Copper nuts and stainless studs

[tref]

It has been suggested that whilst I have the engine out I should replace the exhaust, turbo and wastegate studs and ensure they are fitted with copper nuts.
Given the absolute nightmare getting the exhaust off the turbo was, I'm inclined to agree. (The main problem was the nuts securing the exhaust to the wastegate were rounded, and I didn't want to wreck the wastegate studs grinding them off, so I took the exhaust off with the wastegate attached"¦oh yes you can!).

Are they stainless steel and copper as standard? If not, is replacement with these suitable "" I have used copper exhaust nuts before, and seem to recall them as easier to remove longer term. Any ideas of a suitable supplier? All fixings associated with the exhaust system, or just some of them?

Two of the studs came out of the turbo anyway, the other two seem in good health, and I am inclined to leave alone, in preference to breaking them off and having to drill them out.

Any arguments and opinions gratefully received!

Regards,

Tref.

 

[Lancerlot]

Although it is some time since I had a 924 Turbo, I would imagine that the studs and nuts are manufactured from nimonic material.
Nimonic is a nickel-chromium-cobalt alloy being precipitation hardenable, having high stress-rupture strength and creep resistance at high temperatures (up to about 950°C). It is a widely used and well proven alloy in high temperature conditions.
You would be wise therefore to use OEM replacement items if they are available. At a push you may be able to use high grade stainless steel nuts in high heat applications eg. turbo studs or copper alloy items lower down on exhaust pipes.
Don't use normal steel nuts (even HT mateial) since these have a tendancy to seize in use because the heat buildup "welds" the fixing.
Replacement parts are usually obtainable from Porsche suppliers, turbo specialists or specialist fastener suppliers.
Hope this helps!

 

[tref]

Just the sort of advice I was after, thanks...

But... is there an advantage to using disimlar materials for the studs and nuts? For moving parts, I usually reckon they should be disimilar, but, not sure that the increased corrosion would sway me against it in this case.

Regards,

Tref.

 

[tref]

Trawling the net, I found this graph (no, not from an SS supplier!) which would suggest that 316 Stainless steel is not a million miles from the performance of a Nimonic alloy, but obviously a lot easier to get hold of. The possible difference is at the higher temperature range.

The best I could come up with for temperature was a sparkplug range of typically 400-900 C. How does this compare with turbo exit temperatures?

Regards,

Tref.