A Plug For Bosch
by David Braun
You are looking at a photomicrograph of the tip of a WR7DP Bosch spark plug which came out of a BMW R80G/S after about 9,000 miles of operation. I dunno if you ever saw a platinum plug. But they look nothing like traditional plugs at the electrode.
What the pic shows is this... at the bottom, that sort of rounded thing is the porcelain tip. At the top, that gnarley thing is the ground electrode. So, the plug is oriented upside down in the photo since this is the way you normally INSPECT spark plugs. What's that rivet-looking thing? Good question! In fact, that is *my* question.
My hypothesis is that the platinum filament inside the porcelain fractured and began to hop up and down inside the porcelain. Of course, when it was actually touching the electrode, the "spark" was then inside the porcelain, causing the motor to miss. This was sporadic and got progressively worse over a 50 mile ride. Finally, the plug "fouled" entirely. I believe that the constant hammering of the filament against the ground electrode caused it to deform into it's present rivet appearance. When it finally lost enough of its longitudinal dimension, it came out of its hole in the porcelain far enough to cock. And that is when it failed entirely.
I contacted Bosch to ask about this failure and see if they wanted to look at the photomicrograph. They gave me an address and asked me to send them the spark plug because failure analysis from a photograph is problematic at best. I sent them the plug, without any letter, to the address they suggested. I included my business card, however. About two weeks after I mailed the plug, a package arrived for me at work. Inside were EIGHT WR7DP plugs. That's 40,000 miles worth! The enclosed letter said something about "Your claim for consideration under our Goodwill Policy has been processed... We apologize for any inconvenience this situation may have caused you." Thank you Bosch.
Incidentally, if you plan on using Bosch platinum plugs (which are GREAT plugs, btw) or any other resistor-type plug in a BMW airhead, you'll need to get rid of your 5000 Ohm spark plug caps and get some 0 Ohm caps. It'll RUN just fine with the resistor caps. But your gas mileage will be adversely affected.
