904 LSD open heart surgery

[ajwans]

Thanks Jon. I do appreciate it. While I have the attention of experts, what's the opinion on safety wiring both the cover plate and crown wheel?

I've seen it done and it looks much neater than peening over lock tabs. I would like to know if one has any real advantages over the other.

andy

[Jon B]

Jon, any chance I could bother you to measure the thickness of your steel thrust washer there? I don't trust the one I have as somebody put a 0.6mm shim washer behind it.

Andy, one of my #3 steel thrust washers measures 1.955 mm. I really don't want to empty the differential to measure the other one.
The room temperature is 15°C ;-)

But mine are also used, so this measurement doesn't represent a new dimension.



Jon B.
Vista, CA

[Jon B]

While I have the attention of experts, what's the opinion on safety wiring both the cover plate and crown wheel?
I've seen it done and it looks much neater than peening over lock tabs. I would like to know if one has any real advantages over the other.

None of those fasteners are prone to coming loose in a 2-litre car, and either method is effective. But you'll have to drill them all to use safety wire, and it's really not worth the extra effort.
If your lock tabs don't look "neat" when you've peened them over, then you just need more practice...

Jon B.
Vista CA

[66S]

One weakness of the 904 ZF lsd is the end cover weld can break. The 904 ZF manual shows bolts on that end. I wonder if it could be drilled and tapped for bolts to help reduce the stress on the weld?

[Jon B]

One weakness of the 904 ZF lsd is the end cover weld can break. The 904 ZF manual shows bolts on that end. I wonder if it could be drilled and tapped for bolts to help reduce the stress on the weld?

The common 904 ZF like Andy's is not welded on the end cover, but on the main housing.

I posted these photos a few years ago, of a 904 diff housing that's cracked completely through the original weld. I still have it here, and haven't attempted to repair it.
There's no way to reinforce these with bolts. Partial cracks can usually be repaired.

Another common problem with these housings is that the bearing journals wear down to the point that the bearings become loose, and will then wear down the adjustment shims.





Jon B.
Vista, CA

[Olivier]

Thanks Jon. I do appreciate it. While I have the attention of experts, what's the opinion on safety wiring both the cover plate and crown wheel?

I've seen it done and it looks much neater than peening over lock tabs. I would like to know if one has any real advantages over the other.

andy

I've seen (on a large gear industrial reducer) one head screw which broke (fatigue). It was held by safety wire. So after a whil the head whis was still hanging from the wire, went where it shouldnt: between 2 gears... I let you imagine the result...

If torqued properly, shimed properly, and designed properly, any screw should "untighten" by itself, but it's a debat which could keep busy an army of expert for years.

The only diff I disassembled was a bmw E30. The crown was nearly impossible to unbolt. They used high grade screws, in conjonction with "hard" loctite glue.

Good luck!

Olivier.

[gled49]

I also have seen a 904 diff bolt break and hang on the lock plate, tearing up housing. I always use new bolts and loctite them, sometimes using lock plates, but most of time not. I’ve never had a bolt break or come loose. I believe if a fastener come loose it wasn’t tight. G

[ajwans]

Gordon do you use the OEM bolts or other? If you leave off the plates I suppose you don't need the special machined bolt heads.

andy

[gled49]

I always use Porsche bolts, but new bolts aren’t graded like in the olden days. 10K was the old grading, if you found a contemporary bolt of the same dimensions in 10.9, I suppose it would be ok. You never told us what the application of this diff was for. Almost all these early diffs, like Jon pointed out, have worn S1 bearing journals and are past their high performance application use. G

[ajwans]

I'm finding Molykote G paste hard to find in small quantities, I can have a 5kg tub for $1000 but I'll pass hard on that, it doesn't look like anybody imports the small tubs to Australia. The alternate assembly paste is LM 348, I guess this is a Liqui-Moly product although possibly NLA.

What do you suggest for reassembly in the absence of Molykote G?

andy