LSD rebuild - replace only friction discs?
 

Hi

I have a 915/63 limited slip transmission in my race car. I am rebuilding the transmission, and I am sure that the lsd is getting tired (since everything else in there is!)

I have done a search on here, and the few threads talking about rebuilding limited slips in a 915 talk about replacing the discs with external splines, measuring stack thickness, etc.

Assuming I don't want to change the breakaway torque from what it was when the lsd was fresh, is there any reason why I can't just replace the friction discs? Do the discs with external splines wear down?

What about the belleville washers? Are they a wear item?

Just trying to simplify my life here - Pelican is showing the washers as a special order item, so if I can avoid "fixing what ain't broken", I would like to.

Thanks

Matt Monson of Guard Transmission posts here, ask his advice

Will do!

Hello,

I would advise taking the LSD apart before guessing regarding anything it might need. It could be as simple as a bell. washer broken in half.

Technically, you also want it on the bench when you break test it to see if it is in fact worn out.

That said, breakaway torque is not the end all of determining the tune on an LSD. If using all factory parts, the factory does give you an acceptable range, but that's not a race tune in the first place. That just means it works as the mothership intended.

Sometimes you can replace some plates and not the others. It all depends on wear. It also depends on how it is stacked right now. It might go: steel, steel, friction, friction. Generally on a race diff you would want:steel, friction, steel, friction. Twice as many friction faces.

Yes, I manufacture and stock bell washers that will fit even though I am no longer selling frictions and steels for the ZF LSDs. No need to special order them. And once you know the condition of the rest, and have an action plan of what to replace, I can advise what thickness of bell washer will likely work best. Unlike ZF/Porsche, I manufacture them in a variety of thicknesses to alter their spring rate and thereby the preload on the unit.

Regards,

Matt

What Matt said. Add to it:

- I do a visual check on the clutch disks for condition, then test for break-away once assembled.
- The LSD will frequently need one or more thicker shim plates (the ones that come in three sizes), to address wear to parts notably the ramps; measure and adjust per workshop manual
- Check the clutch disks, shim plates, and thrust rings for any damage from debris: gouging, galling, whatnot
- Measure height of the cup springs (bellville washers), old ones can fatigue and lose height and spring pressure affecting preload
- Check the thrust washers. Their pressed-in lock tab creates a couple classic stress risers which can propagate cracks and fracture the washer.

Good reminder from Kevin. The thrust washers are nla. I have them.

Thanks very much guys. I will do some checking...

I have several questions:

1. Can anyone comment on the effect of a moly type of grease in a 1977 930 with limited slip. I have mine apart - breakaway torque is around 8 lb-ft, while spec is 29-58 lb-ft. The transmission was apart once before, and the mechanic who assembled it seems to have slathered the inners, particularly the differential carrier, in CV grease. There was black grease all over the transmission inners, though the oil remained relatively clear blue (SWEPCO, I assume), so the oil and grease did not appear to mix. Could the CV grease, or whatever it is, affect the functioning of the limited slip?
2. The plates in my limited slip look pretty good. Though the moly surface on the friction plates has clearly worn some, they measure 1.8-2.0 mm thick. None of the outer splined plates show wear or damage. I don't see a spec for the height of the belleville washer. The thickness is 2.0 mm. Where are the specs for these?
3. There were no lock plates on the ring gear bolts, but they were secured with red Loctite. It seems to me that this might be fine, but it's not what the manual calls for. Any thoughts on that?
4. Will I regret it, if I assemble the limited slip plates steel-friction-steel-friction-steel, instead of the stock steel-friction-friction-steel? This is not a track car. The car has 79,000 miles on it. The previous work on the transmission was to install an Andial 8:39 ring and pinion.

I can probably answer some of this for myself on the bench, but I would like to hear what others have found before I go down that path.

Tom, there should not have been any grease in your transaxle or diff. I use gear oil, with just a tiny dab of moly mixed with oil on a few spots (side gear thrust surface, backside of spiders).

Loctite is now SOP over lock plates with most mechanics.

40% is the norm for street (1 active friction disc per side). You may regret setting to 80% for street use.

Setting friction discs back-to-back is silly, IMO. Just get get two more plain plates for your 40% setting when you purchase your new friction discs.

Some of the factory plain plates are soft, and require replacing (if installed alongside a new friction disc). Scored ones are OK when used as spacers only.

I can simply reverse the friction discs, as the sides where they face each other are perfect. This will get me fresh surfaces, at least, which should work if the parts are thick enough.

You probably have all the parts you need. Just clean out that grease and reverse those discs.

Yeah, that grease thing is weird. I've grown to despise the smell of gear oil in recent years so I use straight 50w engine oil on assembly. I grease the thrust washers and under the spiders on the cross posts. But not the clutch assembly!

Frictions should be 2.0mm. If you have some down at 1.8mm they are worn.

Steel outers range from 1.9, 2.0 and 2.1mm. That's how they fine tune them. Replace with like dimension if you don't want to change the tune on the LSD.

Bell washers on ZF LSDs are always the same. We make various sizes to tune with, but OEM has only been one size fits all.

If you don't track the car, don't switch the tune on it.

Very helpful information. Thank you.