Whos is familiar with radial engines?

Below is a picture of part of a Pratt & Whitney R-1830 radial engine. This is the engine that powered the DC-3, C-47, and other aircraft. I have labelled a few parts but there is one part that I am wondering what it is. It is the wire to the right of the spark plug wire in the picture. Does anyone know what it is for?

http://i429.photobucket.com/albums/qq18/Tim_B/R-1830whatsthis.jpg

same thing, there are 2 plugs per cylinder.

Okay, shoulda known with the level of redundancy used in aircraft systems. I kinda thought that's what it might be but because it was so far from the other one it threw me off.

It will run over the head into the other side of the head.
Aero engines normally have 2 plugs, one on either side of the head, similar to the Alfa T-spark system

Since the engine has two magnetos then do each of the two plugs per cylinder work off a different distributor system powered by a different magneto or do both magnetos power both sets of plugs?

I'm no aero mechanic but I hope my Google-fu is up to the task. Try this and look for the section on "ignition." It isn't the end all, be all but it could answer some questions.

If this isn't enough information, try contacting a company that rebuilts vintage P&W engines.

Got to ask -- just doing some research or are you working on a rebuild?

Tim B said:

Since the engine has two magnetos then do each of the two plugs per cylinder work off a different distributor system powered by a different magneto or do both magnetos power both sets of plugs?

Click to expand...

one magneto powers one bank of plugs, its a redundancy thing
an aviation ignition switch is marked Off, Left, Right, Both, Start
part of the preflight runup is turning the switch from "both" back to right, then left to test each bank individually before takeoff

After the engine has been started, on most light aircraft you throttle engine up to about 1800rpm (well beyond idle), and then switch ignition systems. If the RPM drops more than 150rpm on either Left Only or Right Only, the ignition system is suspect, and should be repaired before taking off. Magneto's are used becuase they are simple,reliable and need no external elecrical source. Aircraft engine are also low compression so that the prop can turn them over if they stall, and can also be started by flipping the prop by hand. Having started an O-360 (which I admit isn't a radial) engine a couple of times with my bare hands, I definitely don't recommend it however.

Most light aircraft can be flown with no functioning electrical system and a dead battery. Not recommend, but if you are in the middle of nowhere with a dead battery, the choices are very limited.

Everyone that answered were right on track, so nothing to add other than a pair of PW 985's sitting behind my Track-T in my old neighborhood.