I would suggest using the NGK BKR5EIX – an iridium 14mm long reach plug. I use this on my 1936 model, and for the first time, no misfiring or other plug problems. This type of plug is a bit expensive, and a cheaper variety, NGK BP 5 E can also be used. I have presumed that long reach plugs will be suitable for your cylinder head. If short reach is used, go for a plug in the 5-range. If you look in Yowl for October 2006, pages 462 and 463, you will see more details.