The first few hundred engines were fitted with a fabricated steel pot. I know of one such engine (#40117) that was so fitted. Unfortunately, there being no information that it might be original, it was scrapped. A 1914 advertising postcard shows this style but contains much artistic licence so must be considered with caution.

The first, approximately, 1500, engines had a small exhaust flange on the crankcase, the same size as that of the 2 1/2 hp engine. Following the fabricated pot a cast pot with small flange was fitted. I'm told that the pot is otherwise identical to the "Tulip top" but have no picture. No doubt the flange was designed for a lighter fabricated pot and proved either unequal to the weight of the cast pot or too small for the required gas flow.

From about 1917 until 1923 the Tulip top became the standard. The outlet fitting originally unscrewed and was fitted with a pierced steel baffle.

Briefly in late 1923 and early 1924 an interim style was fitted. I had thought this style appeared on 5hp S types only but have now seen it on 5hp and 8hp M and S within the date range. It is the same basic shape but around half the weight. I can only surmise that it proved beyond even the skilled casters at Petters to get a consistent result with an acceptable failure rate and it was abandoned

From 1924 until 1940 the Bolt top became the standard. The outlet flange rim carries the legend "Reversible" and has two diameters of outlet. The larger was intended for fixed engines whilst the smaller was suitable for the shorter exhaust pipes fitted to portable and semi-portable engines.

External ground silencers and wall spacers were available for fixed engines.