BMW R850 Maintenance Manual page 84

you see, I've been using non-OEM plugs in my oilhead since about March 1996
(about 28K miles) with TOTAL success.
By the way, BMW changed their own plug spec to a two-electrode variety in July
1996 under the auspices of the "Low Friction Throttle Cable" service bulletin
#2748 which was an attempt to solve the surging I was investigating at that time.
In so doing, BMW revised the specified plug to the two-electrode Bosch
FR6DDC, a slightly hotter running tip. Some have reported the newer Bosch
plug is an improvement, but many other riders I have communicated with say the
Autolite or Champion plug is, in most cases, quite superior in operation and
surging minimization.
What's my favorite? Autolite. They exhibit minimal wear during the 12,000 mile
specified service life and the center electrode color (on my RS) is a perfect
medium to light tan on the insulator. My experience with Champions (in K-bikes)
is satisfactory--quite good actually--but the Autolites seem to win out in life
expectancy, electrode wear, and have a performance edge that I "think" I can
feel. Perhaps this is just a prejudice, though.
Some riders have reported that the non-OEM Autolites or Champions are 1/8"
longer than the original Bosch plugs. I don't find this to be the case with the
Autolites. The measured length from the crushed gasket to the tip of the
electrode is 22.5mm on the original 3-electrode FR5DTC and 23mm on the
Autolite 3923. Perhaps the length of the FR6DDC is slightly shorter and/or the
Champion RC9YC is slightly longer equating to the reported difference. I haven't
measured the Champion, but in any case there have been no reported
interference problems and all of these plugs are 14mm, ¾" reach and take a 5/8"
socket. Use a 12 point thinwall socket to clear the small cylinder head access
hole.
So here are your options, gapped at .031" or .8mm:
OEM:
Bosch FR5DTC (3-electrode)
Bosch FR6DDC (2-electrode)
Non-EOM:
Autolite 3923 (1-electrode)
Champion RC9YC or RC9YC4 (1-electrode)
(Note: If you are hard-core Bosch, their 1-electrode Autolite 3923 equivalent is
the F6DC)
One VERY good thing about these non-OEM plugs is their availability and price.
For example, you can purchase the Autolites at a store such as Auto Zone or
84