# Spark Plug Feedthrough?

#### scubasteve_911

Joined Dec 27, 2007
1,203
Hi all,

I need to pass a signal through an aluminum block and have a really tight seal. I need to withstand about 5000PSI and maintain a seal.

Is it possible to use a metal gap filling adhesive along with a sparkplug to do this? I was thinking of machining the same thread and applying a strong adhesive.

Does anyone know how much pressure a spark plug is designed to withstand?

Besides this, I am only worried about the tempco of expansion, since this may break the seal. I am only working with a 25degree delta though, so it may be insignificant.

Steve

#### PackratKing

Joined Jul 13, 2008
847
Doing a quick google through ICE combustion pressures, it showed a high end for nitromethane fuel near 4200 psi, on the low side, approx 600 - 1000 psi for " regular" fuel. Bear in mind that a sparkplug must contain these pressures repeatedly, based on the rpm and acceleration load of the engine. That represents quite a pounding.

How thick is the aluminum block you have to go through ? is the pressure you need built up and maintained constant ?

I would suggest using a sparkplug with a long reach on the threads, at least 12 pitches, and it should contain that pressure. A sealant / adhesive shouldn't be necessary.

#### scubasteve_911

Joined Dec 27, 2007
1,203

The aluminum is about an inch thick, so it isn't the weakest link. The pressure is applied very slowly, I would guess at a rate of 1psi/second.

You don't think a sealant is necessary? I suppose aluminum is fairly soft and if there is sufficient torque, you may get a high pressure seal. I would rather play it safe and use an adhesive to fill any remaining gaps. Maybe a 5mil gap filling adhesive?

Are the mentioned plugs standard-issue? Or, do I need to buy some high performance stuff? Do sparkplugs come in small sizes?

Steve

#### scubasteve_911

Joined Dec 27, 2007
1,203
Oh, I should also mention that I want about 1A continuous flowing through.. Apparently sparkplugs have internal resistors? I really wish I knew more about sparkplugs... it's a funny subject, I just found the wikipedia page, hopefully I will find out if this is feasible.

Steve

#### SgtWookie

Joined Jul 17, 2007
22,220
Steve,
SOME sparkplugs have internal resistors. In the Champion brand of plugs, these were designated with an "R" in the part number. However, they re-vamped their part numbering scheme, and nowadays you really have to look at the datasheets or catalogs.

But really, what you want is a hermetically sealed thru-bulkhead connector.

This company:
http://www.pavetechnologyco.com/index.html
makes such connectors, some rated up to 10kPSI. They would certainly be more expensive than a spark plug, but they are designed specifically for the purpose you have in mind. You sure don't want a failure with 5,000 PSI of force behind it, particularly if the vessel has any significant volume. It'll turn into an unguided missile. If the threads of the spark plug or seat fail, the gas vent will be that much larger, with a much greater potential for lethality.

Whatever size thru-bulkhead connector you wind up using, I suggest you select one that would wind up with the smallest hole possible in case of failure; furthermore plan the mounting of the vessel so that any possible failure of vessel fittings could not result in the vessel being put into motion; ie: bolt that sucker down really well, and I mean more than to just a workbench. I'm talking several large bolts, sunk into reinforced concrete.

When I was on active duty, someone in a neighboring squadron managed to knock over a nitrogen bottle that had 3,000 PSI in it. The valve got broken off the end, which basically turned the bottle into a rocket motor. It went right through a concrete block wall. By some miracle, no one was even injured.

#### scubasteve_911

Joined Dec 27, 2007
1,203
Wookie,

I truly appreciate your concern, you always are on the side of caution. I am actually designing an autonomous underwater vehicle and am trying to seal the thruster motor's electrical connection. It's all low-voltage and the failure mode will be a sunken 5000$vehicle, rather than death to all around I just found some feedthroughs on ebay and they didn't have a part number, but I scored them for 9$ each. They look very similar to the products from pave tech, so I am going to try to cross them and hope for a pressure specification. Thanks! Maybe I can beg them and play the whole "I'm a poor student" thing?

I am still very interested in using some matured and cheap technology to make these feedthroughs. I wish there were some sites on the net about this.

Steve

#### SgtWookie

Joined Jul 17, 2007
22,220
OK then, that makes sense.

You'll find quite a number of hermetic seals are made with glass as an insulator. I've looked at a number of them under microscopes. It appears that the solid wire going through the glass insulator had been bead-blasted or the like to roughen it up prior to being fused with the glass under heat and pressure. The conductor is somewhat vulnerable; if bent it can cause the glass to crack due to the mechanical advantage. Nothing like the insulator on a spark plug, however - those would be relatively easy to break off.

#### scubasteve_911

Joined Dec 27, 2007
1,203
It's very interesting stuff, too bad you need pretty specialized equipment to make such things. I was considering pouring an epoxy or something, but was worried about the water getting absorbed or wearing it away.

Steve

#### PackratKing

Joined Jul 13, 2008
847
Wookie,

I truly appreciate your concern, you always are on the side of caution. I am actually designing an autonomous underwater vehicle and am trying to seal the thruster motor's electrical connection. It's all low-voltage and the failure mode will be a sunken 5000\$ vehicle, rather than death to all around
Steve
OK.....so the pressure is from the exterior. That is the proverbial horse..........

Sounds like you have it covered.