Dilvar & plating
 

I'm thinking of having my used 993 dilvar head studs cadmium plated then repainted. Looks like the old timecerts have transfered a little corrosion to the studs, doesn't look like this has affected the studs but a good cleaning and coating would be nice. What type of coating would be good for these? Looking at a web site they show different types of cadmium plating, QQ-P-416F Type I, II, III Class 1, 2, 3. I will talk to them on monday just looking for some tips before hand, of course depending on the cost of having this done. Is Cadmium the best plating for this app?

Cad is hard to find for environmental reasons, of course it's toxic. The factory uses zinc with a trivalent chromate conversion coating-- this is the more environmentally friendly solution.

Personally I wouldn't monkey with plating and then coating-- just coat them. The hydrochloric acid used as a preparation step for the plating process can cause "hydrogen embrittlement" unless the parts are baked after plating-- there's a lot of chin music on this forum about whether and when this is an issue, but I think folks would stipulate to the fact that a head stud is just about one of the most stressed fasteners in all of Porsche-dom. Add to that the unique properties of Dilavar and I would REALLY not monkey with it.

The one word you use that frightens me is corrosion-- particularly where the studs sit in the case inserts. If there is ANY doubt- replace-- if you can see ANY surface pitting, etc., with a magnifying glass, you should replace that stud. Surface corrosion is a stress raiser and you do NOT want to get this engine back together, in the car, all broken in, only to have a freaking stud break off AND inside the case insert where you can't grab it with vice grips.

Good luck!

Hydrogen embrittlement usually occurs in electroplating when 'atomic' hydrogen is liberated and then diffuses into the grain boundaries of a steel component.

When a single hydrogen atom meets another hydrogen atom the they form a hydrogen molecule and the resultant pressure inside the crystal lattice causes cracking.

Any process that produces atomic hydrogen at the surface of a steel component can cause problems for certain type of alloy.

I am reasonably sure that Dilavar is an Austenitic Alloy and is probably not too sensitive to hydrogen embrittlement but is is very prone to stress corrosion cracking especially in the presence of chlorides

I would agree that electoplating this type of material without significant testing is not a great idea.

I think that there may be a family of epoxy-phenolic resins that could be used as a coating with good results.

+1
I would not add any more complexity to this already-complex alloy steel.

If the material is in good shape and you want to add some corrosion protection, have them powder coated or use an epoxy-based coating.

Sherwood

I've determined that re-plating would not be worth risking. Just repainting, going to try a firearms finish called Duraheat, can with stand 1800 deg f. I understand it is an epoxy based paint. The case was put through a ultra sonic cleaner, when it can back some of the paint was peeling off. At both the ends of the studs at the head nut and threads into the case most of the plating is gone, but no corrosion to the stud. After painting and installing I'll probably use a paint brush to coat the stud to the case and use anti seize at the head nuts. Could just use them as they are but repainting won't hurt.