Original message:
I somehow managed to burr the spark plug threads on one cylinder last night.
All the tin on these air cooled motors makes changing the plugs much harder
than it should be.

Update:
Got lots of good advice on this one.  The cylinder that was damaged was the
rear drivers side (rear of the vehicle).  This is one of the cylinders
buried under lots of cooling tin.  I picked up a thread chaser and tried
cleaning up the threads with the engine in place.  This wasn't working so I
dropped the motor and stripped off the cooling tin so I could get right on
top of the cylinder.  This still didn't work very well.  I eventually got
the threads cleaned up enough that they would accept a spark plug, but it
fit very loosely compared to the other cylinders.  At this point I was in
pretty deep so I just kept digging.  I pulled the head and brought it into a
shop and had a heli coil inserted.  The motor is back together and the heli
coil looks great.  When I pulled the head I found several clumps of metal
shavings stuck in the cylinder despite using liberal amounts of grease on
the chaser and hitting the cylinder with compressed air and a vacuum.  In my
mind that discovery justifies the extra effort I made to pull the head
rather than inserting the heli coil with the motor in place.  So that was
fun.

C-ya,
Jake