Tom M'Guinn

On the 944, a negative battery cable bolts to the firewall to ground the chassis. Porsche actually put a copper boss with copper threads in the firewall for this purpose, presumably to ensure a good electrical connection with less corrosion -- but my threads are stripped.

Can I put a stainless steel helicoil in the copper without causing any problems from dissimilar metals? I wouldn't want to weaken the ground connection over time if those two metals don't get along in that environment. I can always use a nut on the back side to keep it tight. Any metallurgists out there?

951Dreams

My only thought is that the SS isn't the best conductor. It conducts, but not near as well as copper. I would think it would conduct well enough, but that's one connection I'd want to be sure of.

g-50cab

you in a place close to salt water? I'd stick to regular steel bolt - or copper bolt and nut (you can get from McMaster-carr) - a ss helicoil, steel sheet metal and cooper come to three metals - add saltwater and now you need to worry about galvanic corrosion (and making a battery)

Tom M'Guinn

Yeah, that thought crossed my mind as well. Might try my hand at copper coating a helicoil. I found a picture of copper helicoils on google, but can't find an actual source for them. There is an oversized copper landing under the bolt/washer head area on the firewall, however, where the cable actually makes contact, so assume most of the current flows right there. But I hear you...

I may have been unclear, but wasn't planning to use a ss bolt. The firewall has a copper boss inert in the firewall, and the threads in that (made of copper) stripped. I was mulling whether to put a helicoil (made of ss) in the copper to fix the threads. I'm in a dry CA climate, but would rather not bank on that...

I may see if there's enough material to move up to M9 or M10 without an insert...

Voith

Stainless is bad conductor cut new threads and use one size bigger steel bolt.

Tom M'Guinn

Must be Friday. See above -- I was never planning to use a SS bolt. I need to fix the copper threads in the firewall, and traditional thread repair coils (helocoils) are made out of SS.

odonnell

From a galvanic corrosion perspective, SS is probably the closest you'll in terms of metal nobility unless you come across some tin thread-serts, somehow. If you applied dielectric grease to the outside of the connection, to minimize O2 getting in, steel would probably be ok.

Tom M'Guinn

Yeah, been looking at those charts. Unfortunately, all the common helicoils are 304 stainless, which is much further from copper than most other SS.

divil

I just looked up timeserts (which are what I use whenever I have to repair threads) to see what they're made from, and apparently they are available in either stainless or carbon steel. (http://www.timesert.com/html/faq.html#3)

Now, I don't have a clue how well carbon steel conducts compared to stainless, but it might be worth looking in to.

V2Rocket

I'd just drill the hole and put a through bolt/nut in it.

The ground of the battery, connects to a steel lug, in copper threads...in a steel chassis.
Steel is fine..

Tom M'Guinn

Yep, I'm pretty much there. I've had a nut on the back for years, just thought this would be a chance to fix it with the battery box stripped down for rust repair. I'm not inclined to drill it, as the threads make plenty of good contact, they just won't hold the torque. The cable lug sits flush on the big copper landing, so imagine that's where all the current flows anyway. Interesting side note -- the factory cable had a copper lug so it was copper on copper. The Iceshark cables have a plated steel lug, which is showing a little pitting after all these years. Score one for the factory there.

V2Rocket

Maybe a PITA, but I wonder if you could fill the hole edges with liquid solder, let it cool, then re-tap the threads?

Or there's always stuff like this

http://www.ebay.com/itm/15-Rectangul...-/201555479497

Tom M'Guinn

At that point I think it would be easier to braze in a spare copper boss, which I actually have from a junked 944. But even I am not that obsessed. I think I'm down to using a nut or finding a copper insert... I'd tap it up to M10, but there's not a lot of spare material to work with, and the lug would have to be drilled, etc. M9 bolts are rare as hens teeth it turns out.

V2Rocket

3/8" self tapping bolt?
about 1/2 a mm larger diameter than the M9, would probably wiggle its own way into place.

944Phil

Hi tom,

Copper and stainless aren't actually to far off from each other galvanically. Also since the helicoil is relatively small compared to the copper it makes it even better.

That said it is a wet area and usually helicoils would be installed with a corrosion inhibiting compound or primer, which you can't do because you need conductivity.

You can use the stainless helicoils, but you need to seal off the stack-up with sealant or some grease afterwards to prevent water from getting in there, then you should be good.

I'm a materials engineer in aerospace. I do this all day long. We have this issue a lot with stainless helicoils installed in aluminum, which is worse then copper. As a rule of thumb, if you can keep water from getting in then galvanics isn't a concern. All water should be considered as an electrolyte, not just salt water. There are so many contaminants in the engine area and on the road that it's never going to be pure water anyway.