The topic of how many washers are required around the burner/priming ring/riser 2-layer joint has been discussed in several threads. This thread is intended only to provide a quick, easily identifiable, link into the topic. Basically stoves originally came with one or other of 2 different types of assembly at this 2 layer joint. One was designed to use only one washer, the other was intended to use 2. An additional issue, however, is that wear and tear on some originally 'one washer' joints means that these too sometimes now require 2 to achieve an effective seal. Two photos below show a riser top designed for one washer on the left, and on the right the 2 washer type: And with the spirit/priming ring fitted: The spirit rings themselves are different too: The 'one-washer' spirit ring has a larger diameter central hole with almost no lip to sit a washer into, whilst the central hole in the 2-washer spirit ring is only just sufficient to clear the burner threads and with a nice seating area for a washer under the ring as well as one on top. The key difference between the 2 types of riser tube is that the top surface of the riser designed to use only one washer has a stepped profile, with the lower level being on the outer edge as here: , whilst the 2-washer type has a flat top to the riser wall with a concentric groove machined into it as in the 2nd photo above. The 'one-washer' design was used most extensively by Sievert/Svea on their stoves and on quite a lot of Primus stoves too, as well as in a few other cases. The one further complication is that some Primus stoves with the grooved top risers were originally sent from the factory with only a single washer fitted under a wide hole spirit ring and the whole assembly simply crushed when the burner was tightened to a point where the washer was squeezed up to fill the gap! (See this photo in this thread.) Stoves originally assembled this way are frequent candidates for moving to 2 washers on subsequent re-assembling! Further discussion of the topic can be followed up here and here.