Spark Plug feedthroughs 101

[Doug Coulter]

Good point, Joe. Yes, probably it should be removed, and maybe replaced with something lower vapor pressure and more inert (copper, say). Iron is not so great in vacuum, once clean (which vacuum will do) and then exposed to air, it rusts fast -- then believe it or not, the rust decomposes in vacuum and slowly gives off oxygen. If it's going to get hot, Zinc is going to make quite a mess. This hasn't come up in the ones I've done so far -- the model A plug isn't plated (it's gun-blued!) and the other one I did was submarined into the flange I had it in, so wasn't exposed too much. Lucky it's easy to get rid of -- weak HCl is the ticket there.

[Joe Jarski]

I found these at the local hardware store. They have a little longer longer reach than your average spark plug if you need it, but cost 2-3 times as much. This one is a Champion plug used in torpedo heaters.

Champion F121501

[chrismb]

I decided to prep my baseplate with M14x1.25 threads to receive the much more common sizes of spark plug threads. This meant I could go for the BCP7ET triple electrode version. The advantage of this is that because the triple electrodes spark to the sides of the electrode tip the electrode is therefore a bit longer, enough such that I can saw a notch into the electrode, twist some wire around it, and then solder the wire solid.

I guess there are a number of ways to skin this cat. I'll post up my variation in due course once I have received some suitably sized seals and plumbed them into my baseplate.

[Doug Coulter]

Joe, do you know if this one has a built in resistor or not? If not, that looks like the best seen so far -- worth the price of an odd tap if needed.

[Joe Jarski]

Unless I am mistaken, the threads are a M14 X 1.25mm. The resistance is 2 Ohms which is higher than I expected, but I've never measured spark plug resistance before - still not a resistor plug at ~5k.

[Doug Coulter]

Excellent, so not even a weird tap. A few ohms might be ohmmeter error -- that is certainly not something you'd worry about much. Remember, if it's galvanized you'd want to remove that before vacuum use, it really makes a mess when it sputters in the tank -- weak HCl does that quick and easy. Why not pick up a few extra and bring them along when you visit, for trade?

[Joe Jarski]

Why not pick up a few extra and bring them along when you visit, for trade?

Good idea! I just bought one because I wasn't absolutely sure that they weren't resistor plugs, but I will go back and get more now that I know.