
and calculate the press-fit interference?

Best,
Grady

[Joe Bob]

Originally posted by ischmitz
Can be done with the tanny in the car and there is only one side cover

Isn't that where the axles come out of?

[cowtown]

Yes, you can do it in the car. Pull the axle/CV, then unbolt the flange bolt in the center. Pull out the flange/shaft piece, then pull the plate perimeter nuts off. You'll want a new rubbr o-ring when you put on a new plate.

[Joe Bob]

I'm gonna drive over to Ingo's and kick him in the a$$...

[ischmitz]

Originally posted by mikez
I'm gonna drive over to Ingo's and kick him in the a$$...

[Joe Bob]

He ran like a girl....but he did esplain that there is only one cover....since I have never had a 915 apart and he has....I stand corrected.

BUT IT DOES look like there are two side that come off....

I also got to ride my new/old Moto Guzzi over to his house in the rain break we just had.....the next storm be coming in....