Here we have a Champion D18Y spark plug. 18mm thread. I think these were fitted to various old tractors and maybe stationary engines but please check your own engine for suitability of fitment as I can't do returns. Thanks

I have a good stock of new-old-stock Champion spark and glow plugs including some very obscure part numbers. Automotive, motorcycle, racing, marine, stationary engine, tractor, industrial etc etc. Let me know your requirements and if I can help I will list for you.