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Simon, that size would be perfect, and the price looks to be correct. I tried to order some from Amazon in US but they won't ship to UK either.Here in rip of Britain, RS want $37 for a piece 6mm dia x 100mm long, which is $111 for 12"! I reckon I need 3 x 12" pieces to do both engines.......
The PTFE erosion was one of the reasons I opted to keep the PTFE out of the combustion zone. The other is it can give off some rather nasty vapours when exposed to heat above it's comfort zone. This is how I went about making my plugs below. The smaller plug is 4.5 mm thread shown against my standard 1/4-32, this smaller plug uses the same type of construction.My best regardsGray,