What is the thread on the cartridge?
Anyhow, what is the thread? The Lindal company specification for valve B188 says the thread is ‘7/16 NS’, but this is misleading. The ‘NS’ refers to an old (American) National Standard thread type which was replaced by the Unified National Standard, now usually known as UNC (coarse) or UNF (fine). The 7/16 means 7/16″, which matches, sort of, with the diameter I have measured on several cartridges of about 0.415″. I say ‘sort of’ because 7/16″ is 0.437″, which is some 0.022″ (0.56 mm) bigger than the measured 0.415″ on the cartridges. But then, a close inspection of the thread on various cartridges tells you they are all seriously under-size. This aberration is doubtless due to the thin wall of the metal behind the rolled thread, and does not present a problem in my opinion (or apparently in the opinion of either the manufacturers, who would have some liability concerns, or the Standards Authorities). Measuring a range of stoves and cartridges I have found that the pitch is 28 tpi, whi
