Surprisingly it is very difficult to find good quality vulcanised (red fibre) washers to fit 1/4"BSP fittings. These red fibre washers are of the highest quality (from a specialist manufacturer) and have just the right external diameter - but are a very tight fit on the thread. To fit them I place the breather (or similar 1/4" BSP fitting) in soft jaw vice, then place these washers just onto the thread - then I carefully use a pair of large pliers (the circular hole with coarse teeth) to carefully wind them onto the thread. Although this is difficult - the result is the correct size washer that does not look out of place under a Norton breather (unlike many modern ones that are far too wide and look out of place). These are high quality washers from a top manufacturer - sold in pack of 5. Note: do not buy if you are worried you might not be able to fit them!!