Originally Posted by chasbmw
On my 90/6, with 14mm dual plugs, I use hotter running plugs on the bottom as an anti fouling measure, using iridiums on the 90/6 and NGKs on the 1070. The 90/6 was using a bit of oil as the rings did not bed in properly first time round, so last winter had the cylinders lightly glaze busted. Seems better now.
Jim Cray who did the work and is the main man for dual plug setups in the UK, told me straight that he has never had any head cracking problems by using 14mm lower plugs.
I had upper and lower 14mm on my 75-->90/5. On this one I let Ted Porter make the call and he called 12mm lowers.
I can get NGK Irridiums in an 8 for the top, but only the plain janes for the bottom. I'll start with plane janes everywhere. I'm using resistor plugs and could be nice to have fresh resistors every so often.
I always liked the bosch platinums, but you need 14mm to run those on the bottom. I like to burn a little oil, the motor was designed for it. But I got a feeling this is going to be a hot runner. It has the stock oil cooler but no thermostat, the Briel cooler on the pan and I'm considering some dead loss water cooling as well. I Have it fitted with a lot of temperature instrumentation. Will have to see.