Good quality Copper washer suitable for 1/4" BSP fittings on most Norton SOHC/OHV/SV engines - i.e. One Way Valves, Breather Unions and Crankcase Oil unions. They can also be fitted to 1/4 BSP petrol taps if required.
Sold as a pack of 3
Update 2022: These copper washers are far better quality than most others I have seen available - they are a good 'close' fit onto a 1/4" BSP thread (most are not), they are not too wide on their outer diameter - just right for Norton unions, and most importantly are a genuine 1.4mm thick - which is perfect for our unions and thicker than most other similar copper washers available.
These may be slightly more expensive than some others you will see - but are the best available I can find (and I have been to a lot of suppliers over the years!)
Note: for many application of breathers and unions on Norton engines - it is usual to find both copper and red fibre washers fitted (sometimes even both togeher!). I think it is personal preference and have used both types myself - but tend to favour these copper washers over red fibre on breathers and one way valves. However, if you want a washer that will 'Screw' onto the thread of a Norton 1/4" union or sump plug . . . but is then a good fit on the shoulder behind the thread (which some Norton unions have) - we also sell a special Red Fibre Washer, which we specially thread for this purpose - see item 0249