Seal washers, aluminum versus copper?

[dirkbalter]

As I am reassembling the components, I recognized that aluminum as well as copper washers are used in similar applications. What is the theory in one versus another. Does it matter? I was planning on using copper throughout?

[MikeSimon]

Mercedes Benz changed from copper to aluminum. Right now, it eludes me what the reasons were, other than from a certain time on, copper was no longer available. Crush gaskets/seals have a very specific way to be applied. usually copper crush gaskets are not applied with torque but with angle.

[BaronYoungman]

I also am perplexed by it. On the short bosch fuel pumps the inlet has an large aluminum Crush Washer and the outlet has a smaller copper Crush Washer. I have just stayed with the same that Mercedes had but I've always been curious as to what the logic is, maybe someone more knowledgeable about metallurgy could answer the question correctly.
Bob

[wwheeler]

Interesting question and have been wondering this for a long time. There are two properties that I think are significant. 1) compressibility of the material 2) compatibility of seal ring material with base material

So the first, clearly copper can handle more compression from torque than an aluminum seal ring can. But aluminum would seal better with lower fastener torque values. So for items that have a relatively high torque, copper would seem to be the choice. Items that have low torque or are prone to leak, aluminum would seem logical. I have had a couple of places that would not seal with copper rings and aluminum sealed them right up.   

For the next property, I am talking about using an aluminum seal ring on an aluminum surface. Because the two materials are alike, there is a very real chance one of the sealing surfaces will gall. Either the seal ring itself or the sealing surface on the component. I have seen that happen and is not a good thing. I use copper seal rings on aluminum components when I think there is enough torque to seal the copper. 

Aluminum will tend to corrode more than copper in coolant and maybe that is a deciding factor. I am not sure it really makes that much difference and am probably way over thinking this. 

[wwheeler]

Bob brings up yet another point of confusion with metal seal rings. There are two types of seal rings. A solid metal ring and then a seal ring that is hollow. I have always called the hollow seal rings a "crush" washer. When you tighten the fastener with a crush washer, the hollow ring literally collapses and crushes. The solid metal seal rings obviously do not crush.

Hollow crush rings are meant for lower torque applications and are usually a one time use. Two places I have seen the hollow seal rings are on the (3) valve cover bolts and a large diameter seal ring used for the early oil pans with the very large drain plug. There are probably others, but these are the only ones I have seen.