Broken '86 915/930 ZF LSD. What should I do?

PcarPhil · 03-11-2013, 05:09 AM

Hey there Pelicans,

Here's an interesting one for the group. Last year I sent my '86 915 LSD trans to Gary Fairbanks for a rebuild due to a 3rd gear issue. During the tear down he called me with some news. My diff case was broken with the flange piece barely hanging on. He was shocked that this hadn't caused a problem. I think he literally broke the remaining piece off with this hands. I was shocked too as this trans is installed with a 3.6 engine and I've had zero diff issues that I'm aware of. Gary mentioned this was the second time he'd seen this happen in 30+ years. I have a couple ideas of how it may have happened but nothing conclusive.

Anyways I ended up replacing the diff with a new one as at the time I couldn't find a replacement LSD diff housing. So now I have this broken diff and I'd like to figure out what to do with it.

The options I've thought of so far:

1. Find the part number for the case (I've been unsuccessful at this) and replace the case reusing the existing internals (Gary thinks the internal LSD pieces are ok).

2. Sell it as is. The internal LSD parts must have some value to someone.

I could use some help with both these options. As mentioned I haven't been able to find a part number for the LSD case. Do any of you happen to know what this part number may be? Or on the flip side what would be the value of this '86 915/930 ZF LSD as is?

Here are a couple pictures. As always thank you for any help you can provide!

PcarPhil · 03-11-2013, 05:13 AM

Also if anyone has any ideas on how this may have happened I'd love to hear them!

Steve@Rennsport · 03-11-2013, 06:20 AM

I've seen lots of these over the past 38+ years, mostly in race cars and lately with high-powered street cars like yours.

Thankfully, Guard Transmission's chrome-moly cased LSD's (and a redesign) made this problem obsolete, however it still occurs with factory ZF LSD's in high-stress applications.

Those LSD's have cast & machined cases which fracture under shock loads in high-torque situations and there is little one can do to prevent it besides using less-grippy tires. Big engines and good tires create the environment for this to happen.

KTL · 03-11-2013, 06:23 AM

Matt @ Guard or Roger @ CMS might be able to help you with some spare parts.

My understanding is one of the features of Guard's GT chromoly diff is its increased side cover strength in the area of the bearing support areas. Point being is, they are aware of these sorts of failures and probably have seen more than a few broken diff end covers.

Matt Monson · 03-11-2013, 06:35 AM

Hello,
As mentioned, we see this fairly often and actually use an 8620 heat treated end cover on our LSD to help prevent this.

The reason it often doesn't cause a catastrophic failure is because the stub axle boot keeps the flange in place and everything stays centered until you take it apart. Last year Pat Williams had this happen on one of his 600whp racecars. He didn't know until he took the box apart as he was going in to replace a broken gear.

I don't have spare end covers but have a customer who would be interested in your ramps. He has a later g50 with 20/100 ramps and would like your 40/40 ramps. Pm me if interested and I will give you his contact information.

chris_seven · 06:41 AM

Quote:

Originally Posted by Kaefer

Also if anyone has any ideas on how this may have happened I'd love to hear them!

Simple fatigue failure is the only sensible explanation.

Probably 95% of all component failures are associated with fatigue in one form or another.

I have a 3-D model of an Escort ZF end casing as we have manufactured a few spares and it wouldn't be hard to change to suit a 911 unit as the plates are compatiblem and sizes are similar. I have a ZF I could measure if this would help.

Any half decent machine shop should be able to make a new part from a 4140 or 4340 steel.

KTL · 03-11-2013, 06:42 AM

Quote:

Originally Posted by Matt Monson

Hello,
He has a later g50 with 20/100 ramps and would like your 40/40 ramps. Pm me if interested and I will give you his contact information.

Yeah, I needed this broken diff when I sent you that 20/100 to build a few years ago! Ended up selling it to a guy in Denmark.

PcarPhil · 03-11-2013, 06:48 AM

Interesting. I think I know when and what may have caused this. Coincidentally it wasn't too long before I sent the trans to Gary for rebuild.

3.6 + full throttle in upper RPMs of 1st gear + sticky tires + sticky pavement = BANG!

Yes I was doing something I shouldn't have ---> using full throttle in 1st gear with a 3.6 + 915 combo. I remember hearing a BANG and wondering "what was that" but yet nothing seemed to be broken.

Oops.

Thinking about this some more - I think another Pelican mentioned a similar problem last year but his diff was broken on the other side. I'll try and find that thread. Perhaps we can make one good diff out of two broken diffs.

Matt Monson · 03-11-2013, 06:54 AM

Yep, they break on both sides.

PcarPhil · 03-11-2013, 07:00 AM

Is a non-LSD case different, or would that work? I've got a spare open diff '85 915 trans that I could open up if the differential case could be used.

By the way I have zero experience working on transmissions and differentials.

PcarPhil · 03-12-2013, 09:38 AM

Quote:

Originally Posted by chris_seven

I have a 3-D model of an Escort ZF end casing as we have manufactured a few spares and it wouldn't be hard to change to suit a 911 unit as the plates are compatiblem and sizes are similar. I have a ZF I could measure if this would help.

Any half decent machine shop should be able to make a new part from a 4140 or 4340 steel.

Chris,

I may take you up on your offer. I'll send you a PM/email this week to discuss details.

Unfortunately I haven't had any luck finding another broken ZF unit to use to make one good unit. Ideally I'll be able to reuse this LSD in my spare '85 915.

Thank you to all for the info,